In Situ Generated Novel 1H MRI Reporter for β-Galactosidase Activity Detection and Visualization in Living Tumor Cells.

For wide applications of the lacZ gene in cellular/molecular biology, small animal investigations, and clinical assessments, the improvement of noninvasive imaging approaches to precisely assay gene expression has garnered much attention. In this study, we investigate a novel molecular platform in which alizarin 2-O-β-d-galactopyranoside AZ-1 acts as a lacZ gene/β-gal responsive 1H-MRI probe to induce significant 1H-MRI contrast changes in relaxation times T 1 and T 2 in situ as a concerted effect for the discovery of β-gal activity with the exposure of Fe3+. We also demonstrate the capability of this strategy for detecting β-gal activity with lacZ-transfected human MCF7 breast and PC3 prostate cancer cells by reaction-enhanced 1H-MRI T 1 and T 2 relaxation mapping.

Introduction

Due to various advantages such as stability, high turnover rate, and ease of conjugation, the lacZ gene-encoding β-galactosidase (β-gal) has been broadly used in cellular/molecular biology, small animal studies, clinical trials with assays of clonal insertion, transcriptional activation, and protein expression and interaction (Kruger et al., 1999; Haberkorn et al., 2005; Razgulin et al., 2011; Yang et al., 2019). Moreover, overexpressed β-gal has been identified as a vital enzyme biomarker related to cell senescence and cancer progression (Chatterjee et al., 1979; Alam et al., 1990; Dimri et al., 1995; Paradis et al., 2001; Pacheco-Rivera et al., 2016; Lozano-Torres et al., 2017; Sharma and Leblanc, 2017; Kim et al., 2018; Wang et al., 2019; Li et al., 2020b; Gao et al., 2020; Qiu et al., 2020). Thus, β-gal activity detection has been exploited with diverse techniques including colorimetric assays (James et al., 2000; Browne et al., 2010; Zeng et al., 2012; Yeung et al., 2013; Chen et al., 2016; Hu Q. et al., 2017), fluorescence (Tung et al., 2004; Urano et al., 2005; Josserand et al., 2007; Kamiya et al., 2007; Feng et al., 2009; Koide et al., 2009; Kamiya et al., 2011; Oushiki et al., 2012; Han et al., 2013; Sakabe et al., 2013; Lee et al., 2014; Asanuma et al., 2015; Peng et al., 2015; Zeng et al., 2015; Doura et al., 2016; Gu et al., 2016; Zhang C. et al., 2016; Zhang X. X. et al., 2016; Huang J. et al., 2017; Hu Q. et al., 2017; Jiang et al., 2017; Kim et al., 2017; Nakamura et al., 2017; Wei et al., 2017; Zhang et al., 2017; Tang et al., 2017; Chen et al., 2018; Ito et al., 2018; Liu et al., 2018; Yang et al., 2018; Chen et al., 2019; Fu et al., 2019; Gu et al., 2019; Jiang et al., 2019; Kong et al., 2019; Lee et al., 2019; Shi et al., 2019; Singh et al., 2019; Zhang et al., 2019a; Zhang X. et al., 2019; Zhao et al., 2019; Li et al., 2020a; Li Y. et al., 2020; Li Z. et al., 2020; Pang et al., 2020; Wu et al., 2020; Zhu et al., 2020), chemiluminescence (Wehrman et al., 2006; Liu and Mason, 2010; Broome et al., 2015; Green et al., 2017; Huang Y. et al., 2017; Wang et al., 2017; Gorai and Maitra, 2018; Hananya and Shabat, 2019; Lozano-Torres et al., 2019; Zhang et al., 2019b), positron emission tomography or single-photon emission computed tomography (Celen et al., 2008; Van Dort et al., 2008; Rempel et al., 2017), magnetic resonance imaging (MRI) (Louie et al., 2000; Chang et al., 2007; Hanaoka et al., 2008; Cui et al., 2010; Bengtsson et al., 2010; Arena et al., 2011; Yu et al., 2012a; Gulaka et al., 2013; Li et al., 2013; Heffern et al., 2014; Burke et al., 2015; Hingorani et al., 2015; Fernández-Cuervo et al., 2016; Hu J. et al., 2017; Li and Meade, 2019; Xu et al., 2019; Lilley et al., 2020), and 19F-MRS/MRI approaches (Yu et al., 2005; Kodibagkar et al., 2006; Yu et al., 2006; Yu and Mason, 2006; Liu et al., 2007; Yu et al., 2008a; Yu et al., 2008b; Mizukami et al., 2011; Yu et al., 2012b; Yu et al., 2013; Yu et al., 2017). In particular, 1H-MRI molecular imaging approaches for visualization of β-gal activity attract much more attention because 1H-MRI is noninvasive and capable of soft tissue delineation with a high lateral and depth resolution (Terreno et al., 2010; Haris et al., 2015; Wahsner et al., 2019).

