The hepatitis B virus HBx protein modulates cell cycle regulatory proteins in cultured primary human hepatocytes

There are over 350 million people chronically infected with the Hepatitis B virus (HBV); chronic HBV infections are associated with the development of hepatocellular carcinoma (HCC). While the precise mechanism of HBV-associated HCC remains undefined, it is believed to involve a combination of the host immune response to infection and activities of HBV proteins including the nonstructural X protein (HBx). HBx is a multifunctional protein that can modulate various cellular processes including cell proliferation. The exact effect of HBx on cell proliferation has varied depending on the cell line and exact conditions used in the study. Our previously published reports have demonstrated that HBx modulates the levels of cell cycle regulatory proteins in primary rat hepatocytes; however, the effect of HBx on cell cycle regulatory proteins in primary human hepatocytes, the natural host for HBV infection, has not been studied. Here we have examined the effect of HBx on cell cycle regulatory proteins in cultured, primary human hepatocytes. We demonstrate that HBx decreases the levels of cell cycle proteins that prevent progression into G1 phase and increases the levels of cell cycle proteins active in G1 phase. We have also shown that HBx modulation of cell cycle regulatory proteins requires cytosolic calcium, similar to the results we previously obtained in primary rat hepatocytes. Cumulatively, our results are the first demonstration that HBx modulates the levels of cell cycle regulatory proteins in a calcium-dependent manner in primary human hepatocytes.