Mcl-1 suppresses abasic site repair following bile acid–induced hepatic cellular DNA damage

Abstract

In cholestasis, increases in bile acid levels result in the generation of reactive oxygen species and the induction of DNA damage and mutation. It is believed that bile acid accumulation is associated with liver tumorigenesis. However, the mechanism that underpins this phenomenon remains to be elucidated. Mcl-1, which is overexpressed in hepatic cells, is a pro-survival member of the Bcl-2 family. In this study, we observed that Mcl-1 potently suppresses the repair of bile acid-induced abasic (apurinic/apyrimidinic) sites in DNA lesions. Upon exposure of hepatic cells to glycochenodeoxycholate, one of the major conjugated human bile acids, we observed an increase in AP site accumulation along with induction of poly(ADP-ribose) polymerase and XRCC1 ( X-Ray Repair Cross Complementing 1). In addition, accumulation of Mcl-1 was observed in the nuclei of QGY-7703 cells in response to glycochenodeoxycholate stimulation. Knockdown of endogenous Mcl-1 by RNA interference significantly accelerated the repair of DNA lesions in glycochenodeoxycholate-treated cells. However, unlike XRCC1, poly(ADP-ribose) polymerase was induced following Mcl-1 knockdown. Conversely, poly(ADP-ribose) polymerase suppression was observed following glycochenodeoxycholate treatment of cells overexpressing Mcl-1. Moreover, AP-site counting analyses revealed that DNA repair activity was enhanced in cells overexpressing poly(ADP-ribose) polymerase under glycochenodeoxycholate stress conditions. It is well known that poly(ADP-ribose) polymerase plays a crucial role in the base excision repair pathway. Thus, our findings suggest that Mcl-1 suppresses base excision repair by inhibiting poly(ADP-ribose) polymerase induction following glycochenodeoxycholate-induced DNA damage. These results potentially explain how bile acid accumulation results in genetic instability and carcinogenesis.

Bile salt (glycochenodeoxycholate acid) induces cell survival and chemoresistance in hepatocellular carcinoma

“…GCDA is a toxic component of bile salts and is associated with hepatocarcinogenesis and resistance. Previous work showed that GCDA induces hepatocyte apoptosis (Maillette & Beuers, ) and also increases chemoresistance of HepG2 cells after cisplatin and irinotecan treatment (Liao et al, ). In the present study, we investigated whether GCDA increased the resistance of HCC cells.…”

“…Additionally, the tumor microenvironment inducing DNA alterations is associated with the stepwise progression of HCC; DNA mutations increase genetic diversity and reduce chemotherapeutic efficacy (Tu et al, ). Accumulation of bile acids increases DNA mutations (Komichi et al, ; Liao et al, ; Yu et al, ), which can lead to chemoresistance at the gene level. Our results indicate that GCDA can increase the resistance of HCC cells and significantly attenuate the apoptotic effect of 5FU (Figure ), which suggests that GCDA, as a member of the tumor microenvironment, may mediate the survival and drug resistance of liver cancer cells.…”

“…Together with previous research, GCDA enhances the antiapoptotic function of Mcl‐1 at both the posttranscriptional (phosphorylation) and transcriptional (mRNA) level. Furthermore, siRNA interference of Mcl‐1 expression reversed the GCDA‐induced chemoresistance (Liao et al, ), so Mcl‐1 plays vital roles in GCDA‐induced chemoresistance and may be an ideal target for treating HCC with GCDA‐induced chemoresistance.…”

“…GCDA has been found responsible for cholestasis‐associated liver injury because of its direct cytotoxic effects on hepatocytes (Maillette & Beuers, ). Previous reports suggested a direct toxic effect of GCDA on hepatocytes via distinct intracellular signaling mechanisms, including disrupting the cell membrane integrity by their detergent action on lipid components (Billington et al, ), triggering the generation of reactive oxygen species (Reinehr et al, ; Sokol et al, ; Webster & Anwer, ), inducing oxidative damage that causes mitochondrial dysfunction (Perez et al, ; Sokol et al, ), and DNA damage (Komichi et al, ; Liao et al, ; Yu et al, ).…”

“…However, GCDA failed to induce apoptosis or kinase activation in HUH7 and HepG2 cells; it must enter hepatocytes to mediate cell death or activate kinases (Rust, ; Webster & Anwer, ). Our previous research showed that GCDA phosphorylated myeloid cell leukemia 1 (Mcl‐1) at Tyr163 and B‐cell lymphoma 2 (Bcl‐2) at Ser70 via the mitogen‐activated protein kinase/extracellular signal‐regulated kinase 1/2 (MAPK/ERK1/2) pathway, which eventually led to survival and chemoresistance in HCC cells (Liao et al, ; Zhou et al, ). On one hand, GCDA causes apoptosis or necrosis in normal liver cells, which leads to the development of HCC and on the other hand, it promotes the survival of hepatoma cells.…”