β-Galactosidase prompts the hydrolysis of β--galactopyranoside by cleavage of its β-anomeric C-O linkage between β--galactopyranose and aglycone; the hydrolysis reactivity of β--galactopyranosides to β-gal is completely dependent upon the aglycone structure. However, the structure activity relationship of the aglycones in β--galactopyranosides vs. β-gal is not yet clear (Juers et al., 2012; Duo et al., 2017). Therefore, further exploration is still highly needed to discover effective β-gal substrates for functional molecular imaging probes. We believe that the traditional histopathological methods of assaying β-gal activity might be the fruitful resources for developing novel imaging agents for the assessment of lacZ gene expression. In reviewing the histopathological literature, we noticed that the well-established β-gal substrate alizarin 2-O-β--galactopyranoside AZ-1 (Figure 1) is readily hydrolyzed by β-gal to release aglycone alizarin, which chelates with ferric iron Fe3+ to form an intense dark violet Fe complex (James et al., 2000). By comparison of the structural characteristics of the Fe3+–alizarin complex with Fe3+-based 1H-MRI contrast agents (Davies et al., 1996; Richardson et al., 1999; Schwert et al., 2002; Schwert et al., 2005; Haas and Franz, 2009; Yu et al., 2012a; Yu et al., 2012b; Gulaka et al., 2013; Li et al., 2013; Yu et al., 2013; Kuznik and Wyskocka, 2016), we speculated that the Fe3+–alizarin complex could function as an Fe3+-based 1H-MRI contrast agent. If so, the well-established β-gal substrate AZ-1 could work as a lacZ gene or β-gal 1H-MRI reporter. Upon delivery and cleavage at lacZ-transfected or β-gal–overexpressed tumor cells with the presence of Fe3+, the paramagnetic Fe complex could be spontaneously formed in situ and specifically produced the 1H MRI contrast effect while localizing and accumulating 1H-MRI signals at the β-gal activity site. Figure 1 depicts the Fe3+–alizarin complex generated in situ for the 1H-MRI detection of β-gal activity. We now demonstrate the use of exploiting AZ-1 to assess β-gal activity in vitro with lacZ-transfected human MCF7 breast and PC3 prostate cancer cells by 1H MRI T and T relaxation mapping.

FIGURE 1

The proposed mechanism of an in situ generated stable Fe3+–alizarin complex for 1H MRI detection of β-gal activity.

Results and Discussion

Verification of the Fe3+–Alizarin Complex as an 1H-MRI Contrast Agent

Alizarin is 1,2-dihydroxy-9,10-anthraquinone with a tricyclic aromatic planar structure and chelates with Fe3+ to form a thermodynamically stable octahedral Fe3+–alizarin (1:3) complex at physiological pH conditions with the stability constant logβ = 32.21 (Das et al., 1995; Das et al., 2002). To explore the MRI signal–enhancing capability of the Fe3+–alizarin complex, the spin–lattice relaxation time T 1 and spin–spin relaxation time T 2 of the Fe3+–alizarin complex were measured with a 4.7 T MR scanner using a saturation recovery spin echo sequence and multi-spin echo sequence with varying repetition times (TRs) and echo times (TEs), respectively. The images were acquired using a 3-cm diameter solenoid coil (home-built) with 4 × 4 cut section of a 96-well plate containing the different concentrations of alizarin and ferric ammonium citrate (FAC) mixed solutions in 1:1 (V/V’) DMSO/PBS (0.1 M, pH = 7.4) at 37°C. Figure 2 displays the significant changes as expected on the T 1 and T 2 maps and relaxation time values of the Fe3+–alizarin complex at T 1 = 254 ± 3, 131 ± 3, and 92 ± 8 ms, and T 2 = 106 ± 1, 59 ± 1, and 48 ± 1 ms, corresponding to the concentrations of alizarin at 2.5, 6.0, and 9.0 mM, respectively. The comparison with the control FAC of T 1 = 389 ± 6 ms and T 2 = 143 ± 1 ms showed that the Fe3+–alizarin complex formed in situ resulted in substantial signal enhancement on either T 1- or T 2-weighted 1H-MRI, confirming the Fe3+–alizarin complex generated in situ to function as an 1H-MRI contrast agent. Notably, the significantly different T 1 and T 2 values of the Fe3+–alizarin complex suggested the potential to combine T 1 and T 2 data for additional information of imaging evaluation and detection reliability, specifically where there is possibility for misinterpretation in tissue heterogeneity (Zhou et al., 2017).

FIGURE 2

The 1H-MRI of an Fe3+–alizarin complex. MRI acquisition parameters: 1H-MRI, 200 MHz, matrix size: 128 × 128, FOV: 40 mm × 40 mm, slice thickness: 2 mm; receiver bandwidth: 20 kHz, T 1-map: saturation recovery spin-echo sequence, TR = 200, 400, 600, 800, 1,000, 2000, 3,000, and 6,000 ms, respectively, TE = 15 ms; T 2-map: multi-echo SE sequence, TE = 10, 20, 30, 40, 50, 60, 70, 80, 90, 100, 110, 120, 130, 140, 150, and 160 ms, respectively, TR = 2000 ms. (A) Control, FAC (15.0 mM); (B) alizarin (2.5 mM), FAC (15.0 mM); (C) alizarin (6.0 mM) and FAC (15.0 mM); (D) alizarin (9.0 mM), FAC (15.0 mM) in 1:1 (V/V′), and DMSO/PBS (0.1 M, pH = 7.4) at 37°C in 4 h.

