Bcr-Abl Regulates Protein Kinase Cι (PKCι) Transcription via an Elk1 Site in the PKCι Promoter

The protein kinase C (PKC) family of serine/threonine kinases plays an important role in numerous cancer signaling pathways, including those downstream of the bcr-abl oncogene. We demonstrated previously that atypical PKCι is required for Bcr-Abl-mediated resistance of human K562 chronic myelogenous leukemia (CML) cells to Taxol-induced apoptosis. Here, we report that the pattern of PKC isozyme expression characteristic of CML cells is regulated by Bcr-Abl. When Bcr-Abl was expressed in Bcr-Abl-negative HL-60 promyelocytic leukemia cells, expression of the PKCβI, PKCβII, and PKCι genes was induced, whereas expression of the PKCι gene was reduced to levels similar to those found in CML cells. Given the importance of PKCι in Bcr-Abl-mediated transformation, we characterized the mechanism by which Bcr-Abl regulates PKCι expression. A 1200-bp PKCι promoter construct isolated from genomic DNA was highly active in Bcr-Abl-positive K562 cells and was activated when Bcr-Abl-negative cells were transfected with Bcr-Abl. Bcr-Abl-mediated induction of the PKCι promoter was dependent upon MEK1/2 activity, but not phosphatidylinositol 3-kinase or p38 MAPK activity. Mutational analysis of the PKCι promoter revealed a region between 97 and 114 bp upstream of the transcriptional start site that is responsible for Bcr-Abl-mediated regulation. Mutation of a consensus Elkl-binding site within this region abolished Bcr-Abl-mediated regulation. We conclude that Bcr-Abl regulates PKCι expression through the MEK-dependent activation of an Elk1 element within the proximal PKCι promoter. Our results indicate that Bcr-Abl-mediated transformation involves transcriptional activation of the PKCι gene, which in turn is required for Bcr-Abl-mediated chemoresistance.