Decreases in tubulin and actin gene expression prior to morphological differentiation of 3T3 adipocytes.

The differentiation of 3T3-F442A preadipocytes is characterized by numerous enzymatic events and by a programmed change in cell morphology from a fibroblastic form to a nearly spherical shape. Accompanying this morphological change are large and specific decreases in biosynthetic rates for beta and gamma actin, vimentin and alpha and beta tubulin, as detected by one- and two-dimensional gel electrophoresis. In cells undergoing adipose differentiation, actin synthesis decreases by 90%, while the decrease in tubulin synthesis is more than 95%. Translation in vitro of mRNA isolated from differentiating cultures indicates that the decreases in biosynthetic rate for cytoskeletal proteins result from altered levels of active mRNA. Using cloned cDNA probes for beta actin and alpha tubulin, we show that changes in mRNA activity correspond to a specific loss of these sequences during cellular differentiation. Quantitatively, this loss of tubulin and actin mRNA sequences accounts virtually completely for the changes in protein biosynthetic rates. Examination of the synthesis and accumulation of cytoskeletal proteins and of their temporal relation to morphological conversion indicates that the biosynthetic changes are very early events in the differentiation, and suggests strongly that they participate in the development of the adipocyte morphology. The early occurrence of decreased cytoskeletal-protein synthesis also suggests that subsequent biosynthetic events specific to adipocyte differentiation may be influenced by alterations in the cytoskeleton.