Direct Interaction of All-trans-retinoic Acid with Protein Kinase C (PKC)

Abstract

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 [ 3 H]atRA into PKC␣, 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␣, a slight competition with phorbol-12-myristate-13-acetate, and none with diacylglycerol, fatty acids, or Ca 2؉ . At pharmacological concentrations (10 M), atRA decreased PKC␣ activity through the competition with PS but not phorbol-12-myristate-13-acetate, diacylglycerol, or Ca 2؉ . These results let us hypothesize that in vivo, pharmacological concentrations of atRA may hamper binding of PS to PKC␣ and prevent PKC␣ 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.

Anti-apoptotic effect of retinoic acid on retinal progenitor cells mediated by a protein kinase A-dependent mechanism

“…In this paper, we propose a mechanism for the anti-apoptotic activity of RA in RPCs, which comprises activation of PKA and PKC. Interestingly, structural data support the idea that several PKC isoforms, normally activated by increased concentrations of diacylglycerol and calcium ions, could also be directly activated by RA, as they contain potential binding sites for two RA molecules [28]. An interesting model for PKC and PKA double activation (although not RA-dependent) was previously suggested for B lymphocytes: high intracellular calcium concentrations via calmodulin lead to increased cAMP concentration, npg which modulates PKA activation.…”

Protein Kinase Cε Actin-binding Site Is Important for Neurite Outgrowth during Neuronal Differentiation

“…Stimulation of growth factor receptors frequently leads to activation of phospholipase C␥ with subsequent formation of diacylglycerol, which can induce changes in both conformation and localization of PKC. There are recent reports demonstrating that retinoids can directly bind to the PKC molecule (Hoyos et al, 2000;Radominska-Pandya et al, 2000). Alternative pathways leading to F-actin-mediated effects of PKC⑀ could include activation of the small GTPases of the Rho family that have been shown to modulate the actin cytoskeleton and to be involved in the regulation of neurite outgrowth (reviewed in Hall, 1998).…”

Regulation of Retinoic Acid Receptor α by Protein Kinase C in B16 Mouse Melanoma Cells

“…PKC has been shown to phosphorylate RAR in vitro (28,29). On the other hand, one group has reported that RA binds to PKC in a specific manner and inactivates its enzymatic activity (30). We have previously reported that treatment of B16 melanoma cells with RA increases the amount of PKC␣ mRNA and protein (31).…”