Alizarin β--Galactopyranoside Synthesis

After the Fe3+–alizarin complex was shown to be an 1H-MRI contrast agent, we therefore began the β--galactopyranosylation with alizarin at the phenolic hydroxyl groups. Previously, James et al. (2000) reported a modified method for the synthesis of AZ-1, involving the reaction of alizarin potassium salt with acetobromo-α-D-galactose via the nucleophilic substitution procedure followed by deacetylation mediated by the aqueous NaOH solution, but the yield was low (14%). We observed that the phenolic hydroxyl groups at 1,2-positions of alizarin have excellent site-reaction selectivity due to the various electronic deficiency/sterically hindered effects (Mahal et al., 2011) and apparently different pKa values: pKa (2-OH) = 5.98 ± 0.05, whereas pKa (1-OH) = 9.88 ± 0.05 (Das et al., 1995; Das et al., 2002), which suggested that the phase-transfer catalysis method at pH = 8-9 could provide regio- and stereoselective synthesis of AZ-1, as exploited previously for β-gal 19F-MRS/MRI reporters (Yu et al., 2005; Kodibagkar et al., 2006; Yu et al., 2006; Yu and Mason, 2006; Liu et al., 2007; Yu et al., 2008a; Yu et al., 2008b; Yu et al., 2012b; Yu et al., 2013; Yu et al., 2017). To the well-stirred solution of alizarin in CH2Cl2-H2O (pH 8–9) using tetrabutylammonium bromide (TBAB) as a catalyst at 50°C, an equimolar amount of 2, 3, 4, and 6-tetra-O-acetyl-α-D-galactopyranosyl bromide was dropped under N2 atmosphere for around 1 h; alizarin 2-O-2′, 3′, 4′, and 6′-tetra-O-acetyl-β-D-galactopyranoside AZ-M1 was isolated purely at the yield of 78%. The NOESY correlation between anomeric H-1′ and H-3 in AZ-M1 (Supporting Information, Supplementary Figure S7) showed that β--galactopyranosylation took place at the 2-hydroxyl group of alizarin as predicted. The following deacetylation with NH3/MeOH from 0°C to room temperature produced AZ-1 at 91% yield.

The prerequisite for molecular MRI of intracellular targets is that the contrast agents must be effectively taken up by cells in vivo, which requires contrast agents to be sufficiently soluble and capable of entering cells. Carbohydrate-associated prodrugs in clinical applications have widely demonstrated the improved aqueous solubility and permeability, leading to better selectivity and efficacy for diagnosis and therapy (Dwek, 1996; Bertozzi and Kiessling, 2001; Hudak and Bertozzi, 2014; Fernández-Tejada et al., 2015). We hence thought about introducing an additional β-D-galactopyranosyl unit to form alizarin 1,2-di-O-β--galactopyranoside AZ-2. Similarly, a drop of 2.2 equivalent 2, 3, 4, and 6-tetra-O-acetyl-α-D-galactopyranosyl bromide CH2Cl2 solution into alizarin in CH2Cl2-H2O (pH 10–11) employing TBAB as a catalyst at 55°C under N2 atmosphere afforded alizarin 1,2-di-O-2′, 3′, 4′, 6′-tetra-O-acetyl-β-D-galactopyranoside AZ-M2 at 62% yield. Deacetylation accomplished the target molecule alizarin 1,2-di-O-β--galactopyranoside AZ-2 with 94% yield. Figure 3 illustrates the structures of AZ-1/AZ-M1 and AZ-2/AZ-M2. As expected, the free di-β--galactopyranoside AZ-2 is soluble in PBS (0.1 M, pH = 7.4) at the concentration of 15 mM; meanwhile, the free mono-β--galactopyranoside AZ-1 unlikely requires the addition of DMSO for the same concentration. The structures of AZ-M1/AZ-1 and AZ-M2/AZ-2 were confirmed by NMR and HRMS data. The molecular/quasimolecular ions of AZ-M1 and AZ-1, as well as AZ-M2 and AZ-2, showed the introduction of one and two galactopyranosyl units to AZ-M1/AZ-1 and AZ-M2/AZ-2, respectively, in which the β-D-galactopyranoside configuration was determined by 1H/13C NMR data of the anomeric protons at δH-1 ′ = δH-1 ′′ = 4.90–5.30 ppm and their coupling constants J 1 ′ ,2 ′ = J 1 ′′ ,2 ′′≅8.0 Hz while maintaining the corresponding anomeric C-1′/C-1′′ at δC-1 ′ = δC-1 ′′ = 99.5–104.0 ppm in accordance (Supporting Information, Supplementary Figures S4–S13), which are in agreement with the typical characteristics for the identification of the anomeric β--configuration (Yu et al., 2005; Yu et al., 2006; Yu and Mason, 2006; Kodibagkar et al., 2006; Liu et al., 2007; Yu et al., 2008a; Yu et al., 2008b; Yu et al., 2012b; Yu et al., 2013; Yu et al., 2017).

FIGURE 3

The structures of alizarin mono β-D-galactopyranosides AZ-M1/AZ-1 , and alizarin di-β-D-galactopyranosides AZ-M2/AZ-2.

β-Gal Reactivity

AZ-1 has been identified as a highly sensitive substrate for the demonstration of β-gal in a range of Gram-negative bacteria under incubation at 37°C in air for 18 h (James et al., 2000). However, none of the existing data have shown the kinetics of AZ-1 vs. β-gal, which is crucial for further in vivo imaging applications. The absorption spectra of AZ-1 and AZ-2 solutions in 1:1 (V/V′) DMSO/PBS (0.1 M, pH = 7.4) with and without β-gal (E801A) at 20–22°C indicated that upon reactions of AZ-1 and AZ-2 with β-gal, a new absorption around 520 nm, corresponding to the in situ released alizarin mono-/dianions, appeared and increased gradually. Hence, the absorbance measurements at 520 nm following the enzymatic reaction of AZ-1 and AZ-2 with β-gal (E801A) at 20–22°C in different time points showed that both AZ-1 and AZ-2 are very reactive to β-gal (E801A) with varying hydrolytic rates at ν(AZ-1) = 93.3 and ν(AZ-2) = 133.3 μM/min/unit, respectively (Figure 4). Also, the absorption spectra of AZ-1 and AZ-2 by reaction with other enzymes α-galactosidase (Sigma G7163) and β-glucuronidase (Sigma G8295) at 20–22°C; showed that both AZ-1 and AZ-2 remained essentially stable over the period of 60 min, verifying their specificity for reaction to β-gal.

