Upon obesity, adipose tissue is excessively expanded and characterized by pathologic processes like hypoxia, fibrosis, and inflammation. Death ligands belonging to the TNF superfamily such as TNF-α are important contributors to these derangements and exert a pronounced influence on the metabolic and cellular homeostasis of adipose tissue. Here, we sought to identify the effect of the death ligand TNF-related apoptosis-inducing ligand (TRAIL) on the adipose tissue precursor cell pool and therefore investigated its influence on preadipocyte proliferation. Treatment of human preadipocytes with TRAIL resulted in a time- and dose-dependent increase in proliferation (EC50 3.4 ng/ml) comparable to IGF-1. Although no apoptosis was observed, TRAIL triggered a rapid cleavage of caspase-8 and -3. Neither inhibition of caspase activity by zVAD.fmk (20 µM) nor ablation of caspase-8 expression by lentivirus-delivered small hairpin RNA (shRNA) abolished the proliferative response. TRAIL triggered a delayed and sustained activation of ERK1/2, leaving Akt, p38, JNK, and NF-κB unaffected. Importantly, inhibition of ERK1/2 activation by PD0325901 (300 nM) or AZD6244 (5 or 10 µM) completely abolished the proliferative response. We thus reveal a hitherto unknown function of TRAIL in regulating adipose tissue homeostasis by promoting the proliferation of tissue-resident precursor cells.
Keywords: adipocyte progenitor; adipose tissue homeostasis; death ligand; noncanonical signaling.