Abstract

Introduction

Liver cancer is one of the most malignant tumors globally, with high mortality rates, which commonly result from chronic inflammation and hepatic fibrosis (¹1. Chen X, Calvisi DF. Hydrodynamic transfection for generation of novel mouse models for liver cancer research. Am J Pathol 2014; 184: 912–923, doi: 10.1016/j.ajpath.2013.12.002
https://doi.org/10.1016/j.ajpath.2013.12... ). In high-incidence areas of hepatocellular carcinoma (HCC), such as West Africa approximately, 60% of HCC patients are infected with hepatitis B virus (HBV), suggesting that integrated HBV genetic material in hepatocytes may accelerate the process of cirrhosis and cell immortalization (²2. Kirk GD, Lesi OA, Mendy M, Akano AO, Sam O, Goedert JJ, et al. The Gambia liver cancer study: Infection with hepatitis B and C and the risk of hepatocellular carcinoma in West Africa. Hepatology 2004; 39: 211–219, doi: 10.1002/hep.20027
https://doi.org/10.1002/hep.20027... ). Signaling pathways that activate cell proliferation and invasion are the effectors of HBx, a reverse transcription element in the HBV genome for fibrosis and oncogenesis, interacted by TGF-β or other transcriptional factors, such as nuclear factor-κB and Sp-1 (³3. Arbuthnot P, Kew M. Hepatitis B virus and hepatocellular carcinoma. Int J Exp Pathol 2001; 82: 77–100, doi: 10.1111/j.1365-2613.2001.iep178.x
https://doi.org/10.1111/j.1365-2613.2001... 4. Tang H, Da L, Mao Y, Li Y, Li D, Xu Z, et al. Hepatitis B virus X protein sensitizes cells to starvation-induced autophagy via up-regulation of beclin 1 expression. Hepatology 2009; 49: 60–71, doi: 10.1002/hep.22581
https://doi.org/10.1002/hep.22581... –⁵5. Ng SA, Lee C. Hepatitis B virus X gene and hepatocarcinogenesis. J Gastroenterol 2011; 46: 974–990, doi: 10.1007/s00535-011-0415-9
https://doi.org/10.1007/s00535-011-0415-... ). Consequently, although HBV replication in infected hepatocytes cannot be eradicated, acting on the HBx-triggered molecular networks and suppressing the cofactors of HBx might be a potent therapeutic strategy for HBV-infected HCC.

Dysregulated microRNA (miRNA) profiles are the collaborators of Hbx, which aggravate tumor progression involved in carcinogenesis, cell invasion, and metastasis in human malignancies. Let-7a, one of the let-7 family members, was initially characterized in Caenorhabditis elegans with Lin28a, an RNA-binding protein that acted as a suppressor of let-7 expression and controller for development and differentiation (⁶6. Reinhart BJ, Slack FJ, Basson M, Pasquinelli AE, Bettinger JC, Rougvie AE, et al. The 21-nucleotide let-7 RNA regulates developmental timing in Caenorhabditis elegans. Nature 2000; 403: 901–906, doi: 10.1038/35002607
https://doi.org/10.1038/35002607... ). Let-7a is also down-regulated in HCC patients and its low expression in liver tissues may contribute to poor survival rates (⁷7. Khare S, Zhang Q, Ibdah JA. Epigenetics of hepatocellular carcinoma: role of microRNA. World J Gastroenterol 2013; 19: 5439–5445, doi: 10.3748/wjg.v19.i33.5439
https://doi.org/10.3748/wjg.v19.i33.5439... ). Moreover, Lin28-induced cancer cell EMT is dependent on the low let-7 level and overexpression of the EMT-associated let-7a downstream targets, such as K-ras and HMGA2 (⁸8. Wang T, Wang G, Hao D, Liu X, Wang D, Ning N, et al. Aberrant regulation of the LIN28A/LIN28B and let-7 loop in human malignant tumors and its effects on the hallmarks of cancer. Mol Cancer 2015; 14: 125, doi: 10.1186/s12943-015-0402-5
https://doi.org/10.1186/s12943-015-0402-... ). Therefore, seeking an approach to alter the Lin28a/let-7a axis in hepatoma cancer cells may lead to the development of effective strategies for HCC therapy.

