Diabetes mellitus type 2 has been reported to be associated with the development of hepatocellular carcinoma (HCC) [1-5], while the mechanism of the association between these two diseases remains unclear. Hepatitis C virus (HCV) infection is known as one of the major risk factors of HCC [6] and diabetes mellitus [7]. Furthermore, it has been shown that HCV infection and replication lead to autophosphorylation and thus activation of double-stranded RNA-dependent protein kinase (PKR) [8, 9]. This raises the question whether PKR plays a role in modulating tumor growth and insulin action. PKR is best known for triggering cell defense responses by phosphorylating eIF-2α and thus activating cellular apoptosis during virus infection or other stress conditions, for example endoplasmic reticulum (ER) stress [10, 11]. However, evidence is emerging suggesting PKR has a tumor-promoting and anti-apoptotic roles, for example, elevated protein level and activity of PKR have been observed in HCV-related HCC [12], while the mechanism of the anti-apoptotic role of PKR is still unknown. Meanwhile, the role of PKR in insulin signaling also has not been reported. In the present study, we identified the roles played by PKR in both the apoptosis and insulin signaling pathways in liver cells. First, we determined for the first time that PKR exploited an anti-apoptotic role in human hepatocellular carcinoma cells (HepG2) by regulating the Bcl-2 protein [13]. As one of the anti-apoptotic members of the Bcl-2 family, Bcl-2 protects cells against intrinsic (mitochondria) apoptosis by maintaining the integrity of the mitochondrial membrane [14]. We showed for the first time that PKR is required to maintain the proper level of Bcl-2 in HepG2 cells, and the transcription factor, NF-κB, is involved in mediating the effect of PKR on Bcl-2 level. In addition, the phosphorylation of Bcl-2 at different amino acid residues, such as Ser70 and Ser87, is critical in determining the role of Bcl-2 in regulating apoptosis [15]. We showed that the phosphorylation of Bcl-2 at the anti-apoptotic residue, Ser70, was mediated by PKR and JNK, while the phosphorylation of Bcl-2 at the pro-apoptotic residue, Ser87, was not affected by PKR. Second, we found that PKR is involved in down-regulating insulin signaling in primary hepatocytes and HepG2 cells. It is the first time that PKR is shown to induce insulin resistance by up-regulating the phosphorylations at Ser residues of insulin receptor substrate (IRS) -1 and -2. In summary, by uncovering the roles of PKR in regulating both the apoptosis and insulin signaling pathways, we have identified a potential key protein that links insulin resistance and hepatocellular carcinogenesis, and therefore provided a possible mechanism for the enhanced risk of diabetes mellitus and primary liver cancer in HCV infection.

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