Interleukin-4 inhibits platelet-derived growth factor-induced preadipocyte proliferation

Interleukin-4 (IL-4) activates STAT6 in 3T3-L1 preadipocytes but its functional role is not known. In this report, we first assessed interleukin-4 receptor alpha (IL-4Ralpha) expression during adipogenesis. IL-4Ralpha was highly expressed in proliferating 3T3-L1 preadipocytes. Receptor expression was down-regulated in post-confluent growth arrested preadipocytes. Induction of differentiation led to a transient 36-h increase in expression, but then levels decreased to undetectable amounts 3-8 days after induction of differentiation. Depending on the cell type, IL-4 either increases or decreases cell proliferation. In growth arrested preconfluent 3T3-L1 preadipocytes, IL-4 alone had no effect on preadipocyte proliferation. In contrast, IL-4 inhibited platelet-derived growth factor (PDGF-BB) induced preadipocyte proliferation. PDGF-BB, but not IL-4, induced STAT3 tyrosine and AKT serine phosphorylation. Both PDGF-BB and IL-4 induced STAT6 tyrosine phosphorylation, but the bands showed distinct electrophoretic migration patterns. IL-4 alone and IL-4 added to the differentiation cocktail had no effect on adipocyte formation or PPARgamma expression. Collectively, these studies demonstrate that IL-4 inhibits PDGF-BB-induced preadipocyte proliferation, possibly through STAT6 activation. The pattern of IL-4 receptor expression suggests that the effects of IL-4 are targeted primarily towards preadipocytes.