Urolithins, the dibenzopyran-6-one colonic metabolites derived from ellagic acid (EA) or ellagitannins (ETs), have been suggested to be beneficial for human health. Urolithins in target tissues and cells could act on sub-cellular components and activate signaling transduction. These cell responses contribute to the various biological potentials of EA- and ET-rich diets (⁹9. Tomas-Barberan FA, Gonzalez-Sarrias A, Garcia-Villalba R, Nunez-Sanchez MA, Selma MV, Garcia-Conesa MT, et al. Urolithins, the rescue of "old" metabolites to understand a "new" concept: Metabotypes as a nexus among phenolic metabolism, microbiota dysbiosis, and host health status. Mol Nutr Food Res 2017; 61, doi: 10.1002/mnfr.201500901
https://doi.org/10.1002/mnfr.201500901... ). Based on the anti-proliferative effects of urolithin A on HepG2 cells in our previous findings (¹⁰10. Wang Y, Qiu Z, Zhou B, Liu C, Ruan J, Yan Q, et al. In vitro antiproliferative and antioxidant effects of urolithin A, the colonic metabolite of ellagic acid, on hepatocellular carcinomas HepG2 cells. Toxicol In Vitro 2015; 29: 1107–1115, doi: 10.1016/j.tiv.2015.04.008
https://doi.org/10.1016/j.tiv.2015.04.00... ), the function of urolithin A in repressing HepG2.2.15 (HBV-integrated HepG2 cell line) cell proliferation and invasion are discussed in the present study. The results suggested that the regulating effects of urolithin A on the Lin28a/let-7a axis contributed to the inhibition of transcriptional factor Sp-1 and down-regulation of HMGA2 and K-ras in HepG2.2.15 cells.

Material and Methods

Chemicals

Urolithin A was synthesized as previously described (¹¹11. Bialonska D, Kasimsetty SG, Khan SI, Ferreira D. Urolithins, intestinal microbial metabolites of Pomegranate ellagitannins, exhibit potent antioxidant activity in a cell-based assay. J Agric Food Chem 2009; 57: 10181–10186, doi: 10.1021/jf9025794
https://doi.org/10.1021/jf9025794... ). The purity (>94%) of urolithin A was evaluated by HPLC, and its molecular weight was confirmed by mass spectrometry analysis (Figure S1). A hydro-soluble tetrazolium salt WST-8 (2-(2-methoxy-4-nitrophenyl)-3-(4-nitrophenyl)-5-(2,4-disulfophenyl)-2H-tetrazolium, monosodium salt) was obtained from Dojindo Laboratories (Japan) for CCK-8 assay. All other chemicals and reagents were of analytical grade.

Cell culture

HepG2 and HepG2.2.15 cells (HepG2 cells integrated with a stable wild-type full-length HBV genome) were obtained from American Type Culture Collection and cultured in Dulbecco's Modified Eagle Medium (Gibco, USA) supplemented with fetal bovine serum (10%), penicillin (1 mM) and streptomycin (1 mM) at 37°C in a humidified 5% CO₂ atmosphere.

Cell viability assay

The effect of urolithin A on HepG2.2.15 cell viability was evaluated using the CCK-8 assay with WST-8 according to manufacturer's protocol (¹²12. Ji Q, Hao X, Meng Y, Zhang M, Desano J, Fan D, et al. Restoration of tumor suppressor miR-34 inhibits human p53-mutant gastric cancer tumorspheres. BMC Cancer 2008; 8: 266, doi: 10.1186/1471-2407-8-266
https://doi.org/10.1186/1471-2407-8-266... ). Cell viability values were normalized as follows: [Final absorbance (urolithin A) / Final absorbance (control)] × 100%. IC₅₀ value for HepG2.2.15 cells was calculated according to a dose-response curve, which was plotted for each concentration.

