6 Our results show that in hepatocytes, a small amount of PHB1 can be found in the nucleus. Furthermore, reduced PHB1 expression in AML12 cells resulted in increased E2F binding to the cyclin D1 promoter and cyclin D1 expression. However, if PHB1 inhibits cell-cycle progression and induces apoptosis, as these studies suggest, why would it be highly expressed in many human cancers? Although it is highly expressed in many types of cancer,23 it has not been well studied in HCC and whether it is functionally normal has not been examined. Indeed, we found much higher PHB1 protein level in normal human hepatocytes when compared

to two human liver cancer cell lines. PHB1 expression level also does not equate function. A recent study found liver cells and transgenic mice that express hepatitis C virus core protein have increased mitochondrial AZD9668 PHB1 protein level but despite higher level, it is functionally impaired.24 Thus, it would be of interest to see if there is a difference in subcellular localization of PHB1 in various cancers and whether it is functionally VX-809 research buy intact. Consistent with this notion, a recent study found lung cancer cells that have increased membrane-associated PHB1 (cell surface and microsomal membrane fractions) were more resistant to paclitaxel23 and interestingly, total whole-cell

levels of PHB1 were unchanged. We did not observe http://www.selleck.co.jp/products/pembrolizumab.html any difference in sensitivity to sorafenib in Huh-7 cells when PHB1 expression was varied. Thus, the effect of PHB1 may be cancer-specific and cell line–specific. The microarray data revealed that many pathways are affected by reduced PHB1 expression. Many are growth-related and oncogenes. Interestingly, cyclin D1 and PCNA, which are up-regulated in Mat1a KO25 and Phb1 KO livers,

are known E2F targets.26 These findings, along with our current observations, would support PHB1′s repressive role on E2F transcriptional activity in mouse liver. We also examined whether these genes are similarly affected after acute knockdown of PHB1 in AML12 cells. Although many of the growth-related genes are similarly affected, the magnitude of change is much smaller than in vivo. This may be partly due to the fact that PHB1 protein level fell only by 30% after 18 hours of siRNA treatment, even though the mRNA level fell by 90%. PHB1 protein appears to be quite stable, with a half-life that exceeds 10 hours.27 Interestingly, CYP4A12A and CYP2F2 are both increased in the AML12 cells following PHB1 knockdown, which is the opposite of what happened in the KO livers. This likely reflects some adaptive changes that occurred in vivo and the influence of the in vivo microenvironment.