Regulation of adipose cell differentiation . 1 . Fatty acids are inducers of the aP 2 gene expression

The regulation of the expression of adipose-related genes, i.e., aP2, adipsin, and glycerophosphate dehydrogenase (GPDH) by growth hormone (GH) and polyamines, as well as the role of fatty acids, have been investigated in polyaminedependent Ob1754 cells and Ob1771 preadipose cells. Growth hormone acts as an obligatory hormone for adipsin and GPDH gene expression but its presence is not required for the expression of the aP2 gene. In fully differentiated Ob1771 cells, impairment of fatty acid synthesis by glucose deprivation leads to an inhibition of the aP2 gene expression, whereas the expression of adipsin and GPDH genes remains unaffected. Supplementation of the culture medium with fatty acids prevents the decrease of aP2 gene expression, and this effect appears primarily due to an increase in the transcriptional level of aP2 gene. The induction of aP2 gene has been examined in early committed, lipidfree Ob1771 cells in which fatty acid synthesis is very low despite glucose supplementation. Long-chain fatty acids ( 1 C l 2 ) are able to activate the aP2 gene. It is concluded that fatty acids or fatty acid metabolites activate the aP2 gene and subsequently modulate its expression.-Amri, E. Z., B. Bertrand, G. Ailhaud, and P. Grimaldi. Regulation of adipose cell differentiation. I. Fatty acids are inducers of the aP2 gene expression. J Lzpid Res. 1991. 32: 1449-1456. Supplementary key words adipose differentiation growth hormone polyamines The adipose conversion of preadipose cells, i.e., Ob17 (l), 3T3-Ll (2), and 3T3-F442A (3) represents a valid model to delineate in vitro the development of adipose tissue in vivo. Under appropriate culture conditions, these cells can differentiate into adipose cells. This process is accompanied by a large shift in the cellular pattern of protein biosynthesis that reflects activation of the transcription of adipose-related genes (4, 5). The differentiation program can be divided into early and late events that are differently regulated. Early events are triggered by growth arrest and characterized by the activation of a set of genes, among which are pOb24 and lipoprotein lipase genes (6, 7). Expression of the terminal differentiation, characterized by the emergence of lipogenic enzymes and subsequent triacylglycerol accumulation, is under multihormonal regulation. Among genes expressed late during differentiation, the aP2, adipsin, and GPDH genes encode proteins known or postulated to play important roles in adipose cell physiology. Glycerol-3-phosphate dehydrogenase provides the glycerol backbone needed for triglyceride synthesis. The cytosolic aP2 protein is postulated to be an adipocyte lipid-binding protein as suggested by its ability to bind fatty acids (8). The function of adipsin is still unclear, but this secreted serine protease could be a putative systemic regulator of energy balance (9). Among hormones required for terminal differentiation, G H plays a critical role (10, 11). Longterm treatment by G H leads in Ob1771 and 3T3-F442A preadipose cells to a specific rise in the intracellular levels of spermidine and to activation of genes related to terminal differentiation (12, 13). Since the inhibition of spermidine synthesis prevents lipid accumulation (13), it could be hypothesized that spermidine acts as one of the messengers in the GH-dependent pathway regulating the process of terminal differentiation. To delineate more precisely the effects of spermidine accumulation on that process, we have investigated the regulation of the expression of aP2, adipsin, and GPDH genes in the variant cell line Ob1754. In these cells no rise in spermidine level takes place despite the presence of GH; however chronic exposure of the cells to a mixture of putrescine and MGBG, a competitive inhibitor of S-adenosyl-methionine decarboxylase, leads to a rise in the cell content of spermidine similar to that occurring during differentiation of GHtreated Ob1771 and 3T3-F442A cells; this rise is accompanied by terminal differentiation (12, 13). The results presented herein appear to exclude a single mechanism for the regulation of the expression of the three genes under consideration. It was found that changes in spermidine intracellular levels are involved in aP2 gene expresAbbreviations: GH, growth hormone; GADPH, glyceraldehyde-3phosphate dehydrogenase; GPDH, glycerol-3-phosphate dehydrogenase; MGBG, methylglyoxal bis(guany1hydrazone); T3, triiodothyronine. 'To whom correspondence should be addressed. Journal of Lipid Research Volume 32, 1991 1449 by gest, on O cber 9, 2017 w w w .j.org D ow nladed fom sion, whereas G H effects on the expression of adipsin and ~~~l~~~ transcrintion aSSavS GPDH genes appear to be independent or partially dependent, respectively, of polyamine metabolism. The expression of the aP2 gene is closely linked to fatty acid supply in differentiating Ob1754 cells as well as in early confluent, lipid-free Ob1771 cells in which fatty acid synthesis is very low. Fatty acids or fatty acid metabolites appear to activate the aP2 gene and subsequently to modulate its expression. MATERIALS AND METHODS