The Effect of Macrophage-secreted Factors on Preadipocyte Survival

Abstract

Adipose tissue (AT) expansion and remodeling that maintains healthy function relies on stromal preadipocytes capable of differentiating into new adipocytes (adipogenesis). During chronic positive energy balance, a relative deficit in adipogenesis, from either a decrease in preadipocyte number or their capacity to differentiate, leads to excessive adipocyte hypertrophy and AT dysfunction. AT contains macrophages whose number and activation state is dynamically regulated with changes in AT mass. This study aims to investigate the effect of macrophage-secreted factors on preadipocyte survival. To assess the effect of macrophage-secreted factors on preadipocytes, murine 3T3-L1 preadipocytes or human primary preadipocytes were incubated with macrophage-conditioned medium (MacCM), prepared from either murine (J774A.1, RAW264.7, bone marrow-derived) or human (THP-1, monocyte-derived) macrophage models, respectively. MacCM inhibited preadipocyte apoptosis and activated pro-survival signaling in both preadipocyte models. Inhibition of PDGFR, Akt, or ERK1/2 reduced the pro-survival effect of MacCM in 3T3-L1 preadipocytes. Inhibition of reactive oxygen species (ROS) generation, or enhancement of ROS clearance, reduced MacCM-dependent 3T3-L1 preadipocyte survival. Whereas anti-inflammatory activated macrophages retained the ability to prevent preadipocyte apoptosis, pro-inflammatory activated macrophages did not. TNF-α immunoneutralization restored the survival activity of pro-inflammatory MacCM on 3T3-L1 preadipocytes. These studies reveal a novel pro-survival effect of MacCM on preadipocytes, and identify signaling molecules (PDGF, Akt, ERK1/2, and ROS) that underlie this action. Macrophage activation was found to regulate the pro-survival activity of MacCM. These in vitro cell culture studies are consistent with a model in which the extent of preadipocyte apoptosis in vivo may determine preadipocyte number and the ability of AT to expand while maintaining healthy function during chronic positive energy balance.