HIPK2 phosphorylates ΔNp63α and promotes its degradation in response to DNA damage.

Homeodomain-interacting protein kinase 2 (HIPK2) is an emerging player in cell response to genotoxic agents that senses damage intensity and contributes to the cell's choice between cell cycle arrest and apoptosis. Phosphorylation of p53 at S46, an apoptosis-specific p53 posttranslational modification, is the most characterized HIPK2 function in response to lethal doses of ultraviolet (UV), ionizing radiation or different anticancer drugs, such as cisplatin, roscovitine and doxorubicin (DOX). Indeed, like p53, HIPK2 has been shown to contribute to the effectiveness of these treatments. Interestingly, p53-independent mechanisms of HIPK2-induced apoptosis were described for UV and tumor growth factor-ß treatments; however, it is unknown whether these mechanisms are relevant for the responses to anticancer drugs. Because of the importance of the so-called 'p53-independent apoptosis and drug response' in human cancer chemotherapy, we asked whether p53-independent factor(s) might be involved in HIPK2-mediated chemosensitivity. Here, we show that HIPK2 depletion by RNA interference induces resistance to different anticancer drugs even in p53-null cells, suggesting the involvement of HIPK2 targets other than p53 in response to chemotherapy. In particular, we found that HIPK2 phosphorylates and promotes proteasomal degradation of ΔNp63α, a prosurvival ΔN isoform of the p53 family member, p63. Indeed, effective cell response to different genotoxic agents was shown to require phosphorylation-induced proteasomal degradation of ΔNp63α. In DOX-treated cells, we show that HIPK2 depletion interferes with ΔNp63α degradation, and expression of a HIPK2-resistant ΔNp63α-Δ390 mutant induces chemoresistance. We identify T397 as the ΔNp63α residue phosphorylated by HIPK2, and show that the non-phosphorylatable ΔNp63α-T397A mutant is not degraded in the face of either HIPK2 overexpression or DOX treatment. These results indicate ΔNp63α as a novel target of HIPK2 in response to genotoxic drugs.

Isolation and characterization of a murine resident liver stem cell.

Increasing evidence provides support that mammalian liver contains stem/progenitor cells, but their molecular phenotype, embryological derivation, biology and their role in liver cell turnover and regeneration remain to be further clarified. In this study, we report the isolation, characterization and reproducible establishment in line of a resident liver stem cell (RLSC) with immunophenotype and differentiative potentiality distinct from other previously described liver precursor/stem cells. RLSCs, derived from fetal and neonatal murine livers as well as from immortalized hepatocytic MMH lines and established in lines, are Sca+, CD34-, CD45-, alpha-fetoprotein+ and albumin-. This molecular phenotype suggests a non-hematopoietic origin. RLSC transcriptional profile, defined by microArray technology, highlighted the expression of a broad spectrum of 'plasticity-related genes' and 'developmental genes' suggesting a multi-differentiative potentiality. Indeed, RLSCs spontaneously differentiate into hepatocytes and cholangiocytes and, when cultured in appropriate conditions, into mesenchymal and neuro-ectodermal cell lineages such as osteoblasts/osteocytes, chondrocytes, astrocytes and neural cells. RLSC capability to spontaneously differentiate into hepatocytes, the lack of albumin expression and the broad differentiative potentiality locate them in a pre-hepatoblast/liver precursor cells hierarchical position. In conclusion, RLSCs may provide a useful tool to improve liver stem cell knowledge and to assess new therapeutic approaches for liver diseases.