lncRNA AGAP2-AS1 Facilitates Tumorigenesis and Ferroptosis Resistance through SLC7A11 by IGF2BP2 Pathway in Melanoma.

Long noncoding RNAs (lncRNAs) stand as indispensable regulators of initiation and development in melanoma (melanoma). However, the action molecular mechanisms linked to melanoma remain unclear. In the current study, the findings revealed that AGAP2-AS1 was considerably greater in melanoma than in healthy tissues and that the level of AGAP2-AS1 in cancer tissue was significantly linked to the cancerous TNM stage of patients. Individuals with high AGAP2-AS1 had a considerably shorter survival duration than patients with low AGAP2-AS1, regardless of progression-free survival or overall survival. Functionally, downregulating the expression of AGAP2-AS1 can inhibit the growth of melanocytes. Compared with the control group, AGAP2-AS1 knockdown increased Erastin-mediated iron death in melanoma cells. However, iron death inhibitor FERSINT-1 restored this effect, while Erastin induced melanoma cell death. Besides, intracellular iron and Fe2+ levels increased after AGAP2-AS1 knockdown in melanoma cells treated with Erastin compared with the si-NC group. In addition, AGAP2-AS1 silencing resulted in a significant decrease in glutathione (GSH) content in Erastin-treated melanoma cells. The mechanistic results suggest AGAP2-AS1 increases SLC7A11 mRNA stability through the IGF2BP2 pathway. In this investigation, we discovered new activities for AGAP2-AS1 and firstly discovered its mechanistic basis in ferroptosis and melanoma formation that might help in the search for potential therapy options in melanoma.

1. Introduction

Melanoma is a malignant tumor caused by the mutation and proliferation of melanocytes [1]. The incidence of melanoma is not high, accounting for about 4% of malignant tumors, but its malignant degree is generally high; in particular, the survival rate of patients with advanced metastatic melanoma is low [2-4]. Melanoma accounts for more than 70% of skin cancer-related deaths [5, 6]. Oral and maxillofacial malignant melanoma, which can occur in the skin or mucosa, is a relatively high degree of malignant tumor [7, 8]. There are a variety of inducing factors, such as environmental factors (ultraviolet radiation), family genetic history, multiple atypical nevus, and dysplastic pigmented nevus [9, 10]. Patients with early stage melanoma have a high five-year survival rate after surgical treatment, whereas patients with distant metastatic melanoma have a poor five-year survival rate and a poor prognosis [11]. The success rate of chemotherapy and radiotherapy for advanced metastatic ocular or skin melanoma is 15-20% [12], but the survival time of most patients is not long.

Ferroptosis is a different type of controlled cell death (RCD). It is first recognized when the RCD program is limited by Erastin [13], especially in cancer cells with rat sarcoma viral oncogene (RAS) mutations. Mutations of the human RAS genes are linked to tumorigenesis. The induction of ferroptosis may exist in both a Ras-dependent and Ras-independent manner. Artesunate induces pancreatic cancer death in a Ras-dependent manner, whereas it induces leukemia cell death in a Ras-independent manner. Ferroptosis suppression has been shown to increase tumorigenesis, and ferroptosis plays a vital role in malignancy development [14]. The mechanistic explanation of ferroptosis in melanoma growth, however, is still unknown [15, 16].

Except for rRNAs and tRNAs, lncRNAs have always been regarded as “noise” in the process of genome transcription [17]. Long noncoding RNAs (lncRNAs) are a type of RNA, generally defined as transcripts more than 200 nucleotides that are not translated into protein. It has been thought that proteins carry genetic information for a long time, but more and more evidence has demonstrated that lncRNAs are also involved in regulated gene expression and affect growth and development of organisms [18]. Compared with short-chain RNAs, lncRNAs may play more complex biological function. At present, more and more evidence has found that lncRNA regulates gene expression on a variety of levels and plays an important role in many vital mediated processes [19]. More and more data have proven that lncRNAs are the functional ncRNAs with the greatest diversity [20]. Because lncRNAs play a role in many biological mechanisms, their failures will affect the function of the cytoplasm. Studies have found that AGAP2-AS1 knockdown can inhibit the occurrence of melanoma in vitro. Furthermore, SLC7A11 mRNA was modified by m5C and bound by AGAP2-AS1. AGAP2-AS1 knockdown promotes GXP4 mRNA degradation in an m5C-dependent manner. Our data demonstrate the critical oncogenic roles of the AGAP2-AS1 about melanoma tumorigenesis.

