Significance of circulatory DPP4 activity in metabolic diseases.

Dipeptidyl peptidase 4 (DPP4), also known as CD26 is a type II transmembrane protein that is released from the cell membrane in a nonclassical secretory mechanism. This exopeptidase selectively degrades varieties of substrates including incretin hormones, growth factors, and cytokines. A significant detectable amount of DPP4 activity can be measured in plasma as well as in different tissues such as intestinal epithelium, vascular endothelium, lymphocytes, monocytes, kidney, liver, adipose, lung, thymus, spleen, prostate, etc. Enzymatically active circulatory DPP4 is shed from the plasma membrane via proteolytic cleavage, a process responsible for the enhanced plasma DPP4 levels and activity. Elevated circulatory DPP4 activity as well as levels has been found in wide spectrum of metabolic diseases including diabetes, obesity, cardiovascular diseases, and nonalcoholic fatty liver diseases. Moreover, recent preclinical studies have further expanded the repertoire for the usage of DPP4 inhibitors in the treatment of other metabolic diseases and in their consequent complications. In the present review we highlight the reason behind the elevated circulatory DPP4 levels in metabolic diseases with a focus on the tissue of origin. We also underscore the discrepancy of protein levels with enzyme activity of circulatory DPP4 in metabolic diseases.