A Role for Bile Salt-Dependent Lipase in Platelet Activation and in Thrombus Formation in Vivo.

Bile Salt Dependent Lipase (BSDL) is an enzyme secreted by pancreatic acinar cells. BSDL, in the presence of primary bile salts, participates in the hydrolysis of dietary lipid esters in the duodenum lumen. This 105 kDa N and O-glycosylated protein has been detected in the plasma of normal subjects. Recent in vitro and in vivo studies demonstrated that pancreatic BSDL reaches the blood via transcytosis through enterocytes. Other studies showed that pancreatic human BSDL is captured by human umbilical vein endothelial cells and induces the proliferation of smooth muscle cells in vitro at BSDL concentrations found in blood, suggesting that this enzyme may play a role in hemostasis and thrombosis. However the specific role of circulating BSDL is unknown. The goal of this study was to determine the possible involvement of circulating BSDL in thrombus formation. We investigated the participation of circulating mouse BSDL in thrombus formation using widefield intravital microscopy in the cremaster muscle of living mice. Thrombi were formed following laser injury of the vessel wall of an arteriole in the cremaster muscle. Pancreatic mouse BSDL, a 74 kDa glycoprotein, was detected using several antibodies directed against either the whole human BSDL (pAbL64, pAbL32) or a peptide based on a sequence in the N-terminal domain of BSDL (Ser326-Thr350; pAbAntipeptide). Mouse and human BSDL share about 80% sequence homology, the main difference localized in the C-terminal domain, which is truncated to the mouse BSDL compared with the human enzyme. All the antibodies are able to specifically recognize the mouse pancreatic BSDL. Using antibodies pAbL64, pAbL32, or pAbAntipeptide we observed specific accumulation of circulating mouse BSDL into the growing thrombus. The circulating BSDL co-localized with platelets present in the thrombus. These results suggest that circulating BSDL is involved in thrombus formation in vivo. In order to determine if BSDL plays a role in platelet activation and aggregation, we performed in vitro studies on human washed platelets. BSDL increased both the amount of phosphatidylserine exposure on the surface of platelets and the activation of αIIbβ3 induced by thrombin. These results indicate that this enzyme can amplify the activation of platelets in vitro. While BSDL alone cannot induce the aggregation of platelets, this enzyme significantly increases the amount of platelet aggregation induced by SFLLRN peptide or thrombin. Altogether, these data suggeste that circulating BSDL participates in the thrombus formation after laser injury of the arterial wall and can amplify both the activation of platelets and the phosphatidylserine exposure, increasing the thrombotic response after vessel injury. This mechanism may be operative in the development of venous thromboembolic disease in pancreatic cancer.