- Protein kinase C (PKC) is involved in several major signal transduction pathways that control gene expression cell growth and differentiation. The PKCη isoform appears as a candidate regulator of mammary gland proliferation or differentiation, as its expression is up-regulated in the mammary gland in the transit from resting to the pregnant state. The purpose of this study was to examine the hormonal regulation of PKCη. Here we show that estradiol specifically up-regulates the expression of PKCη in the estrogen-responsive lines MCF-7 and T47D but not in the estrogen non-responsive line MDA-MB 231. Interestingly, the presence of progesterone, involved in the differentiation of the mammary gland, reduced the estrogen-induced PKCη expression in a time-dependent manner. Thus, our studies suggest that PKCη has an important role in signalling pathways regulating mammary gland proliferation and its development.