fos Protooncogene and the Regulation of Gene Expression in Adipocyte Differentiation

ELLULAR development occurs in several stages in higher eukaryotic organisms. This process is generally characterized by the loss of pluripotency and restriction of possible cell fates, leading to a highly "determined" cell that can either proliferate or differentiate into a defined end-type cell. The process of cell differentiation can be studied in many cell types, some of which offer convenient culture systems. We are particularly interested in the development of fat cells. In our studies we have had two objectives. First, to use adipocyte differentiation as a model for the process of signal transduction in cellular development, whereby extracellular stimulation leads to the ultimate expression of end-product genes, extinction of genes characteristic of the precursor cells, and phenotypic differentiation. In addition, because the adipose cell is centrally involved in certain pathological syndromes of systemic energy balance, most notably obesity, obesity-linked diabetes, and cachexia (fat and muscle wasting), we are interested in using studies of gene control to improve our understanding of these disorders. Because cancer is a disease involving loss of cellular growth control and disruption of the usual pattern of cell differentiation, there has been much interest in the role of oncogenes and their normal cellular counterparts (protooncogenes) in controlling patterns of gene expression during differentiation. Many studies using the expression of oncogenes introduced into cells have shown profound effects, either promoting or inhibiting differentiation depending on the oncogene and cellular system being analyzed (4, 5, 8, 13, 17, 19, 21, 24). One difficulty with this approach has been that the connection between the oncoprotein and the ultimate alterations in gene expression is usually not clear. We have recently approached the same goals through a different method. By studying in detail the cis- and trans-acting components involved in activating a particular gene during adipocyte differentiation, we have identified the first gene product related to an oncogene (fos) which appears to play a direct role in controlling gene expression during differentiation. This will be the major subject discussed below. Adipose Differentiation in Cultured Cell Lines The study of adipocyte development became much simpler with the isolation of preadipocyte cell lines that differentiate in culture, initially by Green and Kehinde (16) and later by many others. These lines typically resemble fibroblasts when kept in a growing state but undergo morphological and biochemical changes characteristic of fat cells when they stop growing under appropriate culture conditions. Many or most of the key enzymes of fatty acid and triglyceride synthesis are greatly increased during this process. The striking change in cell shape during adipocyte differentiation is independent of lipid accumulation; blocking triglyceride formation by depriving cells of biotin has little or no effect on the alterations in cell morphology (20). The gross changes in cell morphology are accompanied by alterations in the synthesis and assembly of cytoskeletal proteins (28). Experiments which interfered with morphological conversion using exogenous extracellular matrix components suggest that changes in cellular structure may be necessary for the programmed alterations in gene expression (29). The differentiation of adipocytes is accelerated by a number of defined or undefined factors including FCS, insulin, growth hormone, glucocorticoids, sex steroids, thyroid hormones, and indomethacin. Differentiation is inhibited by interferon, retinoic acid, interleukin-1, tumor promoters, and tumor necrosis factor. It is likely that at least some of these factors play a physiological role in adipocyte differentiation in vivo but this remains to be proven.