Bile salt export pump is dysregulated with altered farnesoid X receptor isoform expression in patients with hepatocellular carcinoma

Abstract

As a canalicular bile acid effluxer, bile salt export pump (BSEP) plays a vital role in maintaining bile acid homeostasis. BSEP deficiency leads to severe cholestasis and hepatocellular carcinoma (HCC) in young children. Regardless of the etiology, chronic inflammation is the common pathological process for HCC development. Clinical studies showed that bile acid homeostasis is disrupted in HCC patients with elevated serum bile acid level as a proposed marker for HCC. However, the underlying mechanisms remain largely unknown. In this study, we found that BSEP expression was severely diminished in HCC tissues and markedly reduced in adjacent non-tumor tissues. In contrast to mouse, human BSEP was regulated by farnesoid x receptor (FXR) in an isoform-dependent manner. FXRα2 exhibited a much more potent activity than FXRα1 in transactivating human BSEP in vitro and in vivo. The decreased BSEP expression in HCC was associated with altered relative expression of FXRα1 and FXRα2. The FXRα1/FXRα2 ratios were significantly increased with undetectable FXRα2 expression in one third of the HCC tumor samples. Similar correlation between BSEP and FXR isoform expression was confirmed in hepatoma Huh 7 and HepG2 cells. Further studies showed that intrahepatic proinflammatory cytokines interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-α) were significantly elevated in HCC tissues. Treatment of Huh 7 cells with IL-6 and TNF-α resulted in a marked increase in the FXRα1/FXRα2 ratio concurrent with a significant decrease in BSEP expression. In conclusion, BSEP expression was severely diminished in HCC patients associated with alteration of FXR isoform expression induced by inflammation, and the restoration of BSEP expression through suppressing inflammation in the liver may re-establish the bile acid homeostasis.

FXR and liver carcinogenesis

“…The absence of BSEP can cause severe cholestasis and HCC in young children [86,87] . However, BSEP expression is dramatically reduced in human HCC samples [88] . BSEP is transcriptionally induced by FXR in an isoformspecific manner [88] .…”

“…However, BSEP expression is dramatically reduced in human HCC samples [88] . BSEP is transcriptionally induced by FXR in an isoformspecific manner [88] . FXRα1 and FXRα2 are two main isoforms of FXR expressed in normal human liver [89] , and FXRα2 has a greater ability to transactivate BSEP [88] .…”

“…BSEP is transcriptionally induced by FXR in an isoformspecific manner [88] . FXRα1 and FXRα2 are two main isoforms of FXR expressed in normal human liver [89] , and FXRα2 has a greater ability to transactivate BSEP [88] . The increase in the FXRα1/FXRα2 ratio due to the stimulation of proinflammatory cytokines results in significant downregulation of BSEP [88] .…”

“…FXRα1 and FXRα2 are two main isoforms of FXR expressed in normal human liver [89] , and FXRα2 has a greater ability to transactivate BSEP [88] . The increase in the FXRα1/FXRα2 ratio due to the stimulation of proinflammatory cytokines results in significant downregulation of BSEP [88] . Another FXR targeted tumor suppressor gene is N-myc downstream-regulated gene 2 (NDRG2).…”

“…The elevated levels of proinflammatory cytokines, such as TNFα, IL-1β, and IL-6, in both FXR -/-mice [19,20] and in most human HCC patients [25,88] , may reduce the FXR expression via inhibiting the transactivity of hepatic nuclear factor 1α (HNF1α) on the FXR gene promoter [25] . TNFα and IL-1β alter the relative expression of FXRα1 and FXRα2, which leads to an increase in the FXRα1/ FXRα2 ratio and subsequent reduction of BSEP, indicting a potential interaction between FXR alternative splicing procedure and the inflammatory-cytokine mediated signal pathway [88] . Gadaleta et al [90] demonstrated that a vicious cycle was formed when NF-κB-dependent reduction of FXR expression led to less inhibition of NF-κB-mediated intestinal inflammation (Figure 1).…”