Nitric Oxide Donor DETA/NO Inhibits the Growth of Endometrial Cancer Cells by Upregulating the Expression of RASSF1 and CDKN1A.

Nitric oxide (NO) is implicated in several biological processes, including cancer progression. At low concentrations, it promotes cell survival and tumor progression, and at high concentrations it causes apoptosis and cell death. Until now, the impact of NO donors has not been investigated on human endometrial tumors. Four cancer cell lines were exposed to different concentrations of DETA/NO for 24 to 120 h. The effects of DETA/NO on cell proliferation and invasion were determined utilizing MTS and Boyden chamber assays, respectively. The DETA/NO induced a dose and time-dependent reduction in cell viability by the activation of caspase-3 and cell cycle arrest at the G0/G1 phase that was associated with the attenuated expression of cyclin-D1 and D3. Furthermore, the reduction in the amount of CD133-expressing cancer stem-like cell subpopulation was observed following DETA/NO treatment of cells, which was associated with a decreased expression of stem cell markers and attenuation of cell invasiveness. To understand the mechanisms by which DETA/NO elicits anti-cancer effects, RNA sequencing (RNA-seq) was used to ascertain alterations in the transcriptomes of human endometrial cancer cells. RNA-seq analysis revealed that 14 of the top 21 differentially expressed genes were upregulated and seven were downregulated in endometrial cancer cells with DETA/NO. The genes that were upregulated in all four cell lines with DETA/NO were the tumor suppressors Ras association domain family 1 isoform A (RASSF1) and Cyclin-dependent kinase inhibitor 1A (CDKN1A). The expression patterns of these genes were confirmed by Western blotting. Taken together, the results provide the first evidence in support of the anti-cancer effects of DETA/NO in endometrial cancer.

1. Introduction

Endometrial carcinoma ranks as the fourth most common cancer in women in America, with 61,880 new cases and 12,160 deaths anticipated in 2019 [1]. There is an increase in the number of patients in an advanced stage or with a high histological grade, which reflects poor prognosis. The treatment of early-stage disease, commonly consisting of surgery followed by chemotherapy, is very effective with a cure rate of 90%. However, over two-thirds of patients are diagnosed with advanced stages, in which 5-year survival rates are only 10% to 15%. Thus, additional efforts to define effective intervention strategies against endometrial cancer progression are urgently needed.

Nitric oxide (NO), a free radical, is involved in several diseases including cancer [2]. It is endogenously synthesized in various tissues by the transformation of L-arginine to L-citrulline in three distinct isoforms of nitric oxide synthase (NOS). The neuronal (nNOS) and endothelial (eNOS) are calcium-dependent isoforms that yield low fluxes of NO in the pico-to nanomolar range for a short period of time varying from seconds to minutes for regulating intercellular and extracellular signaling pathways [3]. Inducible nitric oxide synthase (iNOS) is the calcium-independent isoform and is induced by cytokines, endotoxins, and hypoxia under oxidative stress. iNOS produces significant concentrations (micromolar range) of NO for a longer period of time ranging from hours to days [4,5].

A number of in vitro and in vivo studies have established that NO regulates many signaling molecules that control diverse cellular processes, including immune response, angiogenesis, metastasis, and apoptosis [6,7,8,9]. In cancer, NO has concentration-dependent effects. At pico- to nanomolar concentrations, it promotes tumorigenesis, whereas at higher concentrations (micro- to millimolar) it has antitumor effects [4,10]. NO contributes to cancer progression through the stimulation of mitogenic pathways. Studies have shown that NO promotes a number of oncogenic pathways. Activation of the Wnt/β-catenin pathway and inhibition of Dickkopf-1 by NO resulted in the proliferation and migration of breast and colon cancer cells [11]. Similarly, Garrido et al. [12] demonstrated that NO induced the stimulation of the extracellular signal-regulated kinase (ERK) and the epidermal growth factor receptor (EGFR) pathways in breast cancer cells, which subsequently increased their migration and invasive potential. Contrary to the tumorigenic promoting role, NO displays an anti-cancer role by censoring oncogenic or by stimulating tumor attenuating pathways. NO has been shown to inhibit the growth of human neuroblastoma cells by diminishing the expression of the c-Myc [13] and by enhancing the expression of tumor suppressor BRCA1/Chk1/p53 pathway, leading to cell cycle arrest [14]. Importantly, low concentrations of NO stimulated cell proliferation in cancer cells by inhibiting apoptosis, while higher concentrations inhibited proliferation by enhancing apoptosis [4,15,16]. The characteristic concentration-dependent response of NO has been used in pre-clinical models of cancer to diminish tumor growth and to improve the efficacy of both chemotherapy and radiotherapy [17]. Several ongoing investigations are developing strategies for manipulating NO production in vivo and exploring the exogenous delivery of NO donors for therapeutic benefit [18,19].

Several derivatives of NO donors (NONOates, DEA/NO and PAPA/NO, S-nitrosothiols) have been examined for their anticancer influences on a variety of cancer cell lines [6,7,11,12,13]. However, the effects of NO donors on human endometrial cancer have not been investigated yet. The goal of this study was to comprehend the molecular mechanisms of its anti-tumorigenic activities and apply the knowledge acquired in this study for the development of new and effective therapies for endometrial cancer.

