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 p roliferation, differentiation, tumorigenesis, and apoptosis. All-trans-reti noic acid (atRA) modulates PKC activity, but the mechanism of this regulati on is unknown. Amino acid alignments and crystal structure analysis of reti noic acid (RA)-binding proteins revealed a putative atRA-binding motif in P KC, suggesting existence of an atRA binding site on the PKC molecule. This was supported by photolabeling studies showing concentration- and UV-depend ent photoincorporation of [H-3]atRA into PKC alpha, which was effectively p rotected by 4-OH-atRA, 9-cis-RA, and atRA glucuronide, but not by retinol, Photoaffinity labeling demonstrated strong competition between atRA and pho sphatidylserine (PS) for binding to PKC alpha, a slight competition with ph orbol-12-myristate-13-acetate, and none with diacylglycerol, fatty acids, o r Ca2+. At pharmacological concentrations (10 mu M), atRA decreased PKC alp ha 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 P KC alpha and prevent PKC alpha activation. Thus, this study provides the fi rst evidence for direct binding of atRA to PKC isozymes and suggests the ex istence of a general mechanism for regulation of PKC activity during exposu re to retinoids, as in retinoid-based cancer therapy.