Increased adipocyte formation from mesenchymal stem cells (MSCs) is typical for obesity. It is recently observed that bone morphogenetic proteins (BMPs) provide instructive signals for the commitment of MSCs to adipocytes. We examined potential role of retinoic acid (RA) in inhibiting the BMP4 induction of MSC commitment toward adipocyte. BMP4-treated C3H10T1/2 MSCs, when further exposed to adipogenic differentiation media, displayed distinct adipocytic commitment and differentiation. This could be inhibited by RA exposure during the BMP4 treatment stage (commitment stage before adipogenic hormonal inducers were given), as was observed by reductions in key adipogenic genes/transcription factors (C/EBP alpha, PPAR gamma, aP2), lipogenic genes (LPL, FAS, GLUT4), and lipid accumulation. Among RA receptors (RARs) screened, RAR beta was mainly upregulated under RA exposure. BMP4 signaled through both Smad1/5/8 and p38 mitogen-activated protein kinase (MAPK) and RA significantly suppressed the BMP4-triggered phosphorylation of both Smad1/5/8 and p38MAPK. These data suggest that RA has inhibitory effects on the BMP4 induction of C3H10T1/2 adipocytic commitment via downregulating Smad/p38MAPK signaling. How to inhibit MSC adipocytic commitment, as partly revealed in this study, will have a significant impact on treating obesity and related diseases. (C) 2011 Elsevier Inc. All rights reserved.