2. Results

2.1. DETA/NO Suppresses the Growth of Endometrial Cancer Cells

To investigate the inhibitory effects of DETA/NO, four endometrial cancer cells were treated with various doses of DETA/NO for 24, 48, 72, 96, and 120 h, and cell viability was assessed using MTS assay. All of the cell lines showed a dose-dependent decrease in cell number at higher concentrations after 24 h of treatment. However, some cell lines were more responsive to DETA/NO than others. The dose of 250 µm decreased cell proliferation by about 40% to 45% in the first 24 h. About 70% to 75% suppression in cell proliferation was observed at 120 h (Figure 1). Lower concentrations failed to affect cell proliferation.

Figure 1

DETA/NO attenuated growth of endometrial cancer cells. Endometrial cancer cells were treated with various doses of DETA/NO (37.1, 62.5, 125, 250, 500, and 1000 μm) every 24 h for 120 h and cell viability was measured by MTS assay. Results represent the average of six wells expressed as the percentage of untreated controls (mean ± SEM). * p < 0.05 is statistically significant between the control and treatment groups.

2.2. DETA/NO Induced Caspase and Suppressed PCNA in Cancer Cells

To identify the mechanism by which DETA/NO decreases the growth of cancer cells, we investigated its consequences on apoptosis. The expression of the caspase-3 at the protein level was analyzed by Western blotting. The results showed a significant cleavage of caspase-3 in cell cultures following DETA/NO treatment (Figure 2A,B). In addition, we analyzed the enzymatic activity of caspase-3 against its specific substrates DEVD p-NA after 24 h of exposure of cells to DETA/NO. The activity of caspase-3 in cultures of DETA/NO-treated cancer cell lines was significantly higher (2–4-fold) than the activity found in untreated cultures (Figure 2C). The effect of DETA/NO on the expression of PCNA was analyzed by Western blotting. A marked decrease in the PCNA expression was seen in DETA/NO-treated cancer cells (Figure 2A,B).

Figure 2

DETA/NO enhanced caspase-3 and attenuated PCNA expression in endometrial cancer cells. (A) Proteins from vehicle or DETA/NO-treated AN3CA, KLE, Ishikawa, and HEC-1B cells were subjected to electrophoresis, and immunoblots were probed with caspase-3 and PCNA antibodies. β-Actin was used as a loading control. A representative experiment is presented. (B) The expression of cleaved caspase-3 and PCNA in DETA/NO-treated and vehicle-treated cells is shown as bars after normalization to β-actin. Data are expressed as means ± SEM of three independent experiments. An asterisk * (p < 0.05) indicates a statistically significant change between control and treatment groups. (C). The enzyme activity of cell lysates from vehicle or DETA/NO-treated cells toward caspase-3 substrate was assessed using the caspase-3-specific substrate DEVD-pNA. The caspase activity is presented as folds relative to controls. Error bars represent mean ± SEM. Statistically significant increases in levels of caspases-3 are shown by an asterisk (p < 0.05).

2.3. DETA/NO Instigated Cell Cycle Arrest at the G1/S Phase

To establish the effect of DETA/NO on the cell cycle, vehicle and DETA/NO-treated cells were subjected to DNA flow cytometry. The results showed an increased cell population in the G0/G1 phase and a decreased cell number in the S and G2-M phases in DETA/NO-treated cells compared with their respective vehicle-treated cells (Figure 3A and Table 1). To explore the mechanism by which DETA/NO controls the cell cycle of endometrial cancer cells, we evaluated the levels of cell cycle regulatory proteins. The levels of cyclin-D1 and cyclin-D3 decreased while P21 increased in DETA/NO-treated cells, contrary to the control groups (Figure 3B–E).

Figure 3

DETA/NO suppressed cell cycle progression. (A) Cells treated with DETA/NO for 24 h and stained with propidium iodide. DNA content was analyzed using flow cytometry. Results are shown as the percentage of cell population in G0-G1, S, and G2-M phases of the cell cycle. (B) Western blot analysis of cyclin D1, cyclin D3, and P21 in lysates of control and DETA/NO-treated cells. The values above the bands represent the relative density of the bands normalized to β-actin. (C–E) The expression of cyclin D1, D3, and P21 in DETA/NO-treated and vehicle-treated cells is shown as bars after normalization to β-actin. Data are expressed as means ± SEM of three independent experiments. An asterisk * (p < 0.05) indicates a statistically significant change between control and treatment groups.

Table 1

Effect of DETA/NO on cell cycle arrest.

Cell Line	Treatment	G0-G1	S	G2-M
AN3CA	Control	57.32 ± 0.34	24.76 ± 0.21	18.03 ± 0.65
	DETA/NO	68.24 ± 0.27 *	21.34 ± 0.16	10.42 ± 0.22 *
KLE	Control	51.34 ± 0.56	31.43 ± 0.35	16.23 ± 0.28
	DETA/NO	63.22 ± 0.42 *	25.41 ± 0.52	11.36 ± 0.19 *
Ishikawa	Control	52.78 ± 0.35	28.24 ± 0.19	18.54 ± 0.56
	DETA/NO	62.34 ± 0.22 *	22.32 ± 0.52	15.32 ± 0.67 *
HEC-1B	Control	61.01 ± 0.42	26.65 ± 0.71	12.21 ± 0.61
	DETA/NO	69.78 ± 0.66 *	19.87 ± 0.26	10.28 ± 0.19 *

* Statistically significant difference from control, p < 0.05.

