| X protein which is a regulatory protein encoded by hepatitis B virus (HBV)is highly conserved in all mammalian hepadnavirus. To determine the HBV Xprotein in the virus life cycle, we designed an X-minus mutant p3.8IIXm bysite-directed mutagenesis via changing the start codon of the HBx gene inp3.8II without affecting the HBV polymerase gene product. p3.8II contains 1.2times HBV genome. When transfected this mutant into HepG2 cells, wedetected decreased levels of secreted HBsAg, HBeAg and cytoplasmHBe/cAg. Analysis of total cellular RNA identified decreased levels of the2.4/2.1kb preS/S mRNA and 3.4/3.5kb pregenomic/precore mRNA. Theamount of replicating viral DNA in the cell lysate of p3.8IIXm is half of thep3.8II. These data suggest that HBV lack of X protein can still replicate inHepG2 cells, but the absence of HBx will weaken the viral abilities ofreplication and transcription. p73, as a novel member of a family of p53-related transcription factors,shares redundant functions with p53, such as the abilities of inducing apoptosisand suppressing growth. p73α and p73β are derived from the differentalternatively spliced transcripts of p73 gene. It is well known that p53 canrepress HBV expression and transcription efficiently. Using the X-minus mutantp3.8IIXm, we investigated the effect of p73α and p73β on HBV transcription tounderstand the correlation between HBV and p73 preliminarily. Aftercotransfecting p3.8IIXm and p73α or p73β expression vectors into HepG2 cells, 2we found both p73α and p73β could repress HBsAg and HBeAg expressionand viral transcription by downregulating the ENI/Xp and ENII/Cp activities. Butp73β exerted stronger inhibition on the activity of ENI/Xp than p73α, and thisresulted in much lower level of viral transcripts and antigens. Therefore, p73β,as a novel member of p53 family, can efficiently inhibit HBV transcription, andmight help host cells to resist the viral infection.
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