Protein kinase C modulates estrogen receptors in differentiated osteoblastic cells in vitro

Several reports have shown an interaction between the estrogen receptor (ER) and the protein kinase C (PKC) intracellular pathways. Data from our laboratory showed that PKC activation can modulate ER levels and responsiveness in estrogen target tissues such as uterus and bone. In particular, ROS.SMER #14 osteoblastic cells, stably transfected with the mouse ER, undergo specific morphological changes in vitro. ROS.SMER #14 cells at post-confluence express a differentiated phenotype and become unresponsive to estrogenic stimulation. Interestingly, ER mRNA and protein levels were not modified by post-confluence, but ER binding sites/cell (2500-3000/cell at subconfluence) were undetectable. Moreover, PKC activity was significantly increased in post-confluent cells. Inhibition of PKC by H7 or staurosporin (PKC inhibitors) or down-regulation by long-term treatment with 12-O-tetradecanoylphorbol-13-acetate enchanced ER binding capacity in a dose-dependent manner. Since the PKC family includes several different isoforms that play different roles in cell homeostasis, we evaluated whether specific isoenzymes were involved in this event. To address this question, Western blotting analysis was performed on both sub- and post-confluent ROS.SMER #14 cells using antibodies against different PKC isoforms. In conclusion, our preliminary data indicate that estrogen responsiveness of osteoblastic cells can be highly regulated by PKC. Finally, these data suggest that this intracellular interaction might play an important role in modulating hormonal and pharmacological responsiveness of bone tissue.