2.4. DETA/NO Attenuated the Number of Stem-Like Cells in Endometrial Cancer

Previously, we have shown that endometrial cancer cell lines have stem-like cell subpopulations, denoted as a “side population” (SP). These cells have traits of cancer stem cells and are capable of effluxing the DNA-binding dye Hoechst 33342. We investigated the effect of DETA/NO treatment on the SP cell percentages in HEC-1B, Ishikawa, AN3CA and KLE cell lines. The percentages of SP cells in AN3CA, KLE, HEC-1, and Ishikawa cells were 14.26%, 17.12%, 12.64%, and 13.11%, respectively, in untreated cells compared to 4.21%, 5.32%, 6.17%, and 4.37% in DETA/NO-treated cells (Figure 4A). The inhibitory effect of DETA/NO treatment on cancer stem-like cells (CSLCs) was confirmed by analyzing the expression of stem cell markers. A marked decrease in the expression of SOX-2 and ALDH1A1 was observed in DETA/NO-treated cells compared to vehicle-treated cells. However, NANOG expression was not altered by treatment in any cell line (Figure 4B,C). These results implied that DETA/NO reduced proliferation and stemness states in cancer cells through the inhibition of the proliferation markers, PCNA, stemness markers, and ALDH1A1 and SOX2 expressions.

Figure 4

DETA/NO reduced the amount of the stem-like cell subpopulation in endometrial cancer. (A) The percentages of CD133+ (side population, SP) and CD133− (main population, MP) cells are shown in AN3CA, KLE, Ishikawa, and HEC-1B vehicle and DETA/NO-treated cells. The bars are mean ± SEM and * indicates a statistically significant difference between the vehicle and DETA/NO treated-cancer cell lines (p < 0.05). (B) The expression of stem cell markers in AN3CA, KLE, HEC-1B, and Ishikawa cells treated with DETA/NO for 24 h. β-actin was used as a loading control. (C) The expression of stem cell markers in DETA/NO-treated and vehicle-treated cells is shown as bars after normalization to β-actin. Data are expressed as means ± SEM of three independent experiments. An asterisk * (p < 0.05) indicates a statistically significant change between control and treatment groups.

2.5. DETA/NO Alters the Malignant Potential of Endometrial Cancer Cells

Cancer cells have the propensity to grow under anchorage-independent conditions. The malignant potential of cells was assessed using a soft agar colony formation assay. DETA/NO-treated cells formed reduced the number of colonies on soft agar compared to vehicle-treated cell lines (Figure 5A,B). The impact of DETA/NO on cell invasiveness and metastatic potential was investigated using Matrigel invasion chambers. Cells that invaded through the Matrigel were stained with toluidine blue and counted. Of note was the striking inhibition (40% and 60%) of invasiveness caused by DETA/NO treatment in cancer cells compared to vehicle-treated cells (Figure 5C).

Figure 5

DETA/NO decreased the colony formation and invasion of cancer cells. (A) AN3CA, KLE, Ishikawa, and HEC-1B vehicle or DETA/NO-treated cells were cultured on soft agar and imaged to examine their colony-forming ability. (B) Number of colonies on soft agar were counted. Bars are mean ± SEM and * indicates a statistically significant difference (p < 0.05). (C) The number of cells that migrated through the Matrigel was counted following 22 h of plating. Data are mean ± SEM of six wells and * indicates a statistically significant difference (p < 0.05) between control and treated cells.

2.6. Identification of DETA/NO Regulated Genes

Transcriptomic profiling was used to detect genes whose expression was altered after 24 h of DETA/NO treatment in four endometrial cancer cell lines. Principal component analysis (PCA) results revealed distinctive clustering patterns between the DETA/NO-treated samples and vehicle-treated control samples, and this phenomenon was observed in all cell lines. Each treatment group forms its own cluster in the PCA plot without mixing clearly, indicating a very homogeneous transcriptomic profile (Figure 6A). Ingenuity Pathway Analysis (IPA) core analysis identified actin cytoskeleton signaling, calcium signaling, and Cdc42 signaling as the top three canonical pathways in the AN3CA cells; basal cell carcinoma signaling, cardiac β-adrenergic signaling, and cell cycle G1/S checkpoint regulation as the top three canonical pathways in the KLE cells; agranulocyte adhesion and diapedesis, allograft rejection signaling, and altered T cell and B cell signaling in rheumatoid arthritis as the top three canonical pathways in the Ishikawa cells; and actin cytoskeleton signaling, agranulocyte adhesion and diapedesis, and agrin interactions at neuromuscular junction pathways as the top three canonical pathways in the HEC-1B. Furthermore, the common upstream regulator analyses identified Hypoxia Inducible Factor 1 Subunit Alpha (HIF1A), Nuclear Protein 1, Transcriptional Regulator (NUPR1), and Egl-9 Family Hypoxia Inducible Factor 1 (EGLN1) as the top three upstream regulators among these four cell lines. HIF1A and NUPR1 were predicted to be activated and EGLN1 was inhibited in their correspondence pathways (Figure 6B and Table 2).

