Essential Role in Vivo of Upstream Stimulatory Factors for a Normal Dietary Response of the Fatty Acid Synthase Gene in the Liver

Abstract

In the liver, transcription of several genes encoding lipogenic and glycolytic enzymes, in particular the gene for fatty acid synthase (FAS), is known to be stimulated by dietary carbohydrates. The molecular dissection of the FAS promoter pointed out the critical role of an E box motif, located at position ؊65 with respect to the start site of transcription, in mediating the glucose-and insulin-dependent regulation of the gene. Upstream stimulatory factors (USF1 and USF2) and sterol response element binding protein 1 (SREBP1) were shown to be able to interact in vitro with this E box. However, to date, the relative contributions of USFs and SREBP1 ex vivo remain controversial. To gain insight into the specific roles of these factors in vivo, we have analyzed the glucose responsiveness of hepatic FAS gene expression in USF1 and USF2 knock-out mice. In both types of mouse lines, defective in either USF1 or USF2, induction of the FAS gene by refeeding a carbohydrate-rich diet was severely delayed, whereas expression of SREBP1 was almost normal and insulin response unchanged. Therefore, USF transactivators, and especially USF1/ USF2 heterodimers, seem to be essential to sustain the dietary induction of the FAS gene in the liver.Fatty acid synthase (FAS) 1 plays a central role in de novo lipogenesis in mammals, catalyzing all reaction steps in the conversion of acetyl-CoA and malonyl-CoA to palmitate. As for many other lipogenic and glycolytic genes involved in maintenance of energy balance, the expression of the FAS gene is highly dependent on nutritional conditions in liver and adipose tissue. Expression of the gene is barely detectable in starved animals and is stimulated by refeeding a high carbohydrate, fat-free diet (for review see Refs. 1 and 2). This fasting/refeeding transition is accompanied in vivo by an increased circulating insulin level, and it is often difficult to differentiate the effects of insulin from those of carbohydrate metabolism in mediating the regulation of gene expression. To date, the role of insulin, either direct or indirect, in mediating FAS-activated gene expression, is still disputed. It has been proposed that the effects of insulin could be only indirect (3-5), being permissive to allow for effective glucose metabolism (1). In contrast, others have provided evidence for a direct involvement of insulin both ex vivo (6) and in vivo (7). In this respect, Sul and co-workers (8) reported, by transfection experiments, that the region between Ϫ71/Ϫ50 was responsible for mediating the effects of insulin on the rat FAS promoter (see Fig. 1). They further demonstrated that upstream stimulatory factors (USFs), USF1 and USF2, were major components of the complex binding to this region (called IRS for insulin response sequence) (9). USFs are ubiquitous basic helix-loop-helix-leucine zipper (b-HLH-Zip) transcription factors able to interact as homo-and/or heterodimers on E boxes of CANNTG sequence (for review see Ref. 10). The FAS promoter IRS contains such an E box at position Ϫ65 (Fig. 1). Mut...

Cha, a Basic Helix-Loop-Helix Transcription Factor Involved in the Regulation of Upstream Stimulatory Factor Activity

“…USF-1 and USF-2 bind to E-boxes in the promoter regions of several genes, some involved in the metabolism of the glucose response (lipidogenic and glycolytic enzymes), as fatty acid synthase (18), hormone sensitive lipase (19), and pyruvate kinase (20). In agreement with this, USF-1 or USF-2 knockout mice have a diminished glucose response (21) and fatty acid synthase expression (22). Other genes such as follicle-stimulating hormone receptor (23), metallothionein (24), HOXB4 transcription factor (25), and the class II major histocompatibility complex (26) are regulated by USF-1.…”

Hepatic Steatosis in Type 1 Diabetes

“…One of two LXR isoforms, LXRα, is abundantly expressed in lipogenic tissues, and by activating the SREBP-1c promoter, it plays an important role in the transcriptional activation of lipogenic genes [56]. Another group of transcription factors in the liver, upstream stimulatory factors (USF), mediates the effects of insulin on the FAS promoter by interaction with SREBP-1c [57,58].…”

Familial combined hyperlipidemia is associated with upstream transcription factor 1 (USF1)

“…USF1 belongs to the basic helix-loop-helix leucine zipper family, interacts with its target DNA as a homodimer or heterodimer with USF2 and recognizes a CACGTG motif called E box in the promoter of the target genes, resulting in transcriptional activation in response to various stimuli, such as glucose and dietary carbohydrates 14,15 . Target genes of USF1 include several apolipoproteins (CIII, AII and E), hormone-sensitive lipase, fatty acid synthase, glucokinase, the glucagon receptor, ATP-binding cassette sub-family A (ABC1) member 1, and renin, making USF1 a good candidate for involvement with the central clinical features of FCHL and type 2 diabetes mellitus: glucose intolerance, hyperlipidemia, insulin resistance and hypertension.…”