Application of two‐dimensional gel electrophoresis in the study of cytoskeletal protein regulation during growth activation and differentiation

Two‐dimensional gel electrophoresis was used to study the regulation of cytoskeletal protein synthesis during growth activation and development of the differentiated phenotype. We demonstrated a correlation between the state of organization and the expression of the respective cytoskeletal protein by showing that depolymerization of microtubules leads to a rapid decrease in new tubulin synthesis. We found that the synthesis of vimentin in both fibroblasts and epithelial cells correlates with extensive cell spreading on the substrate, while cytokeratin synthesis is maximal when cell to cell contacts are abundant. The analysis of cytoskeletal elements, involved directly in the formation of cell contacts, revealed that the level of vinculin synthesis is dependent on the extent of adherent type of cell contacts formed. Moreover, we found that the transient disappearance of vinculin from adhesion plaques of quiescent fibroblasts in response to serum factors was followed by an induction of vinculin mRNA and protein synthesis. The morphological changes associated with establishment of the differentiated phenotype were also found to include changes in the expression of the cytoskeletal‐extracellular matrix complex. This was demonstrated in several differentiating systems: in 3T3 preadipocytes which change their shape from a fibroblastic to a spherical shape when stimulated to differentiate with adipogenic medium, we observed a decrease in mRNA levels and in the synthesis of fibronectin, β‐integrin, and the microfilament proteins, vinculin, α‐actinin, tropomyosin and actin. The culturing of these cells on a certain extracellular matrix prevented the morphological changes occurring in the presence of adipogenic medium and blocked the shifts in cytoskeletal‐ and differentiation‐related gene expression. Similar changes in the organization and expression of cytoskeletal proteins were identified during maturation of primary ovarian granulosa cell cultures, stimulated with gonadotropic hormones to form highly steroidogenic cells. The cell rounding and aggregation occurring during this process were associated with a decreased synthesis of vinculin, α‐actinin, actin and the nonmuscle tropomyosins. The physiological relevance of these changes was suggested by the observation that the level of tropomyosin mRNA was lower in follicles of animals at late stages of granulosa cell maturation when compared to earlier stages. The expression of tissue‐specific and cytoskeletal proteins was also determined in primary cultures of liver hepatocytes, maintained under conditions either favorable for growth or for expression of liver‐specific functions. When DNA synthesis was elevated, cytoskeletal protein synthesis was high and that of liver‐specific proteins was low. In contrast, when the expression of liver specific genes was maintained at high level, cytoskeletal gene expression and DNA synthesis were inhibited, as in the adult liver hepatocytes. Taken together, these results suggest that: (i) there is a close correlation between the mode of organization and expression of cytoskeletal proteins, (ii) such changes induced by the environment in the extracellular and intracellular matrices are programmed events occurring during growth activation and differentiation.