Figure 6

Comparison of overall expression profiles across all samples. (A) Results of the principal component analyses (PCA) of gene expressions in DETA/NO and vehicle-treated cancer cells are shown. Each color represents one cell line (both vehicle-treated control and DETA/NO-treated samples were the same color). Blue: AN3CA, Turquoise: KLE, Magenta: Ishikawa, Green: HEC-1B. The PCA plot shows each DETA/NO-treated cell line forms its own cluster and is clearly separated from its matching control sample clusters. (B) Ingenuity Pathway Analysis (IPA). The top and common canonical networks identified among fours cell lines (AN3CA, KLE, Ishikawa, and HEC-1B). The pink color indicates upregulation and a light green color indicates downregulation of that particular gene.

Table 2

Top three canonical pathways identified in each cell line.

Cell Lines	Canonical Pathways
AN3CA	Actin Cytoskeleton Signaling
AN3CA	Calcium Signaling
AN3CA	Cdc42 Signaling
KLE	Basal Cell Carcinoma Signaling
KLE	Cardiac β-adrenergic Signaling
KLE	Cell Cycle: G1/S Checkpoint Regulation
Ishikawa	Agranulocyte Adhesion and Diapedesis
Ishikawa	Allograft Rejection Signaling
Ishikawa	Allograft Rejection Signaling
HEC-1B	Actin Cytoskeleton Signaling
HEC-1B	Agranulocyte Adhesion and Diapedesis
HEC-1B	Agrin Interactions at Neuromuscular Junction

Twenty-one genes were identified as statistically significant and differentially expressed in the DETA/NO-treated cells (Table 3). Of the five genes identified in AN3CA cells, four were tumor suppressors and were up-regulated by DETA/NO. In KLE cells, ten genes were upregulated and two tumor suppressors (BIRC5 and DUSP1) were downregulated with DETA/NO. In Ishikawa cells, four genes (DUSP6, MYC, TGFA and TMSB10/TMSB4X) were downregulated and five were upregulated. Seven genes were upregulated and one (FOLR1) was downregulated following DETA/NO treatment of HEC-1B cells. Two tumor suppressor genes (CDKN1A, cyclin dependent kinase inhibitor1A, and RASSF1, Ras associated domain family member 1) were up-regulated in all four cell lines with DETA/NO. The expression pattern of CDKN1A and RASSF1 was confirmed by the Western blotting of DETA/NO-treated endometrial cancer cell lines (Figure 7A,B).

Table 3

DETA/NO regulated genes in endometrial cancer cells.

AN3CA	KLE	Ishikawa	HEC-1B
CDK1A	BIRC3	CDK1A	CDK1A
DUSP1	BIRC5	DUSP6	CYP1A1
IGFBP3	CDK1A	GAPDH	FOLR1
RASSF1	CXCR4	MYC	GAPDH
VEGFA	CYP1A1	RASSF1	RASSF1
	DUSP1	RELA	SPP1
	DUSP6	TGFA	VEGFA
	PLAUR	TMSB10/TMSB4X	VIM
	PTEN	VEGFA
	RASSF1
	RELA
	VEGFA

Genes downregulated by DETA/NO are shown in bold.

Figure 7

DETA/NO treatment of endometrial cancer cells upregulated the expression of tumor suppressor genes. (A) Cell lysates from DETA/NO or vehicle-treated cells were probed with CDKN1A and RASSF1 antibodies. β-Actin was used as a loading control. The values above the bands represent the relative density of the bands normalized to β-actin. (B) The expression of RASSF1 or CDKN1A in DETA/NO-treated and vehicle-treated cells is shown as bars after normalization to β-actin. Data are expressed as means ± SEM of three independent experiments. An asterisk * (p < 0.05) indicates a statistically significant change between control and treatment groups. Effect of RASSF1 or CDKN1A silencing on protein expression (C,D), growth (E,F) and caspase-3 activity (G,H) on DETA/NO-treated and vehicle-treated cells. Data shown are mean ± SEM of values from three independent experiments. * p < 0.05 (statistically significant) between the control and treatment groups.

2.7. Knockdown of RASSF1 and CDKN1A Enhances Growth of Cancer Cells, and Upregulation of RASSF1 and CDKN1A Expression by DETA/NO Inhibits Proliferation

To support the idea that the DETA/NO-induced inhibition of endometrial cancer cell growth is mediated via RASSF1 and CDKN1A expression, cells transfected with either RASSF1 or CDKN1A-targeted siRNA oligonucleotides were treated with or without DETA/NA for 24 h. The expression of RASSF1 or CDKN1A was analyzed by Western blot analysis (Figure 7C,D). SiRNAs of RASSF1 or CDKN1A failed to further suppress RASSF1 or CDKN1A expression in cancer cells. Cells transfected with control siRNA and then treated with DETA/NO showed increased RASSF1 and CDKN1A expression, which was associated with reduced cell proliferation and increased caspase-3 activity. Cancer cells transfected with siRNAs specific for RASSF1 or CDKN1A and then treated with DETA/NO failed to inhibit cell proliferation or enhance caspase-3 activity (Figure 7C–H). The failure of DETA/NO to show growth inhibitory effects on RASSF1 or CDKN1 knockdown endometrial cancer cells alludes to the critical role of RASSF1 or CDKN1 in the DETA/NO-induced growth inhibition of cancer.

