Abstract
Human hepatic bile acid transporter Na+/taurocholate cotransporting polypeptide (NTCP) represents the liver-specific entry receptor for the hepatitis B and D viruses (HBV/HDV). Chronic hepatitis B and D affect several million people worldwide, but treatment options are limited. Recently, HBV/HDV entry inhibitors targeting NTCP have emerged as promising novel drug candidates. Nevertheless, the exact molecular mechanism that NTCP uses to mediate virus binding and entry into hepatocytes is still not completely understood. It is already known that human NTCP mRNA expression is downregulated under cholestasis. Furthermore, incubation of rat hepatocytes with the secondary bile acid taurolithocholic acid (TLC) triggers internalization of the rat Ntcp protein from the plasma membrane. In the present study, the long-term inhibitory effect of TLC on transport function, HBV/HDV receptor function, and membrane expression of human NTCP were analyzed in HepG2 and human embryonic kidney (HEK293) cells stably overexpressing NTCP. Even after short-pulse preincubation, TLC had a significant long-lasting inhibitory effect on the transport function of NTCP, but the NTCP protein was still present at the plasma membrane. Furthermore, binding of the HBV/HDV myr-preS1 peptide and susceptibility for in vitro HDV infection were significantly reduced by TLC preincubation. We hypothesize that TLC rapidly accumulates in hepatocytes and mediates long-lasting trans-inhibition of the transport and receptor function of NTCP via a particular TLC-binding site at an intracellularly accessible domain of NTCP. Physiologically, this trans-inhibition might protect hepatocytes from toxic overload of bile acids. Pharmacologically, it provides an interesting novel NTCP target site for potential long-acting HBV/HDV entry inhibitors.NEW & NOTEWORTHY The hepatic bile acid transporter NTCP is a high-affinity receptor for hepatitis B and D viruses. This study shows that TLC rapidly accumulates in NTCP-expressing hepatoma cells and mediates long-lasting trans-inhibition of NTCP's transporter and receptor function via an intracellularly accessible domain, without substantially affecting its membrane expression. This domain is a promising novel NTCP target site for pharmacological long-acting HBV/HDV entry inhibitors.
Highlights
An effective vaccine is available, hepatitis B virus (HBV) and hepatitis D virus (HDV) infections remain a major global health problem
This pattern was not much different, when Na þ /taurocholate cotransporting polypeptide (NTCP)-HEK293 cells were preincubated with 25 mM taurocholic acid (TC) or tauroursodeoxycholic acid (TUDC) for 30 min followed by thorough washout before the respective experiments
As both bile acids demonstrated a potential for NTCP cis-inhibition of bile acid transport and myr-preS1 peptide binding in our previous study [14], this indicates that the washout of the preincubated bile acids was sufficient to avoid such a cis-inhibition
Summary
An effective vaccine is available, hepatitis B virus (HBV) and hepatitis D virus (HDV) infections remain a major global health problem. More than 250 million people worldwide are chronically infected with HBV, and thousands of them suffer from hepatocellular carcinoma (HCC) or develop terminal liver cirrhosis. About 5% of chronic HBV carriers are coinfected with HDV, a situation often associated with disease progression and increased mortality rates [1]. The hepatitis D virus is an HBV satellite virus, and both share identical envelope proteins [2]. Current standard therapies include nucleoside reverse transcriptase inhibitors and interferon. Both of these are not curative in the majority of cases [3,4,5]
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