Grinnell B W, Berg D T
Department of Cardiovascular Research, Lilly Research Laboratories, Indianapolis, Indiana 46285, USA.
Am J Physiol. 1996 Feb;270(2 Pt 2):H603-9. doi: 10.1152/ajpheart.1996.270.2.H603.
Vascular smooth muscle cells produce the proteolytically activated thrombin receptor. Under certain conditions, they have been reported to synthesize thrombomodulin (TM), another thrombin receptor known to convert the specificity of thrombin from cleavage of procoagulant/proinflammatory substrates to the cleavage of the anticoagulant/anti-inflammatory factor protein C. In this study, we examined the role of TM in modulating thrombin-mediated cellular responses. Using a thrombin receptor-positive TM-negative rabbit intimal smooth muscle cell line (RIC), we isolated cells expressing varying levels of functional surface TM after transfection with an expression vector containing the cDNA for full-length TM. The parent RIC (TM negative) line responded to alpha-thrombin and to agonist peptide (SFLLRN-PNDKYEPF; abbreviated SFLL) with both mitogenic response and phosphoinositol release. However, transfected cells producing high levels of TM, equivalent to the level on rabbit aortic endothelial cells, responded to SFLL but not to alpha-thrombin. Whereas alpha-thrombin, SFLL, and the combination of SFLL and thrombin resulted in a mitogenic response in the TM-negative RIC line, the response to the agonist peptide could be blocked by thrombin in the TM-producing cell line. The degree to which thrombin receptor activation was blocked directly correlated with the level of TM on the cell surface, and high levels of thrombin could overcome the inhibitory effect. Our data demonstrate that the coexpression of TM with thrombin receptor on vascular smooth muscle cells can result in a modulation of cellular responses to thrombin, which could control thrombin-induced proliferative events following vessel injury or insult.
血管平滑肌细胞可产生经蛋白水解激活的凝血酶受体。在某些条件下，据报道它们能合成血栓调节蛋白（TM），这是另一种凝血酶受体，已知其可将凝血酶的特异性从切割促凝血/促炎底物转变为切割抗凝/抗炎因子蛋白C。在本研究中，我们检测了TM在调节凝血酶介导的细胞反应中的作用。利用凝血酶受体阳性但TM阴性的兔内膜平滑肌细胞系（RIC），我们在转染了含全长TM cDNA的表达载体后，分离出了表达不同水平功能性表面TM的细胞。亲代RIC（TM阴性）细胞系对α-凝血酶和激动剂肽（SFLLRN-PNDKYEPF；缩写为SFLL）有促有丝分裂反应和磷酸肌醇释放反应。然而，产生高水平TM（相当于兔主动脉内皮细胞上的水平）的转染细胞对SFLL有反应，但对α-凝血酶无反应。虽然α-凝血酶、SFLL以及SFLL与凝血酶的组合在TM阴性的RIC细胞系中会引起促有丝分裂反应，但在产生TM的细胞系中，激动剂肽的反应可被凝血酶阻断。凝血酶受体激活被阻断的程度与细胞表面TM的水平直接相关，且高浓度的凝血酶可克服这种抑制作用。我们的数据表明，TM与凝血酶受体在血管平滑肌细胞上的共表达可导致细胞对凝血酶反应的调节，这可能控制血管损伤或受刺激后凝血酶诱导的增殖事件。
