The Src homology 3 (SH3) motif is found in numerous signal transduction pro
teins involved in cellular growth and differentiation. We have purified and
cloned a novel protein, DEF-1 (differentiation-enhancing factor), from bov
ine brain by using a Src SH3 affinity column. Ectopic expression of DEF-1 i
n fibroblasts resulted in the differentiation of a significant fraction of
the culture into adipocytes. This phenotype appears to be related to the in
duction of the transcription factor peroxisome proliferator-activated recep
tor gamma (PPAR gamma), since DEF-1 NIH 3T3 cells demonstrated augmented le
vels of PPAR gamma mRNA and, when treated with activating PPAR gamma ligand
s, efficient induction of differentiation. Further evidence for a role for
DEF-1 in adipogenesis was provided by heightened expression of DEF-1 mRNA i
n adipose tissue isolated from obese and diabetes mice compared to that in
tissue isolated from wild-type mice. However, DEF-1 mRNA was detected in mu
ltiple tissues, suggesting that the signal transduction pathway(s) in which
DEF-1 is involved is not limited to adipogenesis. These results suggest th
at DEF-1 is an important component of a signal transduction process that is
involved in the differentiation of fibroblasts and possibly of other types
of cells.