Quantitative PCR assay for miRNA and mRNA

Total RNAs containing miRNAs in cells were prepared using mirVana™ miRNA Isolation Kit (Thermo Fisher Scientific, USA). Real-time quantitative PCR (qPCR) assay was performed on a MiniOpticon™ (Bio-Rad, USA) system using FastStart Universal SYBR Green Master (Roche, USA). With initial denaturation at 95°C for 120 s, amplifications were performed for 40 cycles at 95°C for 5 s and 55°C for 25 s. Primers for qPCR are listed in Table 1.

Western blot

Western blot was performed as described previously (¹³13. Zhou B, Wang J, Zheng G, Qiu Z. Methylated urolithin A, the modified ellagitannin-derived metabolite, suppresses cell viability of DU145 human prostate cancer cells via targeting miR-21. Food Chem Toxicol 2016; 97: 375–384, doi: 10.1016/j.fct.2016.10.005
https://doi.org/10.1016/j.fct.2016.10.00... ). Briefly, separated proteins in SDS-PAGE were transferred onto polyvinylidene fluoride membranes and were sequentially immune-reacted with specific primary antibodies and secondary antibodies. Antibody-conjunct proteins were quantified using SuperSignal™ West Pico Chemiluminescent Substrate (Thermo Scientific, USA). The primary antibodies cleaved caspase-3 (Cell Signaling Technology (CST, USA)), Bcl-2 (CST), Bax (CST), Lin28a (CST), Sp-1 (CST), HMGA2 (CST), K-ras (CST), Snail (CST), and Zcchc11 (Abcam, UK) were applied after the membranes were blocked in either 5% milk or 5% BSA. Anti-β-actin-peroxidase antibody was obtained from Sigma-Aldrich (USA) and used as an internal reference. The protein content was analyzed using Image Lab 5.1 (Bio-Rad, USA).

Vector and miRNA transfection

Cells were cultured in 6-well (qPCR and western blot) or 24-well (luciferase assay) plates and were then transfected with plasmids. The Lin28 open reading frame was inserted into the pcDNA3.1(+) vector (Invitrogen, USA) to express Lin28 (pcDNA3.1-Lin28) with Lipofectamine™ 3000 (Invitrogen) in HepG2.2.15 cells. Let-7a inhibitor (2′-O-methyl antisense oligonucleotide) and the non-specific control were commercially synthesized as previously described (¹⁴14. Khodayari N, Mohammed KA, Goldberg EP, Nasreen N. EphrinA1 inhibits malignant mesothelioma tumor growth via let-7 microRNA-mediated repression of the RAS oncogene. Cancer Gene Ther 2011; 18: 806–816, doi: 10.1038/cgt.2011.50
https://doi.org/10.1038/cgt.2011.50... ). For western blot assay, cells in 6-well plates with approximately 80% confluency were transfected with 50 nM of let-7a miRNA inhibitor negative control (MOCK) (Thermo Fisher, USA) or 100 nM of inhibitor using Lipofectamine™ 3000 (Invitrogen, USA). Transfection efficiency was validated by qPCR to directly assess the expression of let-7a.

Transwell invasion assays

The transwell assay was performed using Matrigel-coated (1:5, BD Biosciences) polycarbonate filters (Corning Costar, USA) (¹⁵15. Zhang L, Tu Y, He W, Peng Y, Qiu Z. A novel mechanism of hepatocellular carcinoma cell apoptosis induced by lupeol via Brain-Derived Neurotrophic Factor Inhibition and Glycogen Synthase Kinase 3 beta reactivation. Eur J Pharmacol 2015; 762: 55–62, doi: 10.1016/j.ejphar.2015.05.030
https://doi.org/10.1016/j.ejphar.2015.05... ). Cells were subjected to urolithin A for 24 h before use, and the chemical was present throughout the assay. Invasive cells in the lower chamber were stained with crystal violet and visualized using an optical microscope. Cell invasion was quantitated via the absorbance (600 nm) of eluted crystal violet with a microtiter plate reader as described previously (¹⁶16. Petitclerc E, Boutaud A, Prestayko A, Xu J, Sado Y, Ninomiya Y, et al. New functions for non-collagenous domains of human collagen type IV. Novel integrin ligands inhibiting angiogenesis and tumor growth in vivo. J Biol Chem 2000; 275: 8051–8061, doi: 10.1074/jbc.275.11.8051
https://doi.org/10.1074/jbc.275.11.8051... ).

