Cellular characterization of human epicardial adipose tissue: highly expressed PAPP‐A regulates insulin‐like growth factor I signaling in human cardiomyocytes

Little is known about the cellular biology of fat surrounding the human heart. In this study, we obtained paired samples of epicardial fat, the visceral fat depot attached to the heart, and subcutaneous skin fat from patients undergoing open heart surgery to test the hypothesis that human epicardial fat cells differentially express bioactive molecules that have the potential to affect cardiac function. First, we characterized the free fatty acids (FFAs), adipocytokines, and growth factors secreted by isolated adipocytes and preadipocytes in cell culture. There was little to distinguish the fat cell secretory products in terms of FFAs and adipocytokines. The most striking finding was that preadipocytes from epicardial adipose tissue expressed high levels of pregnancy‐associated plasma protein‐A (PAPP‐A), a novel metalloproteinase that enhances local insulin‐like growth factor (IGF) action through cleavage of inhibitory IGF binding protein‐4 (IGFBP‐4). PAPP‐A levels were 15‐fold higher in conditioned medium from epicardial preadipocytes than from subcutaneous preadipocytes (P < 0.0001). PAPP‐A was not expressed in mature adipocytes. Next we determined whether PAPP‐A could affect IGF‐I signaling in a human cardiomyocyte cell line. IGF‐I activated receptor‐mediated auto‐phosphorylation, and this was blocked by wild‐type and protease‐resistant IGFBP‐4. Addition of PAPP‐A induced cleavage of wild‐type, but not protease‐resistant, IGFBP‐4 thereby restoring IGF‐I action. A proteolytically defective PAPP‐A had no effect. IGF‐I receptor‐mediated signaling through the phosphatidylinositol 3‐kinase pathway was similarly inhibited by IGFBP‐4 and restored by PAPP‐A. Thus, human epicardial fat cells differentially express PAPP‐A, which has the potential to affect IGF signaling in the heart.