FIGURE 4

The kinetic hydrolysis time courses of alizarin β-D-galactopyranosides AZ-1 (■) and AZ-2 (●) with β-gal. Absorbance measurements at λ = 520 nm following the addition of β-gal (E801A, 3 units) to solutions of AZ-1, AZ-2 each (5.0 mM) in 1:1 (V/V′) DMSO/PBS (0.1 M, pH = 7.4) at 20–22°C in different time points; The time courses of alizarin β-D-galactopyranosides AZ-1 (□) and AZ-2 (○) each (5.0 mM) in 1:1 (V/V′) DMSO/PBS (0.1 M, pH = 7.4) at 20–22°C in different time points without β-gal (E801A).

1H-MRI Detection of β-Gal Activity

The T 1 and T 2 maps and relaxation time values were measured with a 4 × 4 cut section of 96-well plate containing various concentrations of AZ-1 and AZ-2 (4.0–9.0 mM) together with a fixed concentration of FAC (15.0 mM), respectively, in 1:1 (V/V′) DMSO/PBS (0.1 M, pH = 7.4) with or without β-gal (E801A). In the AZ-1/FAC solution at 37°C in 4 h in the absence of β-gal, relaxation times were determined to be T 1 = 368 ± 6 and T 2 = 134 ± 1 ms. In comparison, in the presence of β-gal (E801A, 5 units) in the mixture solution of AZ-1 and FAC at 37°C in 4 h, pronounced shortening in relaxation times T 1 and T 2 caused by the Fe3+–alizarin complex generated in situ was observed at T 1 = 138 ± 3, 115 ± 4, and 84 ± 5 ms, whereas T 2 = 74 ± 1, 54 ± 1, and 44 ± 5 ms, correlating with the increasing concentrations of AZ-1 from 4.0, 6.0 and 9.0 mM, respectively (Figure 5). However, the much more soluble and reactive AZ-2, exhibiting significant advantages for in vivo 1H-MRI applications, produced very unexpected results under similar procedures at the same conditions. In the absence of β-gal at 37°C in 4 h, the AZ-2/FAC solution, as the control, yielded surprisingly reduced T 1 = 230 ± 11 and T 2 = 98 ± 1 ms (Figure 6). However, in the presence of β-gal (E801A, 5 units), the mixture solutions of AZ-2/FAC gave rise to an insignificant decrease in T 1 = 220 ± 7, 198 ± 11, and 177 ± 5 ms and T 2 = 95 ± 1, 78 ± 2, and 72 ± 1 ms (AZ-2 concentrations at 4.0, 6.0, and 9.0 mM, respectively, Figure 6), indicating the much less Fe3+–alizarin complex formed in situ during the β-gal hydrolysis. Comparing the interactions between AZ-1/AZ-2 and Fe3+ with their relaxivities to FAC, we attributed that the larger differences of AZ-2/FAC to FAC solution (T 1: 230 ± 11 vs. 389 ± 6 ms and T 2: 98 ± 1 vs. 143 ± 1 ms; alternatively, ΔR 1 = 1.78 s−1 and ΔR 2 = 3.21 s−1) than AZ-1/FAC solution (T 1: 368 ± 6 vs. 389 ± 6 ms, T 2: 134 ± 1 vs. 143 ± 1 ms; alternatively, ΔR 1 = 0.15 s−1, ΔR 2 = 0.47 s−1) were risen from the formation of the much stronger and more stable molecular tweezer complex AZ-2/Fe3+ due to the adjacent 3′,4′,6′-OH and 3″,4″,6″-OH located at the same side of 1,2-di-O-β--galactopyranosyl rings in the favored configuration for chelation of Fe3+ (Supplementary Figure S3 in the Supporting Information) (Davies et al., 1996; Richardson et al., 1999; Schwert et al., 2002; Schwert et al., 2005; Haas and Franz, 2009; Coskuner et al., 2011; Kuznik and Wyskocka, 2016) which thus simultaneously hindered the reaction with β-gal (E801A) and slowed down the release of alizarin as well as the in situ generation of the Fe3+–alizarin complex. These were confirmed by mixing solutions of AZ-2 and β-gal (E801A) first for hydrolysis, and then followed by adding FAC for complexation at 37°C in 2 h for each step with the same concentrations as in Figure 6. A significant decrease in relaxation times T 1 and T 2 was seen at T 1 = 133 ± 1, 110 ± 2, and 78 ± 2 ms and T 2 = 73 ± 2, 51 ± 3, and 41 ± 1 ms, which were very close to expectations based on AZ-1/FAC T 1 and T 2 data as shown in Figure 5.

FIGURE 5

1H-MRI detection of β-gal activity. 1H-MRI acquisition: using the same parameters as in Figure 2, (A) alizarin 2-O-β-D-galactopyranoside AZ-1 (9.0 mM) and FAC (15.0 mM); (B) alizarin 2-O-β-D-galactopyranoside AZ-1 (4.0 mM), FAC (15.0 mM), and β-gal (E801A, 5 units); (C) alizarin 2-O-β-D-galactopyranoside AZ-1 (6.0 mM), FAC (15.0 mM), and β-gal (E801A, 5 units); (D) alizarin 2-O-β-D-galactopyranoside AZ-1 (9.0 mM), FAC (15.0 mM), and β-gal (E801A, 5 units) in 1:1 (V/V′) DMSO/PBS (0.1 M, pH = 7.4) at 37°C in 4 h.