Statistics

All in vitro assessments were performed at least in triplicate. The data are reported as means±SD. Differences between experimental groups were analyzed by Student's t-test or one-way analysis of variance (ANOVA), followed by Dunnett's t-test for multiple comparisons using SPSS (13.0) statistical program. P-values of less than 0.05 were considered statistically significant.

Results

Urolithin A inhibited cell proliferation and induced cytotoxicity in HepG2.2.15 cells

In the CCK-8 assay, cells were administrated with urolithin A and B by gradual concentrations (1–120 μM) for 24 and 48 h. As shown in Figure 1A, urolithin A repressed cell proliferation in a dose-dependent manner. It was also found that urolithin A induced acute cytotoxicity from 80–120 μM, compared to the corresponding controls. However, urolithin B (1–120 μM) showed a moderately suppressing effect on HepG2.2.15 cells (Figure S2). Because of this, we chose urolithin A for further investigation. To identify whether urolithin A functions through caspase-3 signaling, cells were incubated in the absence or presence of Z-DEVD-FMK (20 μM) for 6 h before urolithin A treatment. Data showed that pretreatment with Z-DEVD-FMK abolished the inhibiting effect of cell proliferation induced by urolithin A (Figure 1B). Furthermore, we observed that the protein level of cleaved caspase-3 was up-regulated and the Bcl-2/Bax ratio was decreased by urolithin A in HepG2.2.15 cells (Figure 1C and D), suggesting that the suppressing effect of urolithin A on HepG2.2.15 viability could be involved in the activation of caspase-dependent apoptotic signaling.

Urolithin A altered the expressing pattern of Lin28a/let-7a axis

Lin28a, which is a repressor of let-7a by recruiting Zcchc11, could be up-regulated by the HBx protein via Sp-1 in HCC cells. Our data indicated that the protein expressions of Lin28a, Zcchc11, and Sp-1 were suppressed in the urolithin A group, compared to the control group (Figure 2A and B). We also observed that let-7a was up-regulated when the cells were subjected to urolithin A (Figure 2C). To further confirm the effects of urolithin A on the Lin28a/let-7a axis, overexpression of Lin28a (pcDNA3.1-Lin28a) by transient transfection was performed in HepG2.2.15 cells (Figure S3). The results showed that the effects of urolithin A on the expression of let-7a were abolished by Lin28a transfection (Figure 2D), indicating that urolithin A increased the expression of let-7a by targeting Lin28a in HepG2.2.15 cells.

Urolithin A repressed Lin28a expression and elevated the expression of let-7a in transient HBx-transfected HepG2 cells

The effect of urolithin A on the Lin28a/let-7a axis was also confirmed in transient HBx-overexpressing HepG2 cells. In this section, HepG2 cells (4×10⁵ cells/well) were seeded in 6-well plates for 24 h and transiently transfected with 2 μg pcDNA-HBx (pc-HBX) plasmids (pcDNA3.1 (-) plasmid with the sequence encoding for the HBx protein; a gift from Prof. Fan Zhu at Wuhan University). After post-transfection for 24 h, cells were incubated with urolithin A (60 and 100 μM) for 48 h. As expected, qPCR assay showed that the elevated mRNA expression of Lin28a in the pcDNA-HBx group was repressed by urolithin A (Figure 3A). To examine whether urolithin A affected Lin28a at the protein level in the HBx-transfected HepG2 cells, western blot analysis was performed; the results demonstrated that Lin28a protein expression was suppressed (Figure 3B and C). Moreover, the HBx-induced let-7a decrease in the pcDNA-HBx group was alleviated by urolithin A treatment (Figure 3D). These data indicated that urolithin A delayed the HBx-induced change of the Lin28a/let-7a axis in HCC cells, further enhancing the findings involved in the effects of urolithin A on Lin28a and let-7a in HepG2.2.15 cells.

