Protein Kinase C and Mammary Cell Differentiation: Involvement of Protein Kinase C a in the Induction of 13-Casein Expression

Abstrad Treatment of HC1 1 mouse mammary epithelial cells with the lactogenic hormones dexamethasone, insulin, and proladin (DIP) leads to cellular differentiation and production of the milk protein 13-casein. The following experimental evidence suggests the involvement of protein kinase C (PKC) in DIP induced signal transdudion. Down-regulation of PKC by 1 2-O-tetradecanoylphorbol-1 3-acetate or addition of CGP 41 251 , a seledive inhibitor of PKC, inhibited 3-casein protein expression induced by DIP in HC1 1 cells. This inhibition occurs at the level of transcription, since the DIP mediated adivation of a -casein promoter-iuciferase reporter construd or of mammary gland specific fador (MGF), an essential transcription fador for -casein promoter adivity, was also inhibited by CGP 41 251 . Inhibition or down-regulation of PKC reduced the adivation of MGF by proladin as well. PKC-a, the only conventional PKC isoform expressed in HC1 1 cells, is most likely involved in the DIP induced fi-casein expression. (a) Only PKC-a and PKC-e are down-regulated by 1 2-O-tetradecanoylphorbol1 3-acetate whereas PKCand PKCare not. (b) Of the PKC isoforms expressed in HC1 1 cells, CGP 41251 inhibits PKC-a more potently than PKC-6, PKC-#{128}, and PKC. The IC50 for the inhibition of f3-casein synthesis, MGF activation, and f3-casein promoter activity by CGP 41 251 correlated well with the IC50 of PKC-a inhibition. (c) Finally, only PKC-a translocated to membrane fradions after DIP or proladin treatment. Taken together, these data indicate that PKC-a plays an important role in the signaling pathway adivated by prolactin during 13-casein indudion.