Prolactin-induced protein kinase C activity in a mouse mammary cell line (NOG-8)

The normal mouse mammary epithelial cells, NOG-8, respond to the mitogenic signal of prolactin with a 2.5-fold increase in cell number within 3 days in vitro. When prolactin is added to subconfluent cells for 5-15 min, there is a 5-fold increase in protein kinase C activity. Upon longer exposure (24 h) to the hormone, the enzyme activity returns to that of control. The potent protein kinase C inhibitor, 1-(5-isoquinolinesulfonyl)-2-methylpiperazine dihydrochloride (H7), blocks both the prolactin-induced enzyme activity and subsequent increase in cell number. Prior to prolactin treatment, 90% of the protein kinase C activity resides in the cytosol with only 10% associated with the membranes. After only 5 min of prolactin treatment, 70% of the enzyme activity is now localized to the membranes. These data suggest that prolactin uses the protein kinase C pathway for signal transduction in NOG-8 cells thus leading to enhanced cell growth.