Protein Kinase C-δ mediates down-regulation of heterogeneous nuclear ribonucleoprotein K protein: involvement in apoptosis induction

We reported previously that NSC606985, a camptothecin analogue, induces apoptosis of acute myeloid leukemia (AML) cells through proteolytic activation of protein kinase C delta (ΔPKC-δ). By subcellular proteome analysis, heterogeneous nuclear ribonucleoprotein K (hnRNP K) was identified as being significantly down-regulated in NSC606985-treated leukemic NB4 cells. HnRNP K, a docking protein for DNA, RNA, and transcriptional or translational molecules, is implicated in a host of processes involving the regulation of gene expression. However, the molecular mechanisms of hnRNP K reduction and its roles during apoptosis are still not understood. In the present study, we found that, following the appearance of the ΔPKC-δ, hnRNP K protein was significantly down-regulated in NSC606985, doxorubicin, arsenic trioxide and ultraviolet-induced apoptosis. We further provided evidence that ΔPKC-δ mediated the down-regulation of hnRNP K protein during apoptosis: PKC-δ inhibitor could rescue the reduction of hnRNP K; hnRNP K failed to be decreased in PKC-δ-deficient apoptotic KG1a cells; conditional induction of ΔPKC-δ in U937T cells directly down-regulated hnRNP K protein. Moreover, the proteasome inhibitor also inhibited the down-regulation of hnRNP K protein by apoptosis inducer and the conditional expression of ΔPKC-δ. More intriguingly, the suppression of hnRNP K with siRNA transfection significantly induced apoptosis. To our knowledge, this is the first demonstration that proteolytically activated PKC-δ down-regulates hnRNP K protein in a proteasome-dependent manner, which plays an important role in apoptosis induction.