3. Discussion

NO has been associated with the pathogenesis of diverse tumor types, acting as either an enhancer or inhibitor of carcinogenesis. Three discrete NO flux scales delineate NO-mediated signaling. Low levels of NO (<100 nm) mediate cGMP-dependent signaling. High levels between 200 and 600 nm NO involve nitrosative signaling that is cGMP-independent. Levels of NO > 600 nm usually encompass a stress response in addition to anti-proliferation mechanisms [20]. Low concentrations of NO donors promote cancer progression by the activation of several mitogenic pathways, including extracellular signal-regulated kinase mTOR and Wnt/β-catenin pathways [11,12,21]. Higher doses of NO donors negatively regulate the proliferation of cancer cells by attenuating the expression of oncogenes and by the upregulation of tumor suppressors (BRCA1/Chk1/p53) culminating in cell cycle arrest via the activation of cell cycle checkpoints [14].

We investigated the influence and underlying mechanism of DETA/NO on the growth and invasiveness of endometrial cancer cells. The findings reported herein indicate that DETA/NO impeded cell proliferation via cell cycle arrest at the G0/G1 phase, the downregulation of cyclin D1, and the upregulation of p21 protein. In the present study, cell cycle analysis showed that DETA/NO-treated endometrial cancer cells displayed a marked increase in the proportion of the G0/G1 phase accompanied by a decrease in the proportion of G2/M phase cells. These results show that DETA/NO arrested cancer cells in the G1 phase. Van de Wouwer et al. [14] observed that short- and long-lived NO donors inhibited the proliferation of human neuroblastoma NB69 cells by controlling G1/S transition via transcriptional repressors pRb and cyclin D1 and also by down-regulating systems regulating the S and G2/M phases. Consistent with these results, sodium nitroprusside (SNP), a NO donor, is shown to cause G0/G1 arrest in HepG2 cells [22].

Tumors are composed of heterogeneous neoplastic cells, among which a subpopulation of cells known as cancer stem cells (CSC) exist with an infinite self-renewal capacity and cell division potential. These cells initiate cancer development, invasion, and metastasis [23,24]. The development of effective therapeutic strategies to eliminate CSCs would significantly enhance cancer treatment outcomes and prolong the survival of cancer patients. Agents that target CSCs have the potential to abrogate tumor development. Previously, we have shown that the endometrial cancer cell lines examined in this study contain cancer stem-like cells (CSLCs). Compared to Ishikawa and HEC-1B cells, a higher number of CSLCs are reported in AN3CA and KLE cells [25]. The silencing of nestin in AN3CA and KLE cells resulted in a decrease in CSLCs and consequently attenuated migration, invasion, and metastasis [25]. In this study, the treatment of endometrial cancer cells with DETA/NO caused a significant reduction in the percentage of CSLCs. Several studies have reported an association between the inhibition of CSLC activity and attenuation of cancer progression and metastasis in human breast, lung, pancreatic, and prostate cancers [26,27]. The manifestation of stemness genes is a key sign for the multipotent characteristic of CSCs [27]. The overexpression of stemness genes (KLF4, SOX2, OCT4, and NANOG) has been shown in many cancers and associated with tumorigenesis [28,29]. The overexpression of ALDH1 in a number of cancers is required for the self-renewal of stem cells and is implicated in tumor metastasis [30]. In colon cancer, high and low NO-producing stem cells are reported. High NO cells are shown to propel stemness-related signaling pathways, which are crucial for tumor initiation and progression [31]. Here, we established an association between NO and tumor behaviors in the context of CSLCs. Our results showed a significant reduction in the expression of stemness genes in DETA/NO-treated endometrial cancer cells. The reduction of stemness markers in DETA/NO-treated endometrial cancer cells suggests that the cells may have lost their multipotent properties, thus promoting the apoptosis of the cells. Therefore, it is logical to assume that DETA/NO can indeed impede the migration and invasion of cancer cells by reducing the number of CSLCs and inhibiting the expression of stem cell markers. An earlier study [32] demonstrated the upregulation of CSC markers (CD133 and ALDH1A1) in lung cancer following NO donor treatment. These findings indicate that NO may mediate its effects by controlling the stemness of cancer cells. Dual roles of NO have been described, and it is recognized that the effects of NO on tumor progression are concentration-dependent. The discrepancy in our results and that of Yongsanguanchai et al. [32] could be explained by the fact that the later study used 25 µm DPTA NONOate, whereas we used 500 µm.

Cyclin-dependent kinase inhibitor 1A (CDKN1A) is a tumor suppressor downstream of p53, which binds to the regulatory region of CDKN1A via its consensus binding sequences in order to trans-activate genes involved in the impediment of the cell cycle via G1/S arrest [33,34]. Furthermore, CDKN1A inhibits the activation of DNA polymerase by binding to PCNA. The activation of DNA polymerase is essential for DNA replication and repair. An aberrant expression of CDKN1A has been often detected in various types of cancer tissues [35,36,37]. CDKN1A is implicated in a number of cellular processes such as cell growth, differentiation, proliferation, DNA repair and apoptosis [38,39,40] and has a crucial role in impeding tumor development by inhibiting cell cycle progression and inducing apoptotic signals. In this study, DETA/NO inhibited endometrial cancer cell proliferation via G0/G1 cell cycle arrest and the upregulation of CDKN1A (cyclic dependent kinase inhibitor 1A). Our results are in agreement with the augmented expression of CDKN1A and increased prevalence of apoptosis in colon cancer cells after the combination of vitamin C and azacytidine (AZA) or decitabine (DAC) treatments [41]. The upregulation of CDKN1A and downregulation of Cyclin B1 is shown in lung cancer cells following aqueous extract of Helicteres angustifolia L. Roots [42].