FIGURE 6

1H-MRI detection of β-gal activity. 1H-MRI acquisition: using the same parameters as in Figure 2, (A) alizarin 1,2-di-O-β-D-galactopyranoside AZ-2 (9.0 mM) and FAC (15.0 mM); (B) alizarin 1,2-di-O-β-D-galactopyranoside AZ-2 (4.0 mM), FAC (15.0 mM), and β-gal (E801A, 5 units); (C) alizarin 1,2-di-O-β-D-galactopyranoside AZ-2 (6.0 mM), FAC (15.0 mM), and β-gal (E801A, 5 units); (D) alizarin 1,2-di-O-β-D-galactopyranoside AZ-2 (9.0 mM), FAC (15.0 mM), and β-gal (E801A, 5 units) in 1:1 (V/V′) DMSO/PBS (0.1 M, pH = 7.4) at 37°C in 4 h.

In Vitro 1H-MRI Detection of lacZ Transfection in Human Tumor Cells

The recombinant vector phCMVlacZ has been successfully created and used to stably transfect human prostate cancer PC3-lacZ cells from PC3-wild-type (WT) cells (Liu et al., 2007). Accordingly, human breast cancer MCF7-lacZ cells were stably transfected from MCF7 wild-type (WT) cells: the β-gal activity and quantification in MCF7-lacZ cells were verified on the basis of Western blot, X-gal and S-gal staining, and the β-gal assay (Figure 7).

FIGURE 7

MCF7-lacZ transfection. (A) Western blot: protein extracts from two transfected MCF7-lacZ cell lines (lanes 2,4), together with MCF7-WT cells (lanes 1,3,5) showing intense bands for β-gal activity in MCF7-lacZ cells and none in MCF7-WT cells; (B) MCF7-WT/MCF7-lacZ cells staining by X-gal and S-gal: deep blue (X-gal) and black (S-gal) staining confirming the intense lacZ expression in MCF7-lacZ cells with essentially no β-gal activity in MCF7-WT cells. Regional magnification ×100; (C) β-Gal assay for activity quantification in MCF7-lacZ cells: 1.0 unit corresponding to the hydrolysis of 1.0 μmol/min O-nitrophenyl β-D-galactopyranoside, β-gal activity was increasingly associated with the number of MCF7-lacZ cells.

Given AZ-2 showed much better aqueous solubility and reactivity to β-gal, the stabilized molecular tweezer complexation AZ-2/Fe3+ obstructed its implementation spreading to effective 1H-MRI assessment of β-gal. So, AZ-1 with a significant signal loss either on T 1 or T 2 upon β-gal hydrolysis was prompted for the further in vitro 1H-MRI evaluation. As an initial demonstration for in vitro 1H-MRI detection of β-gal with lacZ-transfected human cancer cells, we first acquired T 2 * maps on pair mixtures of AZ-1 (10.0 mM) with PC3-WT cells (5 × 105) and PC3-lacZ cells (5 × 105), respectively, in the presence of FAC (10.0 mM) layered between agarose after incubation 4 h at 37°C under 5% CO2/air with 95% humidity. Significant differences confined within the layers were observed between PC3-WT and PC3-lacZ cells at different echo times (Figure 8A), in which there was essentially no signal loss with PC3-WT cells but a remarkable signal decrease with PC3-lacZ cells upon increasing echo times (TEs) (Figure 8B). The relaxation time T 2 * was determined to be T 2 * ( = 96 ± 23 ms in PC3-WT cells, while T 2 * ( = 26 ± 14 ms in PC3-lacZ cells. Again, the β-gal activity was verified based on X-gal, S-gal, and AZ-1 staining (dark violet) (Figure 8C), with each staining method consistently showing intense lacZ expression in PC3-lacZ cells with essentially no β-gal activity in PC3-WT cells.

FIGURE 8

In vitro 1H-MRI detection of lacZ gene expression in PC3-lacZ cells. 1H-MRI acquisition: 1H-MRI, 400 MHz, matrix size: 256 × 128, FOV: 48 mm × 24 mm, gradient echo imaging: TE = multiple values 3–30 ms, TR = 100 ms, flip angle = 10° (A) T 2* maps: A mixture of alizarin 2-O-β-D-galactopyranosides AZ-1 (10.0 mM) and FAC (10.0 mM) with 5 × 105 PC3-WT or PC3-lacZ cells was placed in the interlayer between 1% low-gelling temperature agarose in a 10-mm NMR tube, and then incubated for 4 h at 37°C under 5% CO2/air with 95% humidity, T 2*( = 96 ± 23 ms (top row) and T 2*( = 26 ± 14 ms (bottom row), respectively; (B) relative signal intensity changes at different echo times (TEs) from (A), essentially no signal loss with PC3-WT cells (●) but a significant signal loss with PC3-lacZ cells (■); (C) PC3-WT/PC3-lacZ cells staining by X-gal, S-gal, and AZ-1: deep blue (X-gal), black (S-gal), and dark violet (AZ-1) staining confirming an intense lacZ expression in PC3-lacZ cells with essentially no β-gal activity in PC3-WT cells. Regional magnification ×100; (D) cytotoxicity: PC3-WT/PC3-lacZ cells viability vs. AZ-1 in various concentrations in 1:1 (V/V′) DMSO/PBS (0.1 M, pH = 7.4) at 37°C under 5% CO2/air with 95% humidity for 72 h, PC3-WT cells (in blue), and PC3-lacZ cells (in red)).

