Investigating the extremes of the continuum of paracrine functions in CD34-/CD31+ CACs across diverse populations.

Paracrine function of circulating angiogenic cells (CACs) is thought to contribute to vascular maintenance. We previously identified S100A8 and S100A9 secreted from physically inactive individuals' CD34-/CD31+ CACs as negative regulators of capillary-like network formation. The purpose of this study was to further investigate the extremes of the continuum of CAC paracrine actions using two distinctly different groups representing "healthy" and "impaired" CAC function. We aimed to determine how capillary-like network formation in human umbilical vein endothelial cells (HUVECs) is affected by S100A8 and S100A9 in concentrations secreted by CACs from different ends of the health spectrum. CD34-/CD31+ CACs were isolated and cultured from 10 "impaired function" individuals defined as older (50-89 yrs) non-ST-elevation myocardial infarction (NSTEMI) patients and 10 "healthy" individuals defined as younger (18-35 yrs), healthy individuals, and conditioned media (CM) was generated. CM from the impaired function groups' CACs significantly diminished network formation compared to CM from healthy group (P<0.05). We identified elevations in S100A8, S100A9, and S100A8/A9 in the CM from the impaired function group (P<0.05). Pretreatment of HUVECs with inhibitors to a known S100A8 and S100A9 receptor, TLR4, but not RAGE, improved HUVEC network formation (P<0.05) compared to CM alone in the impaired function conditions. Exposure of HUVECs to the TLR4 signaling inhibitor also blocked recombinant S100A8 and S100A9-mediated reductions in network formation. Collectively, the results suggest that the mechanisms behind impaired CAC CD34-/CD31+ CM-mediated reductions in capillary-like network formation involve secretion of S100A8 and S100A9 and binding of these proteins to TLR4 receptors on HUVECs.