Overexpression of PKCalpha is required to impart estradiol inhibition and tamoxifen-resistance in a T47D human breast cancer tumor model.

We previously reported that stable overexpression of protein kinase C alpha (PKCalpha) in hormone responsive T47D:A18 breast cancer cells produces a hormone-independent/tamoxifen (TAM)-resistant and 17beta-estradiol (E2)-inhibitory phenotype in vivo. Furthermore, overexpression of PKCalpha in T47D:A18 cells also results in cross-upregulation of PKCs beta and delta. In this study, we further characterized the contribution of PKC isozymes alpha, beta and delta to this complex phenotype. To determine whether downregulation of PKCalpha is sufficient to restore the hormone-dependent phenotype in T47D:A18/PKCalpha cells, PKCalpha was selectively knocked down using short hairpin RNA (shRNA). To determine the contribution of PKCbeta or delta to the hormone-independent/TAM-resistant and E2-inhibitory phenotype, stable T47D:A18/PKCbeta and T47D:A18/PKCdelta clones were established. Downregulation of PKCalpha by shRNA in T47D:A18/PKCalpha20 cells also resulted in reduced PKCbeta protein expression in vitro. Tumors established from a T47D:A18/PKCalpha/shRNA stable clone exhibit 50% reduction of PKCalpha protein without concomitant reduction in PKCbeta, and exhibit partial reversal of the TAM-resistant and E2-inhibitory phenotype in vivo. Furthermore, stable overexpression of neither PKCbeta nor PKCdelta in T47D:A18 cells are sufficient to produce hormone-independent growth in vitro or in vivo, nor TAM-resistant and E2-inhibited growth in vivo. Taken together, these results suggest that PKCalpha is required to impart the TAM-resistant and E2-inhibitory phenotype in vivo.