Urolithin A suppressed cell invasion by inhibiting K-ras/HMGA2 signaling in HepG2.2.15 cells

Since let-7a was increased by urolithin A, we assumed that downstream targets of let-7a could respond to urolithin A exposure. Accompanied by an elevation of let-7a (Figure 2C), the expression of HMGA2, a protein enhancing oncogenic transformation and epithelial-mesenchymal transition, was reduced by urolithin A compared to the MOCK group (Figure 4A and B). The effect of urolithin A on cell invasion was evaluated using a transwell assay. As shown in Figure 4C and D, cells in the control groups exhibited approximately 3 times higher invasion efficiency than cells in the urolithin A (UroA100) group, indicating that urolithin A could suppress cell invasion in vitro. To further determine whether let-7a elevation contributed to the suppressing effect of urolithin A on cell invasion, the let-7a inhibitor was transfected for 24 h to abolish the expression of let-7a before urolithin A administration (Figure S4). In Figure 4E, the blockade of let-7a resulted in an increase in HMGA2 protein expression in the western blot assay, compared to the urolithin A group. Moreover, K-ras, which was commonly considered to be a compatible factor of HMGA2 to accelerate cell migration and invasion, was also down-regulated in the urolithin A group (Figure 4A). However, the protein expression of Snail showed no difference between the urolithin A and the MOCK groups. Hence, our data suggested that the regulation of the K-ras/HMGA2 signaling by urolithin A may contribute to the inhibiting effect on cell invasion.

Discussion

Accepted theory claims that the intestinal metabolism of ETs or EAs to dibenzopyran-6-one derivatives (urolithins) may play an indispensable role in the absorption of ETs. From this perspective, urolithins identified in tissues are important, and their molecular modulation in target cells can be more effective in interpreting the nutritional benefits for human health than that of ETs (¹⁷17. Espin JC, Gonzalez-Sarrias A, Tomas-Barberan FA. The gut microbiota: a key factor in the therapeutic effects of (poly)phenols. Biochem Pharmacol 2017, doi: 10.1016/j.bcp.2017.04.033
https://doi.org/10.1016/j.bcp.2017.04.03... ).

Urolithin A is an essential metabolite generated in humans after consumption of EA- and ET-rich food and healthy supplements (¹⁸18. Cerda B, Tomas-Barberan FA, Espin JC. Metabolism of antioxidant and chemopreventive ellagitannins from strawberries, raspberries, walnuts, and oak-aged wine in humans: identification of biomarkers and individual variability. J Agric Food Chem 2005; 53: 227–235, doi: 10.1021/jf049144d
https://doi.org/10.1021/jf049144d... ). Cell signaling transduction (EGFR, β-catenin), essential regulators for cell cycle (cyclin D1, c-Myc, p21), and programmed cell death (p53, PARP, caspase-3) could be regulated by urolithins (mainly urolithin A and B) in cancer cells (¹⁹19. Espin JC, Larrosa M, Garcia-Conesa MT, Tomas-Barberan F. Biological significance of urolithins, the gut microbial ellagic Acid-derived metabolites: the evidence so far. Evid Based Complement Alternat Med 2013; 2013: 270418, doi: 10.1155/2013/270418
https://doi.org/10.1155/2013/270418... ). Though the lipogenic gene in hepatoma Huh7 cells and WNT signaling in HepG2 cells are targets of urolithin A or C (²⁰20. Sharma M, Li L, Celver J, Killian C, Kovoor A, Seeram NP. Effects of fruit ellagitannin extracts, ellagic acid, and their colonic metabolite, urolithin A, on Wnt signaling. J Agric Food Chem 2010; 58: 3965–3969, doi: 10.1021/jf902857v
https://doi.org/10.1021/jf902857v... ,²¹21. Kang I, Kim Y, Tomas-Barberan FA, Espin JC, Chung S. Urolithin A, C, and D, but not iso-urolithin A and urolithin B, attenuate triglyceride accumulation in human cultures of adipocytes and hepatocytes. Mol Nutr Food Res 2016; 60: 1129–1138, doi: 10.1002/mnfr.201500796
https://doi.org/10.1002/mnfr.201500796... ), the functions of urolithins in hepatocytes have not been adequately evaluated. Hence, to explore the capacity of urolithin A on HBx-relevant cell invasion, the Lin28a/let-7a axis and Sp-1, a transcriptional factor that could be elevated by HBx, were assumed in the present study to be potential targets of urolithin A in HepG2.2.15 cells.