2. Materials and Methods

2.1. TCGA Database

The transcriptional level of AGAP2-AS1 in normal tissues and melanoma samples was detected by the TCGA database. In addition, the relationship between the relative level of AGAP2-AS1 in melanoma tissue and the tumor TNM stage of patients was analyzed by the TCGA database data. Then, the survival and prognosis of patients with AGAP2-AS1 and melanoma were analyzed by the TCGA database and KM survival curve.

2.2. Cell Culture

Human melanoma cells were grown in DMEM medium containing 10% fetal bovine serum, and all melanoma cells were routinely added to all media and cultured in cell incubators at 37°C and 5% CO2 with 95% humidity. The morphology of cells was observed regularly under an inverted phase contrast microscope.

2.3. CCK-8 (Cell Counting Kit-8) Assay

All HUVEC cells were seeded into 96-well plates with the concentration of 5.0 × 103 cells/well. Those cells were subjected to Erastin at various concentrations. The cells were incubated for about 24 h, and the supernatant was discarded. And on the basis of the instructions of the manufacturer, the viability of the cell was tested with CCK-8 (Enzo Life Sciences). In the next, the absorbance at 450 nm was calculated with a microplate reader. The assays of proliferation were carried out independently and operated repetitively more than three times.

2.4. Real-Time Quantitative PCR (RT-qPCR) Analysis

Through RT-qPCR analysis, the whole mRNA of cells was analyzed. Total RNA was retrotranscribed from each specimen by using the M-MLV reverse transcriptase (Invitrogen, CA, USA). The reactions of RT-qPCR were conducted, and then, the following data were calculated in accordance with the comparison of the Ct procedure. The data were standardized via the expression of β-actin in every specimen. For AGAP2-AS1, U6, and SLC7A11, the sequences of PCR primers are noted in Supplemental Table 1.

2.5. Flow Cytometric Analysis

After transfection for 48 hours, transfected melanoma cells were collected for flow cytometric analysis via the Click-iT™ Plus TUNEL Assay for In Situ Apoptosis Detection, Alexa Fluor™ 594 dye (Invitrogen) based on the directions. FACSCalibur flow cytometer (FACScan®; BD Biosciences) with FlowJo V10 (BD Biosciences) was employed for detection. Samples were divided into phases, and the proportion of dying, prelethal, and apoptotic cells was assessed to the standard within every trial.

2.6. Staining of ROS Assay

The cells were rinsed within PBS at least 2 times and then resuspended in HBSS (containing Mg2+ and Ca2+) at a density of 1.0 × 106/ml. Moreover, the suspension of cells was combined with a liquor and maintained at the condition of 37°C in a darkroom for 10 min; then, labeling was ceased by supplementing 5-6 vol of HBSS. All the cells were rinsed with ice-cold complete media and were assessed at once with a FACSCalibur.

2.7. Western Blot Assay

At various concentrations, the samples were subjected to Erastin. After finishing rinsing in ice-cold PBS, the extraction was obtained from cells through suspending in a RIPA buffer (0.1% SDS, 1∗PBS, 1% Nonidet NP-40) including protease inhibitors (cocktail) on ice for 30 min. Meanwhile, all lysates were elucidated by centrifugation and assayed for protein concentrations. By electrophoresis, 30 mg total protein were detached in cell lysates on the SDS-PAGE gel, and 5% milk with TBS Tween was added to block the membrane. The different primary antibodies, including SLC7A11 (1 : 1000) and GAPDH (1 : 1000), were applied for incubation. After that, the membranes were grown with specific antibody coupled with peroxide. At last, the Chemiluminescent Detection System was tested.

2.8. RNA Immunoprecipitation (RIP) Assay

RIP was performed with Magna RIP RNA-Binding Protein Immunoprecipitation Kit (Millipore Corporation, Burlington, MA, USA) following to the directions. Melanoma cells at 85% confluency were lysed in a RIP lysis buffer. A total of 100 μl of melanoma cell extract was loaded with the RIP buffer combined with SLC7A11 antibody or IgG (Abcam) at 4°C for 6 h. The RNA concentration was calculated with a multiwell spectrophotometer (Bio-Rad, Richmond, CA, USA), and the integrity of the RNA was determined using a specific focus (Agilent, Santa Clara, CA). In the end, immunoprecipitated RNA was collected and measured via qRT-qPCR.

2.9. Statistics

All data were expressed as mean ± SD. The normal levels of significance were evaluated through one- or two-way ANOVA analysis and Student's t-test. GraphPad Prism 9.0 Software was used to perform the statistics analysis. P < 0.05 was considered to be statistically significant.

3. Results

3.1. The Upregulation of AGAP2-AS1 in Melanoma and Prediction of Worse Patient Prognosis

Firstly, we used the TCGA database to detect the level of AGAP2-AS1 lncRNA in normal samples and melanoma specimens. The levels of AGAP2-AS1 in melanoma were significantly greater in comparison with those in normal tissues (P < 0.05) (Figure 1).

