Absence of phosphorylation of retinoid-binding proteins by protein kinase C in vitro

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 [(3)H]atRA into PKCalpha, 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 PKCalpha, a slight competition with phorbol-12-myristate-13-acetate, and none with diacylglycerol, fatty acids, or Ca(2+). At pharmacological concentrations (10 micrometer), atRA decreased PKCalpha 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 PKCalpha and prevent PKCalpha 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.

Specific binding of [3H]retinoids to cellular retinoid-binding proteins

The specific binding of [3H]retinoids to cellular retinoid-binding proteins was measured directly by the cold acetone filtration method. After incubation of purified cellular retinoid-binding proteins with [3H]retinoids with or without competitors for 2-4 h, bound ligands were separated from free by filtration using cold acetone. Nonspecific binding of the ligands was reduced sufficiently to allow measurement of specific binding of [3H]retinoids to cellular retinoid-binding proteins. This method has the advantages of being rapid and practical and giving reproducible results.

Inhibition of ornithine decarboxylase induction by retinobenzoic acids in relation to their binding affinities to cellular retinoid-binding proteins

Retinobenzoic acids induce differentiation of human promyelocytic leukemia cells (HL-60). Like retinoic acid, 14 retinobenzoic acids inhibited the induction of ornithine decarboxylase (ODC) by teleocidin in mouse skin. The mechanism(s) of inhibition of ODC induction by 7 retinobenzoic acids, Am 80, Am 81, Am 580, Am 590, Am 68, Sa 80, and Ch 55 was compared with those by all-trans-retinoic acid and the arotinoid compound 19. Application of 114 nmol of Am 80, Am 81, Am 580, Am 590, Am 68, Sa 80, or Ch 55, 10 min before 11.4 nmol of teleocidin, resulted in 76.7%, 82.0%, 76.2%, 28.3%, 48.4%, 58.6%, and 85.1% inhibition of ODC induction, respectively. Since all-trans-retinoic acid and compound 19 were also inhibitory, we determined whether retinobenzoic acids bind to cellular retinoic acid-binding protein (CRABP) isolated from bovine adrenal glands. Am 80 and Am 580 inhibited the specific binding of 3H-retinoic acid to CRABP, but also showed less affinity than authentic unlabeled retinoic acid and compound 19. Am 81, Am 590, Am 68, Sa 80, and Ch 55 at up to 10 microM were not effective competitors of the binding of either 3H-retinoic acid or 3H-retinol. These results suggest that the inhibition of ODC induction can be mediated by pathways that do not involve CRABP or the cellular retinol-binding protein.