RASSF1A (Ras association domain family 1 isoform A) is inactivated by methylation in a number of tumors and implicated in the pathogenesis of cancers [43,44]. RASSF1A executes tumor-suppressing functions by regulating apoptosis, microtubule stability and cell cycle regulation [45,46]. Our results indicate that DETA/NO attenuates the proliferation of endometrial cancer cells by upregulating the expression of RASSF1A, which arrested cancer cells in the G1 phase by downregulating the expression of cyclins. These findings are in agreement with a study reporting the role of RASSF1A in suppression of EMT (Epithelial–Mesenchymal Transition), cell migration, invasion, and the metastasis of non-small cell lung cancer cells by inhibiting Yes Associated Protein (YAP) activation through the gef-h1/rhob pathway (Rho/Rac Guanine Nucleotide Exchange Factor 2/Ras Homolog Family Member B) [47]. Natural supplements such as vitamins, methyl donors, and polyphenols have been shown to revert the epigenetic silencing of RASSF1A. Furthermore, plant biomaterial mahanine and its analogs are recognized for their cytotoxic activity against different tumors. Studies have shown that treatment of prostate cancer cells with mahanine resulted in increased RASSF1A expression via the proteasome degradation of DNMT (DNA Methyltransferase) [48,49].

In the current study, lower basal levels of RASSF1 and CDKN1A expression were observed in endometrial cancer cell lines. DETA/NO upregulated RASSF1 and CDKN1A expression in endometrial cancer cells. Experimental changes of RASSF1 and CDKN1A levels perturb cellular processes. We elevated or suppressed RASSF1 and CDKN1A expression in cancer cells with DETA/NO and siRNAs, respectively, and later associated the effect of alterations to cellular functions such as proliferation. The highlight of the study is that RASSF1 and CDKN1A are critical mediators of DETA/NO-induced growth inhibition, as RASSF1 and CDKN1A knockdown diminished DETA/NO-induced growth inhibition.

Our study showed a marked increase in endometrial cancer cell death with higher doses of DETA/NO. Whether DETA/NO exhibits similar effects on primary endometrial cells is not known due to a limitation of their availability. However, immortalized epithelial endometrial (EM-E6/E7-TERT) cells treated with DETA/NO (250 μm/L) had no effect on cell death (results not shown). In addition, Lehmann et al. [50] have shown that Schwann cells treated with high doses of DETANONOate (1 mmol/L) did not show enhanced cell death compared with cell cultures treated with vehicle alone. These findings suggest a specific antineoplastic effect of DETA/NO on cancer cells.

Emerging research highlights the role of NO donors in cancer therapy [51,52,53,54]. The most commonly used nitric oxide donors are S-nitrosothiols, clinical nitrovasodilators, and NONOates (diazeniumdiolates). The clinical use of commercially available NO donors is limited by the development of tolerance, toxicity issues, and massive release in circulation with undesirable systemic side effects. Clinical and nonclinical studies have tested a number of recently developed NO-drug hybrids, whereby a NO moiety is attached to a known anti-cancer agent, which offers additive anti-tumor effects by each compound while diminishing their respective side effects [51,52,53,54]. Several NO-drug donors are investigated for their effect on various tumors. In nitric oxide-donating aspirin (NO-Aspirin), a nitric oxide-donating group is attached to aspirin through a linker molecule. It is reported that sub-or low micromolar concentrations of NO-Aspirin inhibited the proliferation of tumorigenic lung cancer in vitro and in vivo via attenuation of EGFR and its downstream effectors [55]. Lopinavir-NO, a nitric oxide-releasing HIV protease inhibitor, is a strong inhibitor of melanoma cancer cell lines, and NO neutralization reinstated cell viability. Furthermore, the inhibitory action of Lopinavir-NO has been reported on the secretion of IL-6, IL-10, TNF-α, and IFN-γ in ConA-stimulated peripheral blood mononuclear cells [56,57,58]. Basile et al. [59] investigated the effects of NO-derivative Lopinavir-NO (Lopi-NO) on the in vitro growth of human Glioblastoma (LN-229 and U-251) cell lines and found an induction of autophagy and a marked reduction of cell proliferation. Furthermore, treatment of LN-229 and U-251 cells with Lopi-NO reduced cisplatin toxicity in LN-229 cells, while this effect was less noticeable in U-251 cells. Improved safety profile of NO-drug hybrid donors, superior cancer preventive activities, and synergistic combination with chemotherapeutic drugs may have advantages for the long-lasting treatment of cancer patients in the clinical setting.

We acknowledge that the current study examined the effects of DETA/NO only on cell lines in 2D cultures, which does not mimic in vivo conditions. Our goal was to delineate the mechanism of DETA/NO action on cancer cells. We plan to validate these findings in 3D cultures as well in an in vivo setting and publish the results as a separate study.