Figure 1

The level of lncRNA AGAP2-AS1 in normal samples and melanoma tissues in TCGA.

Then, we used the TCGA database data to analyze the association of the relative level of AGAP2-AS1 in melanoma tissue and the tumor TNM stage of patients. Our findings displayed that the level of AGAP2-AS1 in cancer tissue was significantly correlated with the tumor TNM stage of patients (P < 0.05) (Figure 2).

Figure 2

TCGA database data to analyze the association of the relative level of AGAP2-AS1 in melanoma specimens and the tumor TNM stage of patients.

Then, we used the TCGA database and KM survival curve to analyze the survival and prognosis of patients with AGAP2-AS1 and melanoma. The results showed that patients with higher AGAP2-AS1 had a considerably shorter life expectancy than those with lower AGAP2-AS1, regardless of progression-free survival (Figure 3(a)) or overall survival (Figure 3(b)). The above results suggest that strongly expressed AGAP2-AS1 indicates a worse prognosis.

Figure 3

Analysis of progression-free survival and overall survival of melanoma patients with different levels of AGAP2-AS1 using TCGA database and KM survival curve.

3.2. AGAP2-AS1 Promotes Proliferation of Melanoma Cells

The cell proliferation of two kinds of cells in the si-AGAP2-AS1 group and NC group was analyzed. The CCK-8 data showed that the absorbance of the blocked AGAP2-AS1 group was greatly less than that of the NC group and blank group at 48 h and 72 h, and the difference became larger and larger with the duration of time (Figures 4(a) and 4(b)). The results suggest that downregulating the expression of AGAP2-AS1 is capable of inhibition growth of melanocytes.

Figure 4

AGAP2-AS1 promotes proliferation of melanoma cells.

3.3. AGAP2-AS1 Prevents Ferroptosis of Melanoma Cells

Ferroptosis resistance is one of the key factors for tumors to maintain a malignant phenotype and treatment resistance. Next, we investigated the effects of the ferroptosis inducer Erastin on the activity of melanoma cells. After si-NC or si-AGAP2-AS1 was transfected into melanoma cells and treated with Erastin, AGAP2-AS1 knockdown improved Erastin-mediated ferroptosis in melanoma cells compared with the control group (Figures 5(a) and 5(b)). Concomitantly, we observed that the ferroptosis inhibitor FERSINT-1 restored this effect while Erastin induced melanoma cell death (Figures 5(a) and 5(b)). It is suggested that AGAP2-AS1 knockdown has been able to enhance the ferroptosis of melanoma cells. These results suggest that AGAP2-AS1 knockdown inhibits the proliferation of melanoma cells and promotes ferroptosis. Subsequently, we analyzed the AGAP2-AS1 impact on iron accumulation and glutathione (GSH) during ferroptosis. Since Fe2+ is an important factor in ferroptosis, we first used an iron kit to assess the effects of AGAP2-AS1 on intracellular iron and Fe2+ concentrations. The data reported that intracellular iron and Fe2+ levels increased after AGAP2-AS1 knockdown in melanoma cells treated with Erastin compared with the si-NC group (Figure 5(c)). In addition, AGAP2-AS1 silencing resulted in a significant decrease in GSH content in Erastin-treated melanoma cells (P < 0.05) (Figure 5(d)). The knockdown of AGAP2-AS1 could promote ferroptosis in melanoma cells; that is, AGAP2-AS1 promotes ferroptosis resistance in melanoma cells.

Figure 5

AGAP2-AS1 prevents ferroptosis of melanoma cells.

3.4. AGAP2-AS1 Increases SLC7A11 mRNA Stability through IGF2BP2 Pathway

SLC7A11 is an important factor in the regulation of ferroptosis in tumor cells. We explored the regulatory relationship between SLC7A11 and AGAP2-AS1. Posttranscriptional modification regulated by IGF2BP2 is an important factor in the regulation of SLC7A11. Firstly, we used the RIP experiment to detect the interaction between SLC7A11 and IGF2BP2. The results show that there is an interaction between SLC7A11 and IGF2BP2 (Figure 6(a)), and knockout of the expression of AGAP2-AS1 can weaken this interaction (Figure 6(b)). Subsequent mRNA decay also confirmed that knocking down the AGAP2-AS1 level can reduce the stability of SLC7A11 mRNA, while overexpression can reverse the AGAP2-AS1 functions (Figure 6(c)). The above results suggest AGAP2-AS1 increases SLC7A11 mRNA stability through the IGF2BP2 pathway.

