c-Crk, a substrate of the insulin-like growth factor-1 receptor tyrosine kinase, functions as an early signal mediator in the adipocyte differentiation process
Sh. Jin et al., c-Crk, a substrate of the insulin-like growth factor-1 receptor tyrosine kinase, functions as an early signal mediator in the adipocyte differentiation process, J BIOL CHEM, 275(44), 2000, pp. 34344-34352
Differentiation of 3T3-L1 preadipocytes into adipocytes is induced by a com
bination of inducers, including a glucocorticoid, an agent that elevates ce
llular cAMP, and a ligand of the insulin-like growth factor-1 receptor. Pre
vious studies have implicated protein-tyrosine phosphatase (PTPase) HA2, a
homologue of PTPase 1B, in the signaling cascade initiated by the different
iation inducers. Vanadate, a potent PTPase inhibitor, blocks adipocyte diff
erentiation at an early stage in the program, but has no effect on the mito
tic clonal expansion required for differentiation. Exposure of preadipocyte
s to vanadate along with the inducing agents led to the accumulation of pp3
5, a phosphotyrosyl protein that is a substrate for PTPase HA2. pp35 was pu
rified to homogeneity and shown by amino acid sequence and mass analyses of
tryptic peptides to be c-Crk, a known cytoplasmic target of the insulin-li
ke growth factor-1 receptor tyrosine kinase. Transfection of 3T3-L1 preadip
ocytes with a c-Crk antisense RNA expression vector markedly reduced c-Crk
levels and prevented differentiation into adipocytes. Studies with C3G, a p
rotein that binds to the SH3 domain in c-Crk, showed that phosphorylation o
f c-Crk rendered the SH3 domain inaccessible to C3G. Taken together, these
findings indicate that locking c-Crk in the phosphorylated state with vanad
ate prevents its participation in the signaling system that initiates adipo
cyte differentiation.