The cytotoxicity of AZ-1 was studied with PC3-WT and PC3-lacZ cells in PBS (0.1 M, pH = 7.4) incubated 72 h at 37°C under 5% CO2/air with 95% humidity. Cell viability assays showed that neither AZ-1 nor alizarin showed toxicity to PC3 cells, for AZ-1 viability exceeded 96% for both PC3-WT and PC3-lacZ cells at all concentrations tested (Figure 8D).

Furthermore, in vitro 1H-MRI acquisition of AZ-1 (10.0 mM) with PC3-WT cells (5 × 105) and PC3-lacZ cells (5 × 105) in the presence of FAC (10.0 mM) was performed in a 1:1 (V/V′) DMSO/PBS (0.1 M, pH = 7.4) solution. A pronounced signal decrease in the relaxation time T 1 was observed between PC3-WT (T 1( = 245 ± 9 ms) and PC3-lacZ cells (T 1( = 82 ± 7 ms) after incubation 4 h at 37°C under 5% CO2/air with 95% humidity.

Similarly, after incubation of AZ-1 (10.0 mM) with MC7-WT cells (5 × 105) and MC7-lacZ cells (5 × 105), respectively, in the same conditions as the previous study, the relaxation times were observed to be T 1( = 223 ± 11 ms and T 2( = 97 ± 12 ms in MC7-WT cells, whereas T 1( = 75 ± 7 ms and T 2( = 45 ± 9 ms made for MC7-lacZ cells (Figures 9A,B), the T 1 and T 2 values are shown as bars adjacent to T 1 and T 2 maps; both illustrated significant differences after the reaction with β-gal at ΔR 1 = 8.85 s−1 and ΔR 2 = 11.91 s−1. Staining by X-gal, S-gal, and AZ-1 (dark violet) (Figure 9C) all displayed an intense lacZ expression in MC7-lacZ cells but with essentially no β-gal activity in MC7-WT cells. Cell viability assays indicated that both AZ-1 and the released aglycone alizarin have no toxicity to MC7 cells upon the viability above 95% to MC7-WT and MC7-lacZ cells at all concentrations tested for 72 h (Figure 9D).

FIGURE 9

In vitro 1H-MRI detection of the lacZ gene expression in MCF7-lacZ cells. 1H-MRI acquisition: using the same parameters as in Figure 2. T 1 and T 2 Maps: Solution of alizarin 2-O-β-D-galactopyranoside AZ-1 (10.0 mM) and FAC (10.0 mM) in 1:1 (V/V′) DMSO/PBS (0.1 M, pH = 7.4) after incubation 4 h at 37°C under 5% CO2/air with 95% humidity with (A) MCF7-WT cells [5 × 105, T 1( = 223 ± 11 ms, T 2( = 97 ± 12 ms]; (B) MCF7-lacZ cells [5 × 105, T 1( = 75 ± 7 ms, and T 2( = 45 ± 9 ms]; (C) MCF7-WT/MCF7-lacZ cells staining by AZ-1: dark violet staining confirming intense lacZ expression in MCF7-lacZ cells with essentially no β-gal activity in MCF7-WT cells. Regional magnification ×100; (D) Cytotoxicity: MCF7-WT/MCF7-lacZ cells viability vs. AZ-1 in various concentrations in 1:1 (V/V′) DMSO/PBS (0.1 M, pH = 7.4) at 37°C under 5% CO2/air with 95% humidity for 72 h, MCF7-WT cells (in blue), and MCF7-lacZ cells (in red).

Currently, a Gd-based contrast agent–enhanced 1H-MRI has been widely applied for medical diagnosis, offering a noninvasive way to generate anatomical and physiological information while maintaining high spatial and temporal resolution (Terreno et al., 2010; Haris et al., 2015; Wahsner et al., 2019). An Fe-based 1H MRI contrast agent, different from the Gd3+-based 1H MRI contrast agent with very strong relaxivity, exhibited much shorter relaxation times because of the formation of Fe complexes with the complete coordination of Fe3+, eliminating the possibility of inner-sphere to directly coordinate water, leaving outer-sphere and second-sphere coordination water molecules as the only pathways for relaxation (Davies et al., 1996; Richardson et al., 1999; Schwert et al., 2002; Schwert et al., 2005; Haas and Franz, 2009; Kuznik and Wyskocka, 2016). However, an Fe-based contrast agent enhanced 1H-MRI has now become a viable alternative because Fe3+ is extensively present in the tissues of the human body and is involved in transport, storage, compartmentalization, and excretion mechanisms, while Gd3+ is not naturally present in human biochemistry (Beutler, 2004; Weber et al., 2006; Kaplan and Kaplan, 2009; Theil and Goss, 2009). Particularly, cancer cells need a significant amount of Fe3+ for rapid replication, so endogenously abundant Fe3+ in tumors has been recognized as a molecular target for chemotherapeutic treatments through depleting cancer cellular Fe3+ to disrupt cancer cell proliferation and inhibit tumor growth (Fe3+-chelation therapy) (Buss et al., 2003; Richardson, 2005). In this study, we introduced exogenous Fe3+ with the ultimate goal of developing this approach to hunt the elevated Fe3+ level in tumors for the 1H-MRI signal generation. Indeed, alizarin has a very high thermodynamic stability constant logβ = 32.21 (Das et al., 1995; Das et al., 2002), indicating its capability of capturing Fe3+ from tumor to produce the Fe3+–alizarin complex in situ while simultaneously generating the 1H-MRI signal enhancement (Richardson et al., 1999; Davies et al., 1996; Schwert et al., 2002; Schwert et al., 2005; Haas and Franz, 2009; Kuznik and Wyskocka, 2016). Moreover, alizarin has been known to inhibit human cytomegalovirus replication, HIV-1 RT-associated RDDP, and integrase activities (Esposito et al., 2011). Furthermore, alizarin is the core part of anthraquinones, which constitute numerous antitumor drugs widely applied in the treatment of various neoplasms such as Adriamycin and daunorubicin, and their coordination with Fe3+ was shown to diminish cardiotoxicity while improving the antitumor activity in chemotherapy and maintain sound radiosensitizing properties in radiotherapy (Lown, 1993; Nowak and Tarasiuk, 2012; Malik and Müller, 2016). Therefore, this novel molecular platform also indicates the potential for cancer therapy and imaging by utilizing the β-gal responsive turn on pathway to selectively deplete tumor Fe3+, resulting in cancer cell cycle arrest and apoptosis while generating 1H-MRI contrast enhancement, thereby providing insight into the lacZ gene expression, development, location, and magnitude.

