ID3 PREVENTS DIFFERENTIATION OF PREADIPOSE CELLS

We have studied the expression of the Idl, Id2, and Id3 genes during a dipose differentiation of 3T3-F442A cells, All three Id mRNAs are pres ent in preadipose cells, but the mRNA for Id3 is the most abundant. Al l three Id mRNAs sharply decline in the course of adipose differentiat ion, and their virtual disappearance precedes differentiation, The dec rease in Id2 and Id3 is associated with adipose differentiation rather than with growth arrest since it is not observed in 3T3-C2 cells, a f ibroblast line with a very low susceptibility to adipose conversion, T he decline in Id2 and Id3 mRNAs is associated with a reduced transcrip tion rate of the two genes, Idl mRNA is reduced in amount during adipo se conversion of 3T3-F442A cells, but the decrease is also observed in resting 3T3-C2 cells and is associated with very little decrease in t ranscription of the gene, Addition of fresh serum reactivates Id3 gene expression in quiescent 3T3-C2 cells but not in adipose 3T3-F442A cel ls, Stably transformed preadipose cells expressing an Id3 cDNA under t he control of a viral promoter are virtually unable to differentiate, We postulate that the Id3 protein is a negative regulator of fat cell formation and presumably acts by preventing an as yet unidentified bas ic helix-loop-helix protein from activating the program of differentia tion.