Succinate‐GPR‐91 receptor signalling is responsible for nonalcoholic steatohepatitis‐associated fibrosis: Effects of DHA supplementation

Abstract

Background and aims Treatment of non‐alcoholic steatohepatitis (NASH) is challenging, because suppressing fibrotic progression has not been achieved consistently by drug candidates currently in clinical trials. The aim of this study was to investigate the molecular interplays underlying NASH‐associated fibrosis in a mouse NASH model and human specimens. Methods Mice were divided into 4 groups: Controls; NASH (high fat/Calorie diet plus high fructose and glucose in drinking water, HFCD‐HF/G) for 16 weeks; HFCD‐HF/G plus docosahexaenoic acid (DHA) for 16 or 8 weeks. Results Along with NASH progression, fibrotic deposition was documented in HFCD‐HF/G‐fed mice. Liver succinate content was significantly increased along with decreased expression of succinate dehydrogenase‐A (SDH‐A) in these mice; whereas, GPR‐91 receptor expression was much enhanced in histology compared to control mice, and co‐localized histologically with hepatic stellate cells (HSCs). Succinate content was increased in fatty acid‐overloaded primary hepatocytes with significant oxidant stress and lipotoxicity. Exposure to succinate led to up‐regulation of GPR‐91 receptor in primary and immortalized HSCs. In contrast, suppression of GPR‐91 receptor expression abolished succinate stimulatory role in GPR‐91 expression and extracellular matrix production in HSCs. All these changes were minimized or abrogated by DHA supplementation in vivo or in vitro. Moreover, GPR‐91 receptor expression correlates with severity of fibrosis in human NASH biopsy specimens. Conclusion Succinate accumulation in steatotoic hepatocytes may result in HSC activation through GPR‐91 receptor signalling in NASH progression, and the cross‐talk between hepatocytes and HSC through GPR‐91 signalling is most likely to be the molecular basis of fibrogenesis in NASH.

GPR91, a critical signaling mechanism in modulating pathophysiologic processes in chronic illnesses

“…Nonalcoholic steatohepatitis (NASH) is an active stage of NAFLD, which may progress to hepatic fibrosis, cirrhosis, and even hepatocellular carcinoma (HCC) in some patients. [45][46][47] The role of succinate-GPR91 signaling pathway in haptic fibrosis has been extensively studied by us 35 and others. 48 With succinate dehydrogenase being inhibited, succinate accumulation leads to GPR91 overexpression in HSCs and the phosphorylation of c-Jun/ERK, thus, increasing expression of smooth muscle α-actin (α-SMA), procollagen type I/III, tissue inhibitor of metalloproteinases (TIMP-1), and connective tissue growth factor (CTGF).…”

“…48 With succinate dehydrogenase being inhibited, succinate accumulation leads to GPR91 overexpression in HSCs and the phosphorylation of c-Jun/ERK, thus, increasing expression of smooth muscle α-actin (α-SMA), procollagen type I/III, tissue inhibitor of metalloproteinases (TIMP-1), and connective tissue growth factor (CTGF). 35 The upregulation of GPR91 and subsequent effects were blocked by an ERK inhibitor, U0126 or PD098059 [48][49][50] or by inhibition of GPR91 expression in HSCs with RNA interference. 35 In NASH, lipotoxicity causes mitochondrial damage and TCA cycle interruption, which results in the succinate accumulation in steatotic hepatocytes.…”

“…35 The upregulation of GPR91 and subsequent effects were blocked by an ERK inhibitor, U0126 or PD098059 [48][49][50] or by inhibition of GPR91 expression in HSCs with RNA interference. 35 In NASH, lipotoxicity causes mitochondrial damage and TCA cycle interruption, which results in the succinate accumulation in steatotic hepatocytes. Elevated succinate in hepatocytes is released to the extracellular space, and acts on HSCs.…”

“…This cross-talk between steatotic hepatocytes and HSCs through an unique succinate-GPR91 signaling pathway has been postulated to be the molecular basis for the NASH-associated hepatic fibrosis. 35 In this context, the succinate-GPR91 signaling pathway represents a typical example of an intrinsic metabolic intermediate activating a tissue repairing process, and it is considered as a valuable target for molecular intervention for NASH-associated fibrosis 35 (Figure 3).…”

“…32,33 An increase in phosphorylated extracellular signal-regulated kinases (ERK) was been detected after succinate stimulation in various cell types, such as human embryonic kidney (HEK293) cells, Madin-Darby Canine Kidney (MDCK) cells, retinal ganglion cells (RGC-5), as well as hepatic stellate cells (HSCs), and the activation of this pathway was suggested to be associated with Gβγ dimer or Gα q subunit. 27,31,[34][35][36][37] Another pathway is via the activation of phospholipase C (PLC), in turn, leading to the phosphorylation of inositol that produces inositol trisphosphate (IP 3 ) and the increased intracellular calcium levels. 38 This PLCβ-IP 3 pathway was thought to be associated with either Gβγ dimer or Gα q subunit directly in hepatic stellate cells (HSCs), but the latter has been challenged since some results showed that GPR91 may act exclusively through the Gα i pathway in HSCs.…”