Conclusion

In this study, we present a novel responsive molecular platform for β-gal activity detection using 1H-MRI, in which the 1H-MRI signal enhancement is specifically generated, localized, and accumulated in situ at the β-gal activity site. In conjunction with this design, we have successfully produced and characterized alizarin 2-O-β--galactopyranoside AZ-1 and alizarin 1,2-di-O-β--galactopyranoside AZ-2. We have also demonstrated the feasibility of using AZ-1 by spontaneous in situ formation of paramagnetic Fe3+–alizarin complex to assess the β-gal activity in solution with Fe3+ ions existence by 1H-MRI T1 and T2/T2* relaxation mapping. 1H-MRI clearly showed the significant differences in both T 1 and T 2 at WT vs. lacZ gene expressing cells in culture after incubation with AZ-1, signifying the potential of integrating T 1 and T 2 data together to gain the additional certainty in imaging evaluation and detection reliability of β-gal activity.

Experimental

General Methods

NMR spectra were recorded on a Varian Unity INOVA 400 spectrometer (400 MHz for 1H, 100 MHz for 13C). 1H and 13C chemical shifts are referenced to TMS as an internal standard with CDCl3, or DMSO-d as solvents, and chemical shifts are given in ppm. All compounds were characterized by NMR at 25°C. Mass spectra were obtained by positive and negative ESI-MS using a Micromass Q-TOF hybrid quadrupole/time-of-flight instrument (Micromass UK Ltd.). Absorption spectra were taken on a UV-2700 UV-Vis Shimadzu spectrophotometer.

Solutions in organic solvents were dried with anhydrous sodium sulfate and concentrated in vacuo below 45°C. 2, 3, 4, 6-Tetra-O-acetyl-α-D-galactopyranosyl bromide was purchased from the Sigma Chemical Company. β-Gal (E801A) was purchased from the Promega (Madison, WI, United States), and enzymatic reactions were performed at 37°C in the PBS solution (0.1 M, pH = 7.4). Column chromatography was performed on silica gel (200–300 mesh), and silica gel GF254 used for analytical TLC was purchased from the Aldrich Chemical Company. The detection was affected by spraying the plates with 5% ethanolic H2SO4 (followed by heating at 110°C for 10 min) or by direct UV illumination of the plate. The purity of the final products was determined by HPLC with ≥95%.

Alizarin 2-O-2′, 3′, 4′, 6′-Tetra-O-Acetyl-β-D-Galactopyranoside AZ-M1

A solution of 2, 3, 4, 6-tetra-O-acetyl-α-D-galactopyranosyl bromide (1.23 g, 3.0 mmol) in CH2Cl2 (15 ml) was added dropwise to a vigorously stirred CH2Cl2-H2O biphasic mixture (pH 8–9) of alizarin (0.72 g, 3.0 mmol) and tetrabutylammonium bromide (TBAB) (322 mg, 1.0 mmol) in CH2Cl2-H2O (30 ml, 1:1 V/V′) around 1 h at 50°C under N2 atmosphere, with stirring continued for an additional 4–5 h until TLC showed that the reaction was completed. The product was extracted with CH2Cl2 (4 × 30 ml) and subsequently washed (H2O), dried (Na2SO4), and evaporated under reduced pressure to give a syrup, which was purified by column chromatography on silica gel to give alizarin 2-O-2′, 3′, 4′, 6′-tetra-O-acetyl-β-D-galactopyranoside AZ-M1.

Alizarin 1,2-Di-O-2′, 3′, 4′, 6′-Tetra-O-Acetyl-β-D-Galactopyranoside AZ-M2

A solution of 2, 3, 4, 6-tetra-O-acetyl-α-D-galactopyranosyl bromide (2.71 g, 6.6 mmol) in CH2Cl2 (30 ml) was added dropwise to a vigorously stirred CH2Cl2-H2O biphasic mixture (pH 10–11) of alizarin (0.72 g, 3.0 mmol) and TBAB (322 mg, 1.0 mmol) in CH2Cl2-H2O (30 ml, 1:1 V/V′) around 1 h at 55°C under N2 atmosphere, with stirring continued for an additional 4–5 h until TLC showed that the reaction was completed. The product was extracted with CH2Cl2 (4 × 40 ml) and subsequently washed (H2O), dried (Na2SO4), and evaporated under reduced pressure to give a syrup, which was purified by column chromatography on silica gel to give alizarin 1,2-di-O-2′, 3′, 4′, 6′-tetra-O-acetyl-β-D-galactopyranoside AZ-M2.

