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 are 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 µm) of α‐, γ‐, or δ‐tocopherol or α‐, γ‐, or δ‐tocotrienol. Treatment with growth inhibitory doses of δ‐tocopherol (100 µm), α‐tocotrienol (50 µm), or γ‐ or δ‐tocotrienol (10 µm) did not affect EGF‐receptor levels, EGF‐induced EGF‐receptor tyrosine kinase activity, or total intracellular levels of PKCα. However, these treatments were found to inhibit EGF‐induced PKCα activation as determined by its translocation from the cytosolic to membrane fraction. Treatment with 250 µmα‐ or γ‐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 PKCα activation.