Extracellular Ubiquitin(1–76) and Ubiquitin(1–74) Regulate Cardiac Fibroblast Proliferation

SDF-1α (stromal cell–derived factor-1α) is a CXCR4-receptor agonist and DPP4 (dipeptidyl peptidase 4) substrate. SDF-1α, particularly when combined with sitagliptin to block the metabolism of SDF-1α by DPP4, stimulates proliferation of cardiac fibroblasts via the CXCR4 receptor; this effect is greater in cells from spontaneously hypertensive rats versus Wistar-Kyoto normotensive rats. Emerging evidence indicates that ubiquitin(1–76) exists in plasma and is a potent CXCR4-receptor agonist. Therefore, we hypothesized that ubiquitin(1–76), similar to SDF-1α, should increase proliferation of cardiac fibroblasts. Contrary to our working hypothesis, ubiquitin(1–76) did not stimulate cardiac fibroblast proliferation, yet unexpectedly antagonized the proproliferative effects of SDF-1α combined with sitagliptin. In this regard, ubiquitin(1–76) was more potent in spontaneously hypertensive versus Wistar-Kyoto cells. In the presence of 6bk (selective inhibitor of insulin-degrading enzyme [IDE]; an enzyme known to co...