Importance of tissue transglutaminase on the activation of secretory phospholipase A2 in the oxidized LDL-treated vascular smooth muscle cells

Tissue transglutaminase (TG2) is a Ca2+ activated, sulfhydrylrich protein which crosslinks proteins via e-(c glutamyl) lysine bridges and acts as a matrix stabilizer by catalyzing inter- and intra-molecular crosslinks between the extracellular matrix (ECM) proteins. Moreover, TG2 serves as a cell adhesion protein and regulates the cell adhesion mechanism via binding to Syndecan-4 (SDC-4) heparan sulfate proteoglycans. TG2 is found to be highly deposited in fibro-lipid plaques and takes part in the stabilization of the atheromatous plaque. Secretory Phospholipase A2 (sPLA2) is an upstream regulator of inflammatory responses and modifies the LDL (Low density protein) resulting in the retention and oxidation of LDL which is one of the hallmark of atherosclerosis. Evidences show that TG2 is abundantly expressed by vascular smooth muscle cells (VSMCs) and its activity was found to increase concomitantly with sPLA2 in the atherosclerosis lesions. Given that sPLA2 comprises a binding affinity to heparan sulfate chains (HSCs) on the cell surface and ECM, we hypothesized that sPLA2 by binding to HSCs of Syndecan-4 can be activated by TG2 on the cell surface. The aim of the study is to investigate the role of TG2 as an activator of sPLA2 in the oxidized LDL (ox-LDL) induced VSMCs and the role of SDC-4 in this process. Our results showed that TG2 expression and deposition to ECM was increased in the presence of 50, 100, 150, 200 lg/ml of mmox-LDL, which was found to be in parallel with an increase in the sPLA2 activity. Treatment of 200 lg/ml mmox-LDL induced HAVSMCs with TG2 inhibitor resulted in a 50% decrease in sPLA2 activity suggesting that sPLA2 activation was dependent on transamidation activity of TG2. On the other hand silencing of SDC-4 did not lead to any change in sPLA2 activity in mmox-LDL induced HAVSMCs. Taken together, our data suggests that treatment of HAVSMCs with the mmox-LDL not only leads to an increase in TG2 expression and deposition to ECM but also induce the activation of sPLA2. Besides that, the presence of SDC-4 on the cell surface does not have an effect on the sPLA2 induction.