Figure 6

AGAP2-AS1 regulates mRNA stability of SLC7A11 through the IGF2BP2 pathway.

4. Discussion

In recent years, clinical studies on melanoma have found that although immunosuppressive therapy has significant survival benefits in some populations, about 40-60% of patients do not benefit from these treatments [21]. In addition, these treatments may have some related toxicity. It was also reported that the same group of chemotherapeutic drugs had significant benefits for patients with different types of melanoma, while others died in the process [22]. lncRNA has an indispensable role in the proliferation, differentiation, and apoptosis of malignant tumors, among which miRNA has been found to be involved in the tumor cell cycle, cell signal transduction pathway, and gene expression and modification, which makes lncRNA a new research hotspot in recent years, mainly studying its regulatory functions at epigenetic, transcriptional, and posttranscriptional levels [23, 24].

Long chain noncoding RNA (lncRNA) is a gene that cannot encode proteins, but it can directly perform biological functions in the form of RNA [25]. It is more than 200 nucleotides in length, but its open reading frame is incomplete [26, 27]. lncRNAs were transcribed by RNA polymerase II, then modified by cap at the 5′end and splicing and polyadenosine acidification at the 3′end [28]. At first, it was thought to have no biological function, but the results of many experiments tell us that lncRNAs regulate gene expression at many levels in the form of RNA, such as transcription, posttranscriptional level, and epigenetics [29, 30]. lncRNAs work mainly through the following four ways: first, induction, lncRNAs can bind to a specific miRNA, which can indirectly regulate the target gene; second, lncRNAs can act as a signal molecule on the pathway and regulate the upstream- and downstream-related genes of its target gene [31, 32]. Third, by guiding the relevant RNA-binding proteins, lncRNAs are able to mediate the target genes via cis- or trans-regulation to specific protein complex sites [33, 34]. Fourth, scaffolds, lncRNAs can be used as a platform; on this platform, a variety of related molecular complexes can be combined together to enhance the information transmission and interaction between molecular complexes [35, 36]. Studies have found that the growth and development of the body, the occurrence and development of tumors, cell proliferation, and apoptosis are all related to the abnormal expression of lncRNAs, especially in tumorigenesis [37].

Dysregulation of the AGAP2-AS1 expression has been shown to suppress melanocyte growth. AGAP2-AS1 gene inactivation enhanced Erastin-mediated side ferroptosis of melanoma cells compared to the control group. The ferroptosis inhibitor FERSINT-1, on the other hand, recovered this effect, whereas Erastin caused melanoma apoptosis. This research found different initiatives of AGAP2-AS1 and melanoma, as well as the basis of their mechanisms in ferroptosis and melanoma formation for the first time, which may aid in the discovery of great and promising melanoma alternative treatments. The innovation of this study is that we studied the clinical samples and melanoma cell lines of patients with melanoma for the first time to detect the AGAP2-AS1 function of melanoma. The findings released that AGAP2-AS1 was highly expressed in melanoma than healthy adjacent skin specimens, and AGAP2-AS1 in the melanoma metastasis group was remarkably stronger than that in the nondistant metastasis group. This is consistent with the analysis of the expression level of lncRNA in other tumor samples made by other researchers, suggesting that AGAP2-AS1 may be related to the occurrence and development of melanoma. In addition, the expression of AGAP2-AS1 in melanoma tissue can affect the overall survival rate (OS) of melanoma patients. The OS value of patients with higher AGAP2-AS1 in melanoma was significantly lower than that of patients with lower AGAP2-AS1, suggesting that the higher the expression of AGAP2-AS1, the worse the prognosis. The recent meta-analysis has suggested that AGAP2-AS1 overexpression was closely correlated with shorter OS in multiple cancer types, suggesting that AGAP2-AS1 might function as a promising predictor for clinical outcomes in cancer [38]. Our results of AGAP2-AS1 in melanoma is consistent with this previous meta-analysis research. An in vitro functional assay showed that knocking down the expression of AGAP2-AS1 could inhibit cellular growth and induce cellular ferroptosis. In modern medicine, where targeted treatment is becoming more and more accurate, the application of ROCK inhibitors also occupies a place.

In conclusion, this study provides a theoretical basis for ROCK inhibitors in the treatment of wild-type BRAF melanoma. In the next study, we will continue to explore whether inhibition of AGAP2-AS1 can inhibit the growth of melanoma cells with BRAF gene mutation and establish a tumor-bearing model in nude mice by knockout AGAP2-AS1 or overexpressing AGAP2-AS1 melanoma cells. By observing the tumor formation rate and tumor growth rate, we finally provide a theoretical basis for the clinical treatment of human melanoma.