Direct interaction of all-trans-retinoic acid with protein kinase C (PKC) -: Implications for PKC signaling and cancer therapy

Protein kinase C (PKC) regulates fundamental cellular functions including proliferation, differentiation, tumorigenesis, and apoptosis. All-trans-retinoic acid (atRA) modulates PKC activity, but the mechanism of this regulation is unknown. Amino acid alignments and crystal structure analysis of retinoic acid (RA)-binding proteins revealed a putative atRA-binding motif in PKC, suggesting existence of an atRA binding site on the PKC molecule. This was supported by photolabeling studies showing concentration- and UV-dependent photoincorporation of [H-3]atRA into PKC alpha, which was effectively protected by 4-OH-atRA, 9-cis-RA, and atRA glucuronide, but not by retinol, Photoaffinity labeling demonstrated strong competition between atRA and phosphatidylserine (PS) for binding to PKC alpha, a slight competition with phorbol-12-myristate-13-acetate, and none with diacylglycerol, fatty acids, or Ca2+. At pharmacological concentrations (10 mu M), atRA decreased PKC alpha activity through the competition with PS but not phorbol-12-myristate-13-acetate, diacylglycerol, or Ca2+. These results let us hypothesize that in vivo, pharmacological concentrations of atRA may hamper binding of PS to PKC alpha and prevent PKC alpha activation. Thus, this study provides the first evidence for direct binding of atRA to PKC isozymes and suggests the existence of a general mechanism for regulation of PKC activity during exposure to retinoids, as in retinoid-based cancer therapy.