Coordinated translation initiation is coupled with cell cycle progression and cell growth, whereas excessive ribosome biogenesis and translation initiation often lead to tumor transformation and survival. Hepatocellular carcinoma (HCC) is among the most common and aggressive cancers worldwide and generally displays inherently high resistance to chemotherapeutic drugs. We found that RACK1, the receptor for activated C-kinase 1, was highly expressed in normal liver and frequently upregulated in HCC. Aberrant expression of RACK1 contributed to in vitro chemoresistance as well as in vivo tumor growth of HCC. These effects depended on ribosome localization of RACK1. Ribosomal RACK1 coupled with PKCβII to promote the phosphorylation of eukaryotic initiation factor 4E (eIF4E), which led to preferential translation of the potent factors involved in growth and survival. Inhibition of PKCβII or depletion of eIF4E abolished RACK1-mediated chemotherapy resistance of HCC in vitro. Our results imply that RACK1 may function as an internal factor involved in the growth and survival of HCC and suggest that targeting RACK1 may be an efficacious strategy for HCC treatment.

No abstract

One of the serious sequelae of chronic hepatitis B virus (HBV) infection is hepatocellular carcinoma (HCC). Among all the proteins encoded by the HBV genome, hepatitis B virus X protein (HBx) is highly associated with the development of HCC. Although Notch1 signaling has been found to exert a tumor-suppressive function during HCC development, the mechanism of interaction between HBx expression and Notch1 signaling needs to be explored. In this study, we report that HBx expression in hepatic and hepatoma cells resulted in decreased endogenous protein levels of Notch1 intracellular domain (ICN1) and messenger RNA levels of its downstream target genes. These effects were due to a reduction of Notch1 cleavage by HBx through the suppression of presenilin1 (Psen1) transcription rather than inhibition of Notch1 transcription or its ligands' expression. Through transient HBx expression, decreased ICN1 resulted in enhanced cell proliferation, induced G1-S cell cycle progression, and blunted cellular senescence in vitro. Furthermore, the effect of blunted senescence-like growth arrest by stable HBx expression through suppression of ICN1 was shown in a nude mouse xenograft transplantation model. The correlation of inhibited Psen1-dependent Notch1 signaling and blunted senescence-like growth arrest was also observed in HBV-associated HCC patient tumor samples. Conclusion: Our results reveal a novel function of HBx in blunting senescence-like growth arrest by decreasing Notch1 signaling, which could be a putative molecular mechanism mediating HBV-associated hepatocarcinogenesis. (HEPATOLOGY 2010;52:142-154) H epatocellular carcinoma (HCC) is the fifth most common neoplasm and the third leading cause of cancer-related death in humans, with nearly 600,000 deaths annually worldwide. 1,2 Chronic hepatitis B virus (HBV) infection has been identified as a major risk factor for the development of HCC, especially in southeastern Asia and sub-Saharan Africa. [3][4][5] Several processes are involved in the development of HBV-associated hepatocellular carcinoma, including integration of HBV genes into host cell genome, sustained cycles of necrosis-inflammation-regeneration, activation of oncogenic pathways, and inactiAbbreviations: BrdU, 5-bromo-2 0 -deoxyuridine; GAPDH, glyceraldehyde 3-phosphate dehydrogenase; HBV, hepatitis B virus; HBx, hepatitis B virus X protein; HCC, hepatocellular carcinoma; ICN1, Notch1 intracellular domain; mRNA, messenger RNA; NEXT1, extracellular truncated Notch1; Notch1-FL, full-length Notch1; qRT-PCR, quantitative real-time polymerase chain reaction; Psen1, presenilin1; Psen2, presenilin2; SA-b-gal, senescence-associated b-galactosidase; SD, standard deviation; TACE, tumor necrosis factor-a-converting enzyme.From the

