The HIP/PAP (=human Reg-2) C-type lectin encoding gene is activated in primary liver cancers. In normal liver, the protein is undetectable in normal mature hepatocytes and found only in some ductular cells, representing potential hepatic progenitor cells. The aim of this study was to examine the consequences of human HIP/PAP expression in the liver of transgenic mice. We demonstrated that HIP/PAP stimulated liver regeneration after partial hepatectomy. To further investigate the enhanced liver regeneration observed in vivo, primary cultures of hepatocytes were used to evaluate the mitogenic and anti-apoptotic properties of HIP/PAP. HIP/PAP increased hepatocyte DNA synthesis and protected hepatocytes against TNF-alpha plus actinomycin-D-induced apoptosis. HIP/PAP anti-apoptotic effects against TNF-alpha were clearly demonstrated when protein kinase A activity was specifically inhibited by KT5720, and HIP/PAP stimulated PKA-dependent phosphorylation of the proapoptotic Bad protein at Ser-112, suggesting that HIP/PAP may compete with cAMP to stimulate PKA activity. Overall, our results led us to propose a new role for a C-type lectin, HIP/PAP, as a hepatic cytokine that combines mitogenic and anti-apoptotic functions regarding hepatocytes, and consequently acts as a growth factor in vivo to enhance liver regeneration.

Cyclins are major cell cycle regulators which role in malignant transformation remains controversial. In this report we describe a new mechanism of cyclin oncogenic activation. We demonstrate that an altered form of cyclin A2 (S2A) which N-terminal part is replaced by the hepatitis B virus envelope protein transforms normal rat kidney cells and cooperates with ras to transform rat embryo ®broblasts. In contrast, neither the viral moiety, nor a full length or N-terminally deleted cyclin A2 show these oncogenic properties. S2A oncogenicity arises from its binding to cyclin dependent kinases, since mutation in the MRAIL sequence abolishes transformation and correlates with an abnormal cellular localization in the endoplasmic reticulum membrane. Together, these results implicate modi®cation in the cellular distribution of a cell cycle regulator as a mechanism of virally-induced transformation.

Cyclin A2 is predominantly, but not exclusively, localized in the nucleus from G1/S transition onwards. It is degraded when cells enter mitosis after nuclear envelope breakdown. We previously showed that a fusion protein (S2A) between the hepatitis B virus (HBV) surface antigen protein and a non-degradable fragment of human cyclin A2 (D152) resides in the endoplasmic reticulum membranes, escapes degradation and transforms normal rat ®broblasts. The present study investigates whether cytoplasmic cyclin A2 may play a role in oncogenesis. We show that the sequestration of non-degradable cyclin A2-D152 by a cellular ER targeting domain (PRL-A2) leads to cell transformation when coexpressed with activated Ha-ras. REF52 cells constitutively expressing PRL-A2 are found to have a high incidence of multinucleate giant cells, polyploidy and abnormal centrosome numbers, giving rise to the nucleation of multipolar spindles. Injection of these cells into athymic nude mice causes tumors, even in the absence of a cooperating Ha-ras oncogene. These results demonstrate that, independently of any viral context, an intracellular redistribution of non-degradable cyclin A2 is capable of deregulating the normal cell cycle to the point where it promotes aneuploidy and cancer.