In conclusion, our results demonstrate that DETA/NO exerts antitumor effects via the inhibition of cell proliferation, induction of apoptosis, G2/M arrest, attenuation of CSLCs number, reduced expression of CSLCs markers, and induction of tumor suppressor genes. These results suggest the potential use of DETA/NO as an effective antitumor treatment for endometrial cancer patients.

4. Materials and Methods

4.1. Cell Culture and Treatment

Four endometrial cancer cell lines (AN3CA, KLE, HEC-1B, and Ishikawa) were used in this study. The rationale for selecting these lines is that they are type I tumor-derived cell lines. They are generally low-grade, estrogen-dependent, hormone-receptor-positive adenocarcinomas with endometrioid morphology and are often referred to as endometrioid endometrial cancers (EECs). About 85% of all endometrial cancers are EECs. These cell lines harbor alterations in the PI3K/Akt pathway, PTEN and KRAS thereby representing the majority of endometrial tumors in the clinic. HEC-1B, KLE, and AN3CA were purchased from the American Type Culture Collection (ATCC, Manassas, VA, USA). Ishikawa cell line was acquired from Sigma (St Louis, MO, USA). All cell lines were tested for cross-contamination by DNA short-tandem repeat analysis by ATCC and Sigma. Cells were cultured in their respective media. While DMEM complemented with insulin (0.005 mg/mL, Invitrogen, Carlsbad, CA, USA) was used for culturing KLE and Ishikawa cells, HEC-1B and AN3CA were cultured in Eagle minimum essential medium (Invitrogen). Fetal bovine serum (10%, v/v, FBS from Invitrogen), penicillin (100 U/mL) and streptomycin (100 U/mL) were added to all media [25,60]. The cells were cultured for twenty-four hours at 370 °C. Later the media were replaced with the same media but without FBS. After 16–24 h, saline (control), 10% NaOH (vehicle) or DETA/NO (treatment) were added, and cells were incubated for 24 h. When treatment ended, cells were washed with ice-cold 1X PBS twice and collected for RNA or protein extraction. To assess the effect of DETA/NO on cell proliferation, endometrial cancer cells were cultured in 96-well plates at 2000 cells per well. After twenty-four hours, cells were exposed to various doses of DETA/NO (37.1, 62.5, 125, 250, 500, and 1000 μm) for 120 h. Following treatment, cell viability was evaluated using a Cell Proliferation Kit (Promega, Madison, WI, USA).

4.2. Cell-Cycle Analysis

Cells were plated onto tissue culture flasks and the medium was substituted the next day with medium containing DETA/NO, or vehicle. Cell-cycle status was measured after 24 h of exposure to treatments. Cells (1 × 106) were collected and centrifuged (5810R, Eppendorf, Hauppauge, NY, USA) at 250 xg for 5 min. The supernatant was removed, and cells were fixed by slowly adding 1.0 mL of a chilled ethanol while vortexing the tube at a low speed. Cells were kept at −20 °C until DNA staining. On the day of DNA staining, cells were centrifuged, and the supernatant was removed. DNA staining buffer (1 mL containing 100 mg propidium iodide; Sigma, St. Louis, MO. USA) was added to the cell pellet and briefly vortexed. Cells were kept in the dark for 15 min at room temperature. The distribution of cells between phases of the cell cycle was determined using a BD LSR II Flow Cytometer (Becton Dickinson, LSR 2) and ModFit LT (version 4.1, Verity Software House, Topsham, ME. USA).

4.3. Effect of DETA/NO on Endometrial Cancer Side Population (Stem-Like Cells)

Vehicle-treated and DETA/NO-treated AN3CA, KLE, Ishikawa and HEC-1A were cultured in their respective media. Subsequently, cells were collected, counted, and resuspended at 1.0 × 106 cells per milliliter (mL). Hoescht dye efflux assay was used to ascertain the side population percentage in each cell line. The cells were treated with Hoechst 33,342 for 90 min at 37 °C. To appropriately detect the side population, cells were treated with the ABC transporter inhibitor Verapamil for 60 min prior to Hoechst 33,342 treatment. To confirm that the side population expresses CD133, cells were cultured with a CD133 antibody for 15 min at 4 °C. The cells were collected and rinsed twice with cold PBS and then incubated with an AlexaFluor-488 secondary antibody for 30 min at 4 °C. The AlexaFluor-488 labeled cells were pelleted, rinsed twice in cold PBS, and resuspended in cold PBS comprising of propidium iodide (PI) for dead cell exclusion. Cells were subjected to Flow cytometry on a BD LSRII flow cytometer equipped with both a violet and an ultraviolet laser. Hoechst high, Hoechst low, CD133+, and CD133− populations were identified and evaluated using FlowJo software (FlowJo, LLC; Ashland, OR, USA).

4.4. Transcriptome Profiling by RNA-Seq

Total RNA was extracted from DETA/NO-treated cancer cell lines using an RNeasy kit (Qiagen) according to the manufacturer’s instructions. After RNA quantification using a NanoDrop (Thermo Fisher Scientific) samples were sent for parallel sequencing to the Collaborative Health Initiative Research Program (CHIRP) based at Uniformed Services University (USU). Total RNA integrity was evaluated by automated capillary electrophoresis on a Fragment Analyzer (Agilent Technologies, Santa Clara, CA., USA). Sequencing libraries were prepared using the TruSeq Stranded mRNA Library Preparation Kit (Illumina, San Diego, CA, USA) with barcoded adapters, and the yield and concentration were determined using the Illumina/Universal Library Quantification Kit (KAPA) on the Light Cycler 480 (Roche, Indianapolis, IN., USA). Library size distribution was established by using the Fragment AnalyzerTM (Agilent Technologies). Clustering was done on cBotTM 2 (Illumina), and sequencing was performed on the HiSeq 3000 (Illumina) with paired-end reads of 75 bp in length with an intended depth of 40 million reads per sample. The resulting RNA-Seq data were exported as FASTQ files for the subsequent data analysis pipeline.