Androgen receptor (AR) is essential for the maintenance of the male reproductive systems and is critical for the carcinogenesis of human prostate cancers (PCas). D-type cyclins are closely related to the repression of AR function. It has been well documented that cyclin D1 inhibits AR function through multiple mechanisms, but the mechanism of how cyclin D3 exerts its repressive role in the AR signaling pathway remains to be identified. In the present investigation, we demonstrate that cyclin D3 and the 58-kDa isoform of cyclin-dependent kinase 11 (CDK11 p58 ) repressed AR transcriptional activity as measured by reporter assays of transformed cells and prostate-specific antigen expression in PCa cells. AR, cyclin D3, and CDK11 p58 formed a ternary complex in cells and were colocalized in the luminal epithelial layer of the prostate. AR activity is controlled by phosphorylation at specific sites. We found that AR was phosphorylated at Ser-308 by cyclin D3/CDK11 p58 in vitro and in vivo, leading to the repressed activity of AR transcriptional activation unit 1 (TAU1). Furthermore, androgen-dependent proliferation of PCa cells was inhibited by cyclin D3/CDK11 p58 through AR repression. These data suggest that cyclin D3/CDK11 p58 signaling is involved in the negative regulation of AR function.Androgen receptor (AR), a member of the nuclear receptor family, directly regulates patterns of gene expression in response to the steroids testosterone and dihydrotestosterone (DHT) and is subsequently involved in the regulation of the development and differentiation of the male reproductive system (10). Similar to other steroid receptors, AR contains a transactivation domain (TAD), also named AF-1, in its N terminus, a ligand binding domain (LBD) in its C terminus, a DNA-binding domain (DBD), and a hinge region between the TAD and LBD. The transcriptional activation unit 1 (TAU1) and TAU5 motifs in the AR N-terminal domain (NTD) (residues 101 to 370 and 360 to 485, respectively) as well as the AF-2 motif in the AR LBD have been implicated in directly contacting p160 proteins and mediating transcription (1,23,31,48).AR is a phosphoprotein whose function is regulated by the modulation of its phosphorylation status at different sites (4). The consensus phosphorylation sites found in AR indicate that AR could be a substrate for DNA-dependent kinase, protein kinase A, protein kinase C, mitogen-activated protein kinase, and casein kinase 2 (4). Ser-16, Ser-81, Ser-94, Ser-256, Ser-308, Ser-424, and Ser-650 have been identified as being phosphorylation sites of AR by mutagenesis, peptide mapping, and mass spectrometry (6, 60). Recently, several Ser/Thr protein kinases have been found to phosphorylate AR at the abovementioned sites in vitro and in vivo. For example, AR Ser-515 is phosphorylated by mitogen-activated protein kinase, Ser-213 and Ser-791 are phosphorylated by Akt, and Ser-650 is phosphorylated by p38␣ and JNK1 (17,30,57).More and more studies suggest that cyclins and cyclin-dependent kinases (CDKs) are also involved in th...

Of the three envelope glycoproteins encoded by hepatitis B virus (HBV) that are collectively referred to as HBV surface antigen (HBsAg), the large HBsAg (LHBs) glycoprotein is expressed preferentially in HBV-associated hepatocellular carcinoma. LHBs can act as an oncogene in transgenic mice, but how it contributes functionally to hepatocarcinogenesis remains unclear. In this study, we determined the molecular and functional roles of LHBs during HBV-associated hepatocarcinogenesis. LHBs increased tumor formation of hepatoma cells. Moreover, expression of LHBs but not other HBV envelope glycoproteins specifically promoted proliferation of hepatoma and hepatic cells in vitro. Mechanistic investigations revealed that these effects were caused by activation of the Src/PI3K/Akt pathway through proximal stimulation of PKCa/Raf1 signaling by LHBs. Proliferation induced by stable LHBs expression was associated with increased G 1 -S cell-cycle progression and apoptosis resistance mediated by Src kinase activation, as established in hepatocellular carcinoma clinical specimens. Importantly, LHBs-induced cellular proliferation and tumor formation were reversed by administration of the Src inhibitor saracatinib. Together, our findings suggest that LHBs promotes tumorigenesis of hepatoma cells by triggering a PKCa/Raf1 to Src/PI3K/Akt signaling pathway, revealing novel insights into the underlying mechanisms of HBVassociated hepatocarcinogenesis. Cancer Res; 71(24); 7547-57. Ó2011 AACR.