Isakov N
Department of Microbiology and Immunology, Ben Gurion University of the Negev, Beer Sheva, Israel.
Cell Immunol. 1988 Sep;115(2):288-98. doi: 10.1016/0008-8749(88)90182-7.
Protein kinase C (PKC) is a ubiquitous enzyme linked to transmembrane signal transduction. It regulates agonist-mediated activation of intracellular events that result in growth and differentiation in a variety of cells and tissues. PKC is the cellular receptor for phorbol ester tumor promoters, such as 12-O-tetradecanoylphorbol-13-acetate (TPA), that bind to, and directly activate, this enzyme. Vitamin A analogs (retinoids) have been known to antagonize biologic effects of phorbol esters, e.g., promotion of skin tumor formation; however, the extract mechanism(s) of this action is not clear. To analyze the effects of retinoids on T-cell-derived PKC, we partially purified the enzyme from human leukemic T cells (Jurkat) and examined the effects of different vitamin A analogs on its activity. Furthermore, the regulatory effects of retinoids on PKC activity were compared with those of common membrane phospholipids. Retinal inhibited PKC activation induced by TPA, as well as by diacylglycerol, the physiologic activator of PKC. The observed inhibition resulted from competition with phospholipid (phosphatidylserine) and was selective for the phospholipid-dependent C kinase; cAMP-dependent protein kinase, which is phospholipid-independent, was not affected by retinal. The inhibitory effect of retinal on PKC activity was similar to that of phosphatidylcholine. Retinoic acid, in contrast to retinal, induced a Ca2+-dependent activation of PKC, thus substituting for phosphatidylserine. Furthermore, PKC activation by retinoic acid was similar to that by phosphatidylserine, the natural phospholipid cofactor, in that both could be inhibited by phosphatidylcholine and augmented by phosphatidylinositol. The inhibition or activation of PKC by retinal or retinoic acid, respectively, was independent of whether the terminal aldehyde (retinal) or carboxyl (retinoic acid) groups were in the trans or cis configuration. Other vitamin A analogs tested did not affect PKC activity. The results demonstrate that different retinoids and phospholipids may have positive or negative cooperativity in PKC activation, thereby regulating its enzymatic activity and affecting the resulting intracellular activation events. These findings suggest that at least part of the biologic effects of retinoids in general, and their modulation of T-cell function in particular, may be mediated via the influence of their intracellular metabolites on PKC, and that this mechanism may account for some of the antagonistic effects of retinoids on TPA-mediated responses in cells.
蛋白激酶C（PKC）是一种与跨膜信号转导相关的普遍存在的酶。它调节激动剂介导的细胞内事件的激活，这些事件导致多种细胞和组织的生长和分化。PKC是佛波酯肿瘤启动子的细胞受体，如12-O-十四烷酰佛波醇-13-乙酸酯（TPA），它能结合并直接激活这种酶。维生素A类似物（类视黄醇）已知可拮抗佛波酯的生物学效应，例如促进皮肤肿瘤形成；然而，这种作用的具体机制尚不清楚。为了分析类视黄醇对T细胞来源的PKC的影响，我们从人白血病T细胞（Jurkat）中部分纯化了该酶，并研究了不同维生素A类似物对其活性的影响。此外，还将类视黄醇对PKC活性的调节作用与常见膜磷脂的调节作用进行了比较。视黄醛抑制了由TPA以及二酰基甘油（PKC的生理激活剂）诱导的PKC激活。观察到的抑制作用是由于与磷脂（磷脂酰丝氨酸）竞争所致，并且对磷脂依赖性C激酶具有选择性；不依赖磷脂的cAMP依赖性蛋白激酶不受视黄醛影响。视黄醛对PKC活性的抑制作用与磷脂酰胆碱相似。与视黄醛相反，视黄酸诱导了PKC的Ca2+依赖性激活，从而替代了磷脂酰丝氨酸。此外，视黄酸对PKC的激活作用与天然磷脂辅因子磷脂酰丝氨酸相似，因为两者都可被磷脂酰胆碱抑制，并被磷脂酰肌醇增强。视黄醛或视黄酸对PKC的抑制或激活作用，分别与末端醛基（视黄醛）或羧基（视黄酸）是反式还是顺式构型无关。测试的其他维生素A类似物不影响PKC活性。结果表明，不同的类视黄醇和磷脂在PKC激活中可能具有正协同或负协同作用，从而调节其酶活性并影响由此产生的细胞内激活事件。这些发现表明，一般而言，类视黄醇的生物学效应，尤其是它们对T细胞功能的调节作用，至少部分可能是通过其细胞内代谢产物对PKC的影响介导的，并且这种机制可能解释了类视黄醇对细胞中TPA介导的反应的一些拮抗作用。
