Tamoxifen elicits its anti-estrogen effects in growth plate chondrocytes by inhibiting protein kinase C

17β-Estradiol (E₂) regulates growth plate cartilage cells via classical nuclear receptor mechanisms, as well as by direct effects on the chondrocyte membrane. These direct effects are stereospecific, causing a rapid increase in protein kinase C (PKC) specific activity, are only found in cells from female rats and are mimicked by E₂-bovine serum albumin (BSA), which cannot penetrate the cell membrane. E₂ and E₂-BSA stimulate alkaline phosphatase specific activity and proteoglycan sulfation in female rat costochondral cartilage cell cultures, but traditional nuclear receptors do not appear to be involved. This study examined the effect of the anti-estrogen tamoxifen on these markers of chondrocyte differentiation; the gender-specificity of tamoxifen's effect on PKC, if tamoxifen has an effect on vitamin D metabolite-stimulated PKC, which is mediated via specific membrane receptors (1,25-mVDR; 24,25-mVDR) and whether the effect of tamoxifen is mediated by nuclear estrogen receptors. Tamoxifen dose-dependently inhibited the effect of E₂-BSA on PKC, alkaline phosphatase and proteoglycan sulfation in confluent cultures of female resting zone (RC) cells and growth zone (GC) (prehypertrophic/upper hypertrophic zones) cells, suggesting that its action is at the membrane and not cell maturation-dependent. Neither the estrogen receptor (ER) antagonist ICI 182780 nor the ER agonist diethylstilbesterol affected E₂ or E₂-BSA-stimulated PKC in female chondrocytes. Tamoxifen also inhibited the increase in PKC activity due to 1α,25-(OH)₂D₃ or 24R,25-(OH)₂D₃ in growth plate cells derived from either female or male rats. Inhibition of PKC by tamoxifen may be a general property of membrane receptors involved in rapid responses to hormones.