ELK1

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 previously demonstrated that atypical PKCiota 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 is expressed in Bcr-Abl-negative HL-60 promyelocytic leukemia cells, expression of the PKC betaI, betaII, and iota genes is induced, whereas expression of the PKCdelta gene is reduced to levels similar to those found in CML cells. Given the importance of PKCiota in Bcr-Abl-mediated transformation, we characterized the mechanism by which Bcr-Abl regulates PKCiota expression. A 1200 base pair PKCiota promoter construct isolated from genomic DNA is highly active in Bcr-Abl-positive K562 cells and is activated when Bcr-Abl-negative cells are transfected with Bcr-Abl. Bcr-Abl-mediated induction of the PKCiota promoter is dependent upon MEK1/2 activity, but not PI3 kinase or p38 MAP kinase activity. Mutational analysis of the PKCiota promoter reveals a region between 114 and 97 base pairs upstream of the transcriptional start site that is responsible for Bcr-Abl-mediated regulation. Mutation of a consensus Elk-1 binding site within this region abolishes Bcr-Abl-mediated regulation. We conclude that Bcr-Abl regulates PKCiota expression through the MEK-dependent activation of an Elk1 element within the proximal PKCiota promoter. Our results indicate that Bcr-Abl-mediated transformation involves transcriptional activation of the PKCiota gene, which in turn is required for Bcr-Abl-mediated chemoresistance.