We have previously shown that PKC inhibitors interfere with the Ets1/Smad3-dependent regulation of parathyroid hormone-related protein (PTHrP) P3 promoter activity by TGFβ in invasive MDA-MB-231 breast cancer cells. By examining PKC expression in a variety of breast cancer cell lines, the protein level of PKCα was found to be much higher in Ets1-expressing MDA-MB-231 and MDA-MB-435 breast cancer cells than in Ets1-deficient MCF-7 and SK-BR3 cells. No correlation of Ets1 expression with the expression of other PKC subtypes (PKCβ1, PKCβ2, PKCδ or PKCε) could be observed. In contrast to MDA-MB-231 cells, PKCα-deficient MCF-7 cells do not support Ets1-induced activation of the PTHrP P3 promoter suggesting that PKCα may be important for Ets1 activity. A constitutively active form of PKCα was found to potentiate the P3 promoter activation by Ets1 alone and in synergy with Smad3. PKCα, but not PKCε, also induced phosphorylation of the Ets1 protein. Both PKCα effects on Ets1 depended on the exon VII domain of Ets1. Using verapamil and ionomycin, we could show that PKCα induces Ets1 phosphorylation independent of calcium mobilization. Collectively, our data suggest that PKCα may regulate Ets1 activity in invasive breast cancer cells.

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