The involvement of Cathepsin B and L in inflammation and cholesterol trafficking protein NPC2 secretion in macrophages

Obesity and its related chronic inflammation are the major risk factors for developing metabolic disturbances. The roles of cathepsin cysteine proteases have been tied to inflammation and atherosclerosis. Cathepsins are important functional links between inflammation, cholesterol metabolism, and atherosclerosis in obesity. NPC2, a lysosomal protein, plays an important role in cholesterol trafficking. The objective of this study was to examine the regulation of cathepsins and NPC2 in adipose tissue and macrophages in obesity, and the effect of modifying cathepsin activity in cholesterol metabolism and trafficking in macrophages. We found that high fat diet (HFD) feeding altered the mRNA and protein expression levels of cathepsin B and L and NPC2 in adipose tissue in mice; the differential regulation of these proteins was observed between adipose depots. In vitro studies showed that TNF-α reduces intracellular protein levels of CtB, CtL, and NPC2, but increases their secretion in 3T3-L1 adipocytes. Likewise, LPS stimulated the secretion of CtB and NPC2 in Raw264.7 macrophages. Using the inhibitors of cathepsin enzymatic activity, we found that cathepsin B and L regulate TNFα production, the expression and secretion of NPC2 protein, and the mRNA levels of the genes involved in cholesterol trafficking in macrophages. Our findings suggest that cathepsin B and L have a significant involvement in mediating the inflammatory response, in cholesterol trafficking, and in regulating NPC2 secretion.