Alizarin 2-O-β-D-Galactopyranoside AZ-1 and Alizarin 1,2-Di-O-β-D-Galactopyranoside AZ-2

General procedure: A solution of alizarin 2-O-2′, 3′, 4′, 6′-tetra-O-acetyl-β-D-galactopyranoside AZ-M1 or alizarin 1,2-di-O-2′, 3′, 4′, 6′-tetra-O-acetyl-β-D-galactopyranoside AZ-M2 (1.30 g) in anhydrous MeOH (120 ml) containing 0.5 M NH3 was vigorously stirred from 0°C to room temperature overnight until TLC showed that the reaction was complete and then evaporated to dryness in vacuo. Chromatography of the crude syrup on silica gel with ethyl acetate-methanol afforded the corresponding alizarin 2-O-β-D-galactopyranoside AZ-1 and alizarin 1,2-di-O-β-D-galactopyranoside AZ-2 in high yields.

MRI

MRI studies were performed using a 4.7T horizontal bore magnet or a 9.4T vertical bore magnet equipped with a Varian INOVA Unity system (Palo Alto, CA, United States). T 1 and T 2 (or T 2 *) maps were acquired using a spin echo (or gradient echo) sequence with varying repetition times (TRs) or echo times (TEs), respectively. The raw data were acquired using a centric k-space reordering scheme, followed by the phase encoding steps with higher phase encoding gradient amplitudes. Data acquisition parameters of the FLASH readout were TR/TE/Flip angle = 10 ms/5 ms/10°. The standard multi-echo Carr–Purcell–Meiboom–Gill pulse sequence was used for measuring T 2 from a single echo train. The T 2 and T 2 * maps were obtained on a voxel-by-voxel basis using a nonlinear least-squares fit equation M = M0e−TE/T2 from the images taken at each echo time. Images were reconstructed and analyzed by using MatLab (MathWorks, Natick, MA).

lacZ Transfection in Human Tumor Cells

The E. coli lacZ gene (from pSV-β-gal vector, Promega, Madison, WI, United States) was inserted into a high expression human cytomegalovirus (CMV) immediate early enhancer/promoter vector phCMV (Gene Therapy Systems, San Diego, CA, United States), producing a recombinant vector phCMV/lacZ. This was used to transfect wild-type MCF7 (human breast cancer) and PC3 (human prostate cancer) cells (ATCC, Manassas, VA, United States) using GenePORTER2 (Gene Therapy Systems, Genlantis, Inc., San Diego, CA, United States). The highest β-gal expressing colony was selected using the antibiotic G418 disulfate (800 μg/ml, Research Products International Corp, Mt Prospect, IL, United States), and G418 (200 μg/ml) was also included for routine culture. The cells were maintained in Dulbecco’s modified Eagle’s medium (DMEM, Mediatech Inc., Herndon, VA, United States) containing 10% fetal bovine serum (FBS, 0.1 M, pH = 7.4, Atlanta Biologicals, Inc., Lawrenceville, GA, United States) with 100 units/mL of penicillin and 100 units/mL streptomycin, and cultured in a humidified 5% CO2 incubator at 37°C. The β-gal activity of lacZ-transfected tumor cells was measured using a β-Gal Assay Kit with o-nitrophenyl-β-D-galactopyranoside (Promega, Madison, WI, United States) and confirmed by X-gal or S-gal staining. Cells were fixed in PBS plus 0.5% glutaraldehyde (5 min) and rinsed in PBS prior to staining. Staining was performed using standard procedures for 2 h at 37°C in PBS plus 1 mg/ml X-gal (Sigma, St. Louis, MO, United States), 1 mM MgCl2, 5 mM K3Fe(CN)6, and 5 mM K4Fe(CN)6 or with 1.5 mg/mL S-gal (Sigma, St. Louis, MO, United States) and 2.5 mg/ml FAC.

Western Blot

The protein extracted from the wild-type and lacZ-expressing MCF7 and PC3 cancer cells was quantified using the Bradford method by a protein assay (Bio-Rad, Hercules, CA, United States). Protein (30 μg) was added to each well, separated by 10% sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and transferred to a polyvinylidene fluoride membrane. A primary monoclonal anti–β-gal antibody (Promega, Madison, WI, United States) and anti-actin antibody (Sigma, St. Louis, MO, United States) were used as probes at a dilution of 1:5,000, with the reacting protein detected using a horseradish peroxidase–conjugated secondary antibody and ECL detection (Amersham, Piscataway, NJ, United States).

Cytotoxicity

The cytotoxicity for the free β-D-galactopyranoside AZ-1 and the released aglycone alizarine was assessed in both wild-type and lacZ- expressing MCF7 and PC3 cells using a colorimetric CellTiter 96 Aqueous Non-Radioactive Cell Proliferation Assay (MTS) (Promega, Madison, WI, United States). Assays were performed in triplicate using 24-well plates seeded with 103 cells per well in 500 μL of RPMI-1640 without phenol red and supplemented with 10% FCS and 2 mM glutamine (Urano et al., 2005; Kamiya et al., 2007).