4.5. First Phase In-House Data Analysis

FASTQ files were first imported into the Bioconductor package (http://bioconductor.org/packages) for preliminary data mining. All reads were normalized to TPM (transcripts per million) and log2 transformed. Followed by the genome mapping algorithm, the Bowtie module was used to map the reads to the latest reference human genome (GRch38). The DESeq2 module was then applied for the differentially expressed gene detection among four endometrial cancer cell lines’ control and treated groups. Consensus clustering plus analysis was also applied for the most variable gene detection. Moreover, non-supervised hierarchical clustering analyses were performed to explore the clustering patterns between treatment groups and cell lines and to validate the stringency of the data set.

4.6. Second Phase Data Analysis

FASTQ were imported into the CLC BioMedical Genomic Workbench version 5 for QC and data mining following the RNA-seq data analysis workflow. Reads were first mapped onto the latest reference human genome (hg38), and then all variants were annotated. Next, the principal component analysis was performed to examine the data set stringency. Followed by the differentially expressed (DE) gene detection, the differentially expressed gene lists generated were exported to the Ingenuity Pathway Analysis (IPA) platform for pathway analysis.

4.7. Western Blot Analysis

Cell extracts from endometrial cancer cells treated with DETA/NO or vehicle were examined for the expression of PCNA, Caspase-3, ALDH1A1, cyclin D1, cyclin D3, RASSF1, CDKN1A, p21 and β-actin. Equal amounts of protein from DETA/NO or vehicle-treated cells were subjected to gel electrophoresis. The level of intracellular β-actin was evaluated as a loading control. The protein bands were visualized by enhanced chemiluminescence system as recommended by the manufacturer (Pierce, Piscataway, NJ, USA).

4.8. Cell Proliferation Assay

The proliferation of endometrial cancer cells treated with various doses of DETA/NO (50, 100, 200, 300, 400, and 500 μM) was assessed using the CellTiter 96 AQueous One Solution from Promega as previously reported [25,50]. Briefly, 20 µL reagent was added into each well of the 96-well assay plate containing the samples in 100 µL of culture medium. Absorbance was assessed at 490 nm using an ELX800 microtiter Reader (BioTek Instruments inc., Winooski, VT, USA). Relative cell viability was expressed as % change of treated cells over vehicle-treated cells.

4.9. Cell Invasion Assay

The invasive potential of vehicle or DETA/NO-treated endometrial cancer cells was assessed by seeding 2.5 × 104 cells/well in 100 µL of serum-free medium without DETA/NO on the upper chambers of BD BioCoat™ Matrigel Invasion Chambers (BD Biosciences, Bedford, MA, USA). Medium comprising 10% fetal bovine serum was applied to the lower chambers as chemoattractant. The chambers were incubated for 16–18 h at 37 °C. Subsequently, non-invading cells were removed from the upper surface of the membranes by scrubbing and membranes were fixed for 2 min in 100% methanol and stained in 1% toluidine blue for 2 min. Extra stain was removed by washing the inserts with water. Finally, membranes were removed from the inserts and placed on a microscope slide. Cells were counted in five random fields per slide. All slides were coded to avoid biased counting. The assay was run in triplicate.

4.10. Soft-Agar Colony Formation Assay

The tumorigenic potential of vehicle or DETA/NO-treated endometrial cancer cells was assessed by their ability to grow independently on a solid surface. The cells were seeded on 0.3% soft-agar on top of a base layer of 0.6% agar. Cells were allowed to settle down on 0.3% soft-agar before the plates were covered with 2 mL of fresh RPMI medium containing 10% FBS with or without DETA/NO. Plates were incubated at 37 °C in the CO2 incubator for four to five weeks. Cell growth media were replaced every fourth day. At the completion of incubation period, colonies were counted under a Zeiss Axiovert 4 °C microscope (company, city, state abbr. if USA, country) at ×150.

4.11. Knockdown of RASSF1 or CDKN1A in Endometrial Cancer Cells

To establish that RASSF1 or CDKN1A is a mediator of DETA/NO-induced growth inhibition, endometrial cancer cells were seeded in 6-well plates and transfected the following day with either RASSF1 or CDKN1A siRNA or scrambled siRNA) using the Lipofectamine 2000 reagent (Invitrogen). Transfected cells were treated with or without DETA/NO for 24 h. Cell extracts were used to assess the expression of RASSF1 or CDKN1A. The overall transfection efficiency for endometrial cells assessed by luciferase assay was 76% to 89%.

4.12. Statistical Analysis

Data shown are the means ± SEM of six wells/plates. Each experiment was performed three times. Two-way ANOVAs followed by Tukey post-hoc tests were utilized to analyze statistically significant differences between control and treatment groups. A p value of less than 0.05 was considered statistically significant.