Gorla-Bajszczak A, Juge-Aubry C, Pernin A, Burger A G, Meier C A
Division d'Endocrinologie et Diabétologie, Hôpital Cantonal Universitaire de Genève, Geneva, Switzerland.
Mol Cell Endocrinol. 1999 Jan 25;147(1-2):37-47. doi: 10.1016/s0303-7207(98)00217-2.
The peroxisome proliferator-activated receptors (PPARs) are members of the nuclear hormone receptor superfamily. These ligand-activated transcription factors are implicated in the regulation of lipid metabolism and adipocyte differentiation and in the regulation of anti-inflammatory processes. In order to bind to DNA and activate transcription PPAR requires the formation of heterodimers with the retinoid X receptor (RXR). We have previously reported that replacement of a single leucine by an arginine at position 433 of hPPAR alpha (L433R), located in a highly conserved region of the ninth heptad repeat of a leucine-zipper-like motif in the ligand binding domain, abolished heterodimerization of PPAR with RXR and hence its trans-activating capacity. The aim of our present work was to investigate if other conserved amino acids of the ligand binding domain are important for heterodimerization of PPAR with RXR. We found that conserved leucines, L370 and L391, in a leucine-zipper-like motif of hPPAR alpha, as well as a highly conserved aspartic acid (D304) in the tau(i) domain are necessary for heterodimerization with RXR. In contrast, mutations of non-conserved amino acids within the leucine-zipper-like motif do not affect PPAR:RXR heterodimerization. Surprisingly, we found that some mutants deficient in heterodimerization with RXR (hPPAR alpha-L370R and -L391R) were still functional on specific peroxisome proliferator-activator response elements (PPREs). Both mutants could trans-activate on a PPRE from the P450 cytochrome promoter CYP4A1, whereas only the hPPAR alpha-L391R mutant could trans-activate from the acyl-CoA oxidase PPRE (ACOA) and, when stimulated with the peroxisome proliferator Wy14643, also from the bifunctional enzyme PPRE. We therefore hypothesize either that: (i) these mutants might be able to heterodimerize with a protein other than RXR and the affinity for this novel partner may depend on the nature of the PPRE and to some degree on the choice of the activator, or alternatively; (ii) that additional nuclear proteins might compensate in vivo for the decreased binding of RXR to these mutant PPARs observed in vitro.
过氧化物酶体增殖物激活受体（PPARs）是核激素受体超家族的成员。这些配体激活的转录因子参与脂质代谢和脂肪细胞分化的调节以及抗炎过程的调节。为了结合DNA并激活转录，PPAR需要与视黄酸X受体（RXR）形成异二聚体。我们之前报道过，在人PPARα（L433R）的433位将单个亮氨酸替换为精氨酸，该位置位于配体结合域中类似亮氨酸拉链基序的第九个七肽重复序列的高度保守区域，消除了PPAR与RXR的异二聚化，从而消除了其反式激活能力。我们目前工作的目的是研究配体结合域的其他保守氨基酸对于PPAR与RXR异二聚化是否重要。我们发现，人PPARα类似亮氨酸拉链基序中的保守亮氨酸L370和L391，以及τ（i）结构域中的高度保守天冬氨酸（D304）对于与RXR的异二聚化是必需的。相反，类似亮氨酸拉链基序内非保守氨基酸的突变不影响PPAR：RXR异二聚化。令人惊讶的是，我们发现一些与RXR异二聚化缺陷的突变体（人PPARα - L370R和 - L391R）在特定的过氧化物酶体增殖物激活剂反应元件（PPREs）上仍然具有功能。这两个突变体都可以在细胞色素P450启动子CYP4A1的PPRE上进行反式激活，而只有人PPARα - L391R突变体可以从酰基辅酶A氧化酶PPRE（ACOA）进行反式激活，并且当用过氧化物酶体增殖剂Wy14643刺激时，也可以从双功能酶PPRE进行反式激活。因此我们推测：（i）这些突变体可能能够与RXR以外的蛋白质形成异二聚体，并且对这种新伙伴的亲和力可能取决于PPRE的性质，并在一定程度上取决于激活剂的选择，或者；（ii）其他核蛋白可能在体内补偿体外观察到的RXR与这些突变PPAR结合减少的情况。
