CD40L figures prominently in atherogenesis. Recent data demonstrate elevated levels of sCD40L in the serum of patients with the metabolic syndrome (MS). This study investigated the role of CD40L in pro-inflammatory gene expression and cellular differentiation in adipose tissue to obtain insight into mechanisms linking the MS with atherosclerosis. Human adipocytes and preadipocytes expressed CD40 but not CD40L. Stimulation with recombinant CD40L or membranes over-expressing CD40L induced a time- and dose-dependent expression of IL-6, MCP-1, IL-8, and PAI-1. Supernatants of CD40L-stimulated adipose cells activated endothelial cells, suggesting a systemic functional relevance of our findings. Neutralising antibodies against CD40L attenuated these effects substantially. Signalling studies revealed the involvement of mitogen-activated protein kinases and NFkB. Furthermore, stimulation with CD40L resulted in enhanced activation of C/EBPa and PPARg and promoted adipogenesis of preadipose cells in the presence and absence of standard adipogenic conditions. Finally, patients suffering from the metabolic syndrome with high levels of sCD40L also displayed high levels of IL-6, in line with the concept that CD40L may induce the expression of inflammatory cytokines in vivo in this population. Our data reveal potent metabolic functions of CD40L aside from its known pivotal pro-inflammatory role within plaques. Our data suggest that CD40L may mediate risk at the interface of metabolic and atherothrombotic disease.

Background: CD40L figures prominently as marker and mediator of atherosclerosis and its clinical complications. Recent data also demonstrate an association between CD40L and the metabolic syndrome. In the light of these data we hypothesized a functional pro-inflammatory role of CD40L in adipose tissue that provokes systemic pro-inflammatory responses and potentially mediates some of the high cardiovascular risk associated with the metabolic syndrome. Methods and Results: Adipocytes and preadipocytes isolated from adipose tissue obtained from bariatic surgery did not express CD40L but its receptor CD40 as assessed by PCR and immunohistochemistry. Furthermore, sections of human adipose tissue from obese donors contained more macrophages and T cells colocalizing with CD40L than those from lean controls. Stimulation of both adipocytes and preadipocytes with recombinant CD40L resulted in a concentration- and time-dependent release of the pro-inflammatory cytokine IL-6, the chemokines MCP -1 and IL-8 as well as the inhibitor of fibrinolysis PAI-1 into the supernatant as quantified by ELISA. Membranes isolated from a Murine cell line overexpressing human CD40L reproduced these pro-inflammatory effects compared to respective controls. Interestingly, fenofibrate but neither rosiglitazone, metformin, nor atorvastatin attenuated the CD40L-inducible expression of these pro-atherogenic mediators. Finally, supernatants from adipocytes and preadipocytes stimulated with CD40L activated endothelial cells and macrophages, typical cell types resident in atherosclerotic lesions, as assessed by FACS for tissue factor, Mac-1, and ICAM-1, respectively. Conclusions: Our data suggest that CD40L induces inflammatory cytokine production in adipose tissue resulting in activation of endothelial cells and macrophages. This new mechanism may contribute to the high cardiovascular risk of patients suffering from the metabolic syndrome.