Ishaq Mohammad, Fan Ming, Wigmore Kip, Gaddam Arunasri, Natarajan Ven
Laboratory of Molecular Cell Biology, Science Applications International Corporation-Frederick, Frederick Cancer Research and Development Center, National Cancer Institute, National Institutes of Health, Frederick, MD 21702, USA.
J Immunol. 2002 Jul 15;169(2):732-8. doi: 10.4049/jimmunol.169.2.732.
T lymphocyte activation signals regulate the expression and transactivation function of retinoid X receptor (RXR) alpha through an interplay of complex signaling cascades that are not yet fully understood. We show that cellular Ser/Thr protein phosphatases (PPs) play an important role in mediating these processes. Inhibitors specific for PP1 and PP2A decreased basal expression of RXR alpha RNA and protein in T lymphocyte leukemia Jurkat cells and prevented activation-induced RXR alpha accumulation in these cells. In addition, these inhibitors attenuated the RXR responsive element (RXRE)-dependent transcriptional activation in transient transfection assays. Inhibitors of calcineurin (CN), by contrast, did not have any effect on the basal RXR alpha expression and even augmented activation-induced RXR alpha expression. Expression of a dominant-active (DA) mutant of CN together with a DA mutant of protein kinase C (PKC)theta;, a novel PKC isoform, significantly increased RXRE-dependent transcription. Expression of catalytically inactive PKC theta; or a dominant-negative mutant of PKC theta; failed to synergize with CN and did not increase RXRE-dependent transcription. Expression of a DA mutant of PKC alpha or treatment with PMA was found to attenuate PKC theta; and CN synergism. We conclude that PP1, PP2A, and CN regulate levels and transcriptional activation function of RXR alpha in T cells. In addition, CN synergizes with PKC theta; to induce RXRE-dependent activation, a cooperative function that is antagonized by the activation of the conventional PKC alpha isoform. Thus, PKC theta; and PKC alpha may function as positive and negative modulators, respectively, of CN-regulated RXRE-dependent transcription during T cell activation.
T淋巴细胞激活信号通过尚未完全了解的复杂信号级联相互作用来调节视黄酸X受体（RXR）α的表达和反式激活功能。我们发现细胞丝氨酸/苏氨酸蛋白磷酸酶（PPs）在介导这些过程中起重要作用。对PP1和PP2A特异的抑制剂降低了T淋巴细胞白血病Jurkat细胞中RXRα RNA和蛋白质的基础表达，并阻止了这些细胞中激活诱导的RXRα积累。此外，这些抑制剂在瞬时转染实验中减弱了RXR反应元件（RXRE）依赖性的转录激活。相比之下，钙调神经磷酸酶（CN）抑制剂对RXRα的基础表达没有任何影响，甚至增强了激活诱导的RXRα表达。CN的显性激活（DA）突变体与蛋白激酶C（PKC）θ的DA突变体（一种新的PKC亚型）共同表达，显著增加了RXRE依赖性转录。催化无活性的PKCθ或PKCθ的显性负性突变体的表达未能与CN协同作用，也没有增加RXRE依赖性转录。发现PKCα的DA突变体表达或用佛波酯（PMA）处理可减弱PKCθ和CN的协同作用。我们得出结论，PP1、PP2A和CN调节T细胞中RXRα的水平和转录激活功能。此外，CN与PKCθ协同作用以诱导RXRE依赖性激活，这种协同功能受到传统PKCα亚型激活的拮抗。因此，在T细胞激活过程中，PKCθ和PKCα可能分别作为CN调节的RXRE依赖性转录的正性和负性调节因子发挥作用。
