Milakovic Tamara, Tucholski Janusz, McCoy Eric, Johnson Gail V W
Department of Psychiatry, University of Alabama at Birmingham, Alabama 35294-0017, USA.
J Biol Chem. 2004 Mar 5;279(10):8715-22. doi: 10.1074/jbc.M308479200. Epub 2003 Dec 11.
Tissue transglutaminase (tTG) is a unique member of the transglutaminase family as it is both a transamidating enzyme and a GTPase. In the cell tTG is mostly cytosolic, however it is also found in the nucleus and associated with the plasma membrane. tTG can be proapoptotic, however anti-apoptotic activities of the enzyme have also been reported. To determine how the intracellular localization and transamidating activity of tTG modulates its effects on apoptosis, HEK293 cells were transiently transfected with tTG or [C277S]tTG (which lacks transamidating activity) constructs that were targeted to different intracellular compartments. Apoptosis was induced by thapsigargin treatment, which results in increased intracellular calcium concentrations. Cytosolic tTG was pro-apoptotic, while nuclear localization of [C277S]tTG attenuated apoptosis. Membrane-targeted tTG had neither pro- nor anti-apoptotic functions. This finding indicates for the first time that intracellular localization is an important determinant of the effect of tTG on apoptosis. Previous studies have suggested that tTG may modulate retinoblastoma (Rb) protein, an important suppressor of apoptosis. tTG interacted with Rb and after induction of apoptosis, the interaction of nuclear-targeted [C277S]tTG with Rb was increased significantly concomitant with an attenuation of apoptosis. In contrast, the interaction of nuclear-targeted tTG with Rb was significantly decreased and apoptosis was not attenuated. These data suggest that tTG protects cells against apoptosis in response to stimuli that do not result in increased transamidating activity by translocating to the nucleus, and that complexing with Rb may be an important aspect of the protective effects of tTG.
组织转谷氨酰胺酶（tTG）是转谷氨酰胺酶家族中的独特成员，因为它既是一种转酰胺酶，也是一种GTP酶。在细胞中，tTG主要存在于细胞质中，但也存在于细胞核中并与质膜相关。tTG可以促凋亡，然而也有该酶抗凋亡活性的报道。为了确定tTG的细胞内定位和转酰胺活性如何调节其对凋亡的影响，用靶向不同细胞内区室的tTG或[C277S]tTG（缺乏转酰胺活性）构建体瞬时转染HEK293细胞。通过毒胡萝卜素处理诱导凋亡，这会导致细胞内钙浓度升高。细胞质中的tTG具有促凋亡作用，而[C277S]tTG的核定位减弱了凋亡。靶向膜的tTG既没有促凋亡功能也没有抗凋亡功能。这一发现首次表明细胞内定位是tTG对凋亡影响的重要决定因素。先前的研究表明，tTG可能调节视网膜母细胞瘤（Rb）蛋白，这是一种重要凋亡抑制因子。tTG与Rb相互作用，在诱导凋亡后，靶向核的[C277S]tTG与Rb的相互作用显著增加，同时凋亡减弱。相反，靶向核的tTG与Rb的相互作用显著降低，且凋亡未减弱。这些数据表明，tTG通过转运至细胞核来保护细胞免受不导致转酰胺活性增加的刺激引起的凋亡，并且与Rb结合可能是tTG保护作用的一个重要方面。