Sp-1 is an HBx-combinative activator and a member of the Sp/KLF family of transcription factors, and is widely overexpressed in neoplasms, including liver carcinoma (²²22. Shen Q, Uray IP, Li Y, Krisko TI, Strecker TE, Kim HT, et al. The AP-1 transcription factor regulates breast cancer cell growth via cyclins and E2F factors. Oncogene 2008; 27: 366–377, doi: 10.1038/sj.onc.1210643
https://doi.org/10.1038/sj.onc.1210643... ,²³23. Safe S, Abdelrahim M. Sp transcription factor family and its role in cancer. Eur J Cancer 2005; 41: 2438–2448, doi: 10.1016/j.ejca.2005.08.006
https://doi.org/10.1016/j.ejca.2005.08.0... ). Based on the anti-proliferative effects of urolithin A in HepG2.2.15 cells, we investigated whether urolithin A could affect the expression of Sp-1. The results revealed that the protein expression level of Sp-1 was repressed by urolithin A treatment. Lin28a, the transcriptional target of Sp-1, which could further elevate the levels of certain cancer-related miRNAs (²⁴24. You X, Liu F, Zhang T, Lv N, Liu Q, Shan C, et al. Hepatitis B virus X protein upregulates Lin28A/Lin28B through Sp-1/c-Myc to enhance the proliferation of hepatoma cells. Oncogene 2014; 33: 449–460, doi: 10.1038/onc.2012.618
https://doi.org/10.1038/onc.2012.618... ), was also suppressed by urolithin A in HepG2.2.15 cells. Therefore, we speculated that suppression of Sp-1 could be responsible for the decrease of Lin28a by urolithin A.

Let-7a functions as a tumor suppressor and is biologically deleted in several cancers, including in HCC patients with HBV infection (²⁵25. Li J, Shi W, Gao Y, Yang B, Jing X, Shan S, et al. Analysis of microRNA expression profiles in human hepatitis B virus-related hepatocellular carcinoma. Clin Lab 2013; 59: 1009–1015, doi: 10.7754/Clin.Lab.2012.120901
https://doi.org/10.7754/Clin.Lab.2012.12... ). Moreover, Lin28a suppresses let-7a miRNA biogenesis in tumor cells by recruiting Zcchc11 (TUT4) in the cell cytoplasm to degrading pre-let-7 (²⁶26. Hagan JP, Piskounova E, Gregory RI. Lin28 recruits the TUTase Zcchc11 to inhibit let-7 maturation in mouse embryonic stem cells. Nat Struct Mol Biol 2009; 16: 1021–1025, doi: 10.1038/nsmb.1676
https://doi.org/10.1038/nsmb.1676... ). In our study, urolithin A elevated the let-7a expression, which was delayed by Lin28a overexpression in HepG2.2.15 cells. The similar effects of urolithin A on the Lin28a/let-7a axis were further observed in HBx-overexpressed HepG2 cells. Some evidence has also highlighted that the Zcchc11 down-regulation is Lin28b-independent, and should be restricted to Lin28a-positive carcinomas, such as the T47D breast cancer cells (²⁷27. Piskounova E, Polytarchou C, Thornton JE, LaPierre RJ, Pothoulakis C, Hagan JP, et al. Lin28A and Lin28B inhibit let-7 microRNA biogenesis by distinct mechanisms. Cell 2011; 147: 1066–1079, doi: 10.1016/j.cell.2011.10.039
https://doi.org/10.1016/j.cell.2011.10.0... ). In the present study, the protein expression of Zcchc11 was reduced by urolithin A treatment, suggesting that Zcchc11 inhibition plays a bridging role for the effects of urolithin A on the Lin28a/let-7a axis.

