RETINOIC ACID INDUCES TRANSLOCATION OF PROTEIN-KINASE-C (PKC) AND ACTIVATION OF NUCLEAR PKC (NPKC) IN RAT SPLENOCYTES

Retinoic acid (RA), a vitamin A metabolite, has marked effects on grow th of normal and malignant cells; however, the exact mechanism of acti on remains unclear. The effect of two RA analogs, 13-cis-RA and all-tr ans-RA, on transmembrane signalling processes was investigated in rat splenocytes. Treatment of rat splenic cells with these retinoic acid a nalogs resulted in translocation of protein kinase C (PKC) from the cy tosol to the membrane. Previous studies have described nuclear RA rece ptors (RARs and RXRs) for several species and the biologic activity of RA has been shown to be mediated by specific interaction with these n uclear receptors. Thus, activation of nuclear pool(s) of protein kinas e C (nPKC) by RA analogs was also studied. Rat splenocyte nuclei pure by enzymatic and electron microscope criteria demonstrated a biphasic pattern of bell-shaped curves for both cis- and trans-RA with maximum statistically significant peak of phosphate incorporation into endogen ous substrates at 10(-16) M cis-RA and 10(-16)-10(-17) M trans-RA. A m onoclonal antibody to PKC and the PKC inhibitors, H-7, sphingosine, an d staurosporine, blocked the RA-stimulated nuclear phosphorylation. Th e ability of RA to activate cell membrane PKC resulting in an increase in particulate PKC activity correlates well with the activation of nP KC since the particulate fraction would include nuclear enzyme systems . This ability of RA to activate nPKC and possibly affect the growth s tatus of a cell may provide a missing link to our understanding of the cellular sites of action for this vitamin.