Phosphatidylinositide 3-kinase and protein kinase C zeta mediate retinoic acid induction of DARPP-32 in medium size spiny neurons in vitro

Mature striatal medium size spiny neurons express the dopamine and cAMP-regulated phosphoprotein, 32 kDa (DARPP-32), but little is known about the mechanisms regulating its levels, or the specification of fully differentiated neuronal subtypes. Cell extrinsic molecules that increase DARPP-32 mRNA and/or protein levels include retinoic acid (RA), brain-derived neurotrophic factor, and estrogen (E₂). We now demonstrate that RA regulates DARPP-32 mRNA and protein in primary striatal neuronal cultures. Furthermore, DARPP-32 induction by RA in vitro requires phosphatidylinositide 3-kinase, but is independent of tropomyosin-related kinase B, cyclin-dependent kinase 5, and protein kinase B. Using pharmacologic inhibitors of various isoforms of protein kinase C (PKC), we also demonstrate that DARPP-32 induction by RA in vitro is dependent on PKC zeta (PKCζ). Thus, the signal transduction pathways mediated by RA are very different than those mediating DARPP-32 induction by brain-derived neurotrophic factor. These data support the presence of multiple signal transduction pathways mediating expression of DARPP-32 in vitro , including a novel, important pathway via which phosphatidylinositide 3-kinase regulates the contribution of PKCζ.