8 We have also failed to observe alterations of SREBP/FAS expression or triglyceride biosynthesis in Huh-7 cells transduced with HCV core 3a (data not shown). Thus, it remains to be established whether HCV core 3a–mediated PTEN down-regulation both promotes the formation of large cytoplasmic lipid droplets and stimulates lipogenesis. In this respect, the evidence PD-0332991 purchase indicates
that a nonstructural viral protein 5A (NS5A) also promotes lipid accumulation.26 The synergistic effects of multiple HCV proteins on the biogenesis of lipid droplets and the lipid metabolism in hepatocytes remain to be evaluated. We have already shown that the genotype 3a core protein induces IRS1 degradation in hepatocytes.20 As previously reported,8, 27 we have found that IRS1 down-regulation is triggered by low levels of PTEN expression and is crucial for core 3a–induced lipid droplet formation. In agreement with a role for IRS1 in hepatic lipid metabolism,21, 28 we have found IRS1 to be down-regulated in the livers of HCV-infected patients. Furthermore, IRS1 overexpression prevented the formation of large lipid droplets in core 3a–expressing cells. Because IRS1 depletion in cultured cells did not lead to the formation of enlarged lipid droplets, it is likely that, in addition see more to IRS1 down-regulation, other core 3a–dependent and/or PTEN-dependent mechanisms are required. Notably,
it is unlikely that Akt2, which is overactivated in liver-specific PTEN knockout mice and promotes lipogenesis,29 is involved in this process because Akt2 activity was not exacerbated by core 3a expression in our model (data not shown). Further studies are necessary to delineate the precise role of IRS1 versus other effects of core 3a in the generation of large lipid droplets. Mechanisms regulating PTEN expression have been intensively investigated because of the tumor suppressor activity of PTEN. Posttranscriptional modifications such as phosphorylation, ubiquitination, and redox
mechanisms have been shown to control the stability and degradation of the PTEN protein.11 Our data indicate that none of these mechanisms are likely responsible for MG 132 the core 3a–mediated down-regulation of PTEN. Instead, HCV core 3a expression appears to repress PTEN mRNA translation via PTEN 3′-UTR–dependent mechanisms. Noncoding microRNAs play important roles in protein expression by hybridizing to complementary sites on the 3′-UTR sequences of target mRNAs and thereby inhibiting their translation or triggering their degradation.30 Several microRNAs have been reported to inhibit PTEN expression.11 Interestingly, because the levels of PTEN mRNA are unchanged between control and HCV core 3a–expressing cells, it is likely that core 3a induces the expression of microRNAs, which prevent the translation of this mRNA.