Vitamin E inhibition of normal mammary epithelial cell growth is associated with a reduction in protein kinase Cα activation

Tocopherols and tocotrienols represent the two subclasses within the vitamin E family of compounds. However, tocotrienols arc significantly more potent than tocopherols in suppressing epidermal growth factor (EGF)-dependent normal mammary epithelial cell growth. EGF is a potent mitogen for normal mammary epithelial cells and an initial event in EGF-receptor mitogenic-signalling is protein kinase C (PKC) activation. Studies were conducted to determine if the antiproliferative effects of specific tocopherol and tocotrienol isoforms are associated with a reduction in EGF-receptor mitogenic signalling and/or PKC activation. Normal mammary epithelial cells isolated from midpregnant BALB/c mice were grown in primary culture, and maintained on serum-free media containing 10 ng/mL EGF as a mitogen, and treated with various doses (0-250 muM) of alpha-, gamma-, or delta -tocopherol or alpha-, gamma-, or delta -tocotrienol. Treatment with growth inhibitory doses of delta -tocopherol (100 muM), alpha -tocotrienol (50 muM), or gamma- or delta -tocotrienol (100 muM) did not affect EGF-receptor levels, EGF-induced EGF-receptor tyrosine kinase activity, or total intracellular levels of PKC alpha. However, these treatments were found to inhibit EGF-induced PKC alpha activation as determined by its translocation from the cytosolic to membrane fraction. Treatment with 250 muM alpha- or gamma -tocopherol had no affect on EGF-receptor mitogenic signalling or cell growth. These findings demonstrate that the inhibitory effects of specific tocopherol and tocotrienol isoforms on EGF-dependent normal mammary epithelial cell mitogenesis occurs downstream from the EGF-receptor and appears to be mediated, at least in part, by a reduction in PKC alpha activation.