Why is the female population more susceptible to cholestasis‐induced liver injury—Could it be long noncoding RNA H19?
Authors: Grace L. Guo
Journal: Hepatology (2017)

Potential conflict of interest: Nothing to report. See Article on Page 869 The female population is more susceptible to hepatic cholestasis; however, the underlying mechanisms are not entirely clear. Several factors have been reported to contribute to the onset and severity of cholestasis in female subjects, including elevated estrogen; mutations in or decreased functions of canalicular transporters of bile acids (ABCB11), phospholipids (ABCB4), or organic anions (ABCC2); altered activation of signaling pathways (AKT, protein kinase C); and mutations or reduced functions of nuclear receptors of bile acids (farnesoid X receptor) and estrogen (estrogen receptor).1 The Abcb4 gene knockout (Mdr2 KO) mouse, which is an animal model to exemplify the hepatobiliary diseases in humans with mutations in the MDR3 gene, has been widely used to study human primary sclerosing cholangitis (PSC).4 Female Mdr2 knockout (KO) mice are more susceptible to cholestatic liver injury and fibrosis than male mice, and this gender disparity has been shown to be due to increased levels of estrogen and bile acids, especially higher levels of taurine‐conjugated cholic acid (TCA).5 However, the underlying molecular mechanism responsible for this gender disparity has not been elucidated. In the current issue of hepatology, Li et al. elegantly showed that H19, a long noncoding RNA, was markedly induced in the livers of Mdr2 KO mice.6 Several critical features for this increased H19 expression include (1) age dependence, (2) occurrence mainly in cholangiocytes and not hepatocytes, and (3) most strikingly, being found at much higher levels in female mouse livers. Furthermore, the current study suggests that the underlying molecular mechanism responsible for the more severe cholestasis and fibrosis in female Mdr2 KO mice is due to the induction of H19, as revealed by the results that show that both TCA and estrogen significantly activated the extracellular signal–regulated kinase 1/2 pathway, which subsequently induced H19 expression in cholangiocytes. A loss of H19 function by knockdown technology led to reduced expression of the genes involved in fibrosis and the gene that encodes the sphingosine 1‐phosphate receptor 2, a G protein–coupled membrane receptor that is activated by TCA and promotes cholangiocyte growth.7 Interestingly, the induction of H19 gene expression in the current study was found to be c‐myc‐independent and inversely related with the expression of small heterodimer partner, which is not fully consistent with previous reports,8 suggesting that the regulation of H19 levels is complicated in diverse pathological situations. The authors propose that H19 synthesized in cholangiocytes is secreted by extracellular vesicles that are taken up by hepatocytes and further reduces the levels of small heterodimer partner, a nuclear receptor, reported to be an epigenetic regulator of gene expression and a repressor of inflammation (Fig. 1).Figure 1: H19 induction in cholangiocytes is a major mechanism responsible for increased cholestasis‐induced liver injury in female mice. In female mice, under Mdr2 deficiency and cholestasis, the increased bile acids, particularly TCA, activate sphingosine 1‐phosphate receptor 2, along with an increase in estrogen level, leading to H19 induction in cholangiocytes. H19 is secreted by extracellular vesicles that are taken up by hepatocytes, which results in reduction of small heterodimer partner in hepatocytes and subsequent up‐regulation of bile acid synthesis. This may further exacerbate the accumulation of bile acids and damage to the bile duct epithelial cells. Abbreviations: CTP7A1, cytochrome P450 7A1; ERK, extracellular signal–regulated kinase; HDAC, histone deacetylase; PI3K, phosphoinositide 3‐kinase; PLC, phospholipase C; S1P, sphingosine 1‐phosphate; S1PR2, S1P receptor 2; SHP, small heterodimer partner; Sph, sphingosine; SphK1, Sph kinase 1.The H19 gene is located on human chromosome 11 and murine chromosome 7, and it is an imprinted and maternally expressed gene that was originally termed "adult skeletal muscle."10 The H19 gene encodes an RNA product that is spliced and polyadenylated but not translated into a protein. Maternal hypomethylation and paternal hypermethylation in a differentially methylated region upstream of the H19 promoter is the most important mechanism responsible for the imprinting. The classical function of H19 has been described to be repression of the expression of the gene encoding insulin‐like growth factor 2.11 Loss of H19 results in increased insulin‐like growth factor 2 expression and cellular overgrowth.12 Overexpression of H19 is found in many forms of cancer, and H19 has been shown to strongly promote cell proliferation. The link between H19 and the gender disparity of cholangiocyte proliferation has been revealed by the current study. More importantly, the gender disparity found in Mdr2 KO mice is consistent with results of tissue samples obtained from human PSC patients. Therefore, this study is significant and novel as it provides clear evidence demonstrating that H19 plays a critical role in the disease progression of cholestasis and represents a key factor that causes the gender disparity of cholestatic liver injury not only in Mdr2 KO mice but probably in human PSC patients as well. One limitation of this study is that the authors did not examine hepatic H19 expression in human primary biliary cirrhosis (PBC) patients. PSC mainly affects the male population, and the female population is more susceptible to PBC.13 It will be of interest to know if H19 expression level is correlated to disease progression and a gender difference in PBC patients. The most recent study from Song et al. reported that H19 is up‐regulated in the livers of PBC patients. But this study did not compare gender differences.14 PBC is a chronic cholestatic liver disease of autoimmune origin. Studies of sex hormones and X chromosome on gender differences of PBC are of interest, but none of these studies have yielded conclusive results to date. There is evidence indicating that inactivation of the X chromosome contributes to the onset of autoimmune diseases including PBC, systemic lupus erythematosus, and autoimmune thyroid disease.15 In addition, recent studies indicate that genomic imprinting may play a role in autoimmune diseases, such as rheumatoid arthritis, type 1 diabetes, systemic lupus erythematosus, and Crohn's disease.16 Whether the imprinted long noncoding RNA H19 plays a critical role in disease development and progression of different autoimmune diseases remains to be determined. Overall, the results of this study are remarkable and provide evidence supporting the importance of long noncoding RNA H19 in cholestatic liver diseases. In order to further elucidate the molecular/cellular mechanisms by which H19 contributes to cholestatic liver injury, liver‐specific and cell type–specific genetic modified animal models, systemic and advanced molecular approaches, and clinical studies should be helpful. Identification of the links between H19 expression and cholestatic disease progression will provide valuable information for the development of useful early diagnostic biomarkers and potential therapeutics for PSC and PBC.