High mobility group AT-hook 2 (HMGA2) is a transcription factor highly expressed in the embryonic stage. The exceptional re-expression of HMGA2 by chromosomal rearrangements at chr12q13-15 in neoplasia of mice or humans, such as HCC and lung cancer, is involved in the let-7 deletion or loss of let-7-binding sites (²⁸28. Mayr C, Hemann MT, Bartel DP. Disrupting the pairing between let-7 and Hmga2 enhances oncogenic transformation. Science 2007; 315: 1576–1579, doi: 10.1126/science.1137999
https://doi.org/10.1126/science.1137999... 29. Lee YS, Dutta A. The tumor suppressor microRNA let-7 represses the HMGA2 oncogene. Genes Dev 2007; 21: 1025–1030, doi: 10.1101/gad.1540407
https://doi.org/10.1101/gad.1540407... –³⁰30. Heo I, Joo C, Cho J, Ha M, Han J, Kim VN. Lin28 mediates the terminal uridylation of let-7 precursor MicroRNA. Mol Cell 2008; 32: 276–284, doi: 10.1016/j.molcel.2008.09.014
https://doi.org/10.1016/j.molcel.2008.09... ). In this study, the western blot analysis supported the notion that urolithin A can down-regulate HMGA2 expression and the capacity is let-7a-dependent. We also found that the invasive potential of HepG2.2.15 cells could be repressed by urolithin A treatment. Let-7a also affected oncogenic K-ras expression by binding the 3'-UTR of K-ras. Exogenous let-7a miRNA in HepG2 cells could specifically reduce abundant K-ras expression (³¹31. Tsang WP, Kwok TT. Let-7a microRNA suppresses therapeutics-induced cancer cell death by targeting caspase-3. Apoptosis 2008; 13: 1215–1222, doi: 10.1007/s10495-008-0256-z
https://doi.org/10.1007/s10495-008-0256-... ). The protein expression of K-ras was decreased by urolithin A in HepG2.2.15 cells at the repressible concentration in the transwell assay. Therefore, it is possible that the functions of urolithin A in HepG2.2.15 cells may partly match the above-identified proposal involved in let-7a/HMGA2/K-ras signaling.

In the present study, we described the effects of urolithin A in repressing the proliferation and invasion of the HBV-overexpressed HepG2.2.15 HCC cell line. In fact, the virus replication in HBV-infected patients could not be eliminated, but only alleviated with antiviral agents, such as adefovir. The data in this study could be considered an update of our previous data (¹⁰10. Wang Y, Qiu Z, Zhou B, Liu C, Ruan J, Yan Q, et al. In vitro antiproliferative and antioxidant effects of urolithin A, the colonic metabolite of ellagic acid, on hepatocellular carcinomas HepG2 cells. Toxicol In Vitro 2015; 29: 1107–1115, doi: 10.1016/j.tiv.2015.04.008
https://doi.org/10.1016/j.tiv.2015.04.00... ) on HepG2 cells because urolithin A has suppressed cell invasion in HepG2.2.15 cells. In other words, our findings are not only involved in the targets of carcinogenesis but are also based on the initial observation of the HBx-interactional cancerous transcriptional factor, Sp-1. Therefore, our data demonstrated that urolithin A suppressed the HepG2.2.15 cell proliferation and invasion via regulating the Lin28a/let-7a axis and EMT-involved targets, such as HMGA2 and K-ras.

Supplementary Material

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Acknowledgments

This study was supported by a grant from Research Project of Hubei Provincial Department of Education (Q20162001) to Dr. Zhenpeng Qiu.

References

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