BMC Mol Biol. 2007 Jul 17;8:60.
Hormonal and nutritional regulation of alternative CD36 transcripts in rat liver - a role for growth hormone in alternative exon usage.
Cheung L, Andersen M, Gustavsson C, Odeberg J, Fernandez-Perez L, Norstedt G, Tollet-Egnell P.
Department of Molecular Medicine, Karolinska Institutet, Karolinska Hospital, Stockholm, Sweden.
[ Free full text ]

Background. CD36 is a multiligand receptor involved in various metabolic pathways, including cellular uptake of long-chain fatty acids. Defect function or expression of CD36 can result in dyslipidemia or insulin resistance. We have previously shown that CD36 expression is female-predominant in rat liver. In the present study, hormonal and nutritional regulation of hepatic CD36 expression was examined in male and female rats. Since alternative transcription start sites have been described in murine and human Cd36, we investigated whether alternative CD36 transcripts are differentially regulated in rat liver during these conditions.

Results. Sequence information of the rat Cd36 5¡ä-UTR was extended, showing that the gene structure of Cd36 in rat is similar to that previously described in mouse with at least two alternative first exons. The rat Cd36 exon 1a promoter was sequenced and found to be highly similar to murine and human Cd36. We show that alternative first exon usage is involved in the female-predominant expression of CD36 in rat liver and during certain hormonal states that induce CD36 mRNA abundance. Estrogen treatment or continuous infusion of growth hormone (GH) in male rats induced CD36 expression preferentially through the exon 1a promoter. Old age was associated with increased CD36 expression in male rats, albeit without any preferential first exon usage. Intermittent GH treatment in old male rats reversed this effect. Mild starvation (12 hours without food) reduced CD36 expression in female liver, whereas its expression was increased in skeletal muscle.

Conclusion. The results obtained in this study confirm and extend our previous observation that GH is an important regulator of hepatic CD36, and depending on the mode of treatment (continuous or intermittent) the gene might be either induced or repressed. We suggest that the effects of continuous GH secretion in females (which is stimulatory) and intermittent GH secretion in males (which is inhibitory) explains the sex-different expression of this gene. Furthermore, a female-specific repression of hepatic CD36 in response to food deprivation was found, which was in contrast to a stimulatory effect in skeletal muscle. This demonstrates a tissue-specific regulation of Cd36.

From the full text article:

Background. Fatty acid translocase (FAT or CD36) is a cell-surface glycoprotein that functions as a multiligand receptor/transporter involved in various diverse physiological processes and disorders, including atherosclerosis, dyslipidemia, insulin resistance and diabetes (reviewed in [1]). In peripheral tissues active in fatty acid metabolism, such as muscle and adipose tissues, CD36 facilitates the uptake of long-chain fatty acids (LCFA) across the plasma membrane. A null mutation of Cd36 reduces fatty acid (FA) uptake rates and metabolism in these tissues [2], while its over-expression results in the opposing effects [3,4]. Although CD36-deficient mice display reduced triacylglycerol (TG) in muscle, lipid accumulation is increased in the liver. The latter is probably due to increased plasma FA concentrations and CD36-independent hepatic FA uptake. As a consequence of these effects, insulin sensitivity is enhanced peripherally, while it appears impaired in the liver [5]. Since muscle and adipose tissues shift to high glucose utilization in CD36-deficiency, hypoglycemia and hypoinsulinemia develop in the fasted state in CD36-deficient mice [6]. Also, over-expression of CD36 in muscle tissues is associated with hyperglycemia and hyperinsulinemia, as a result of enhanced FA utilisation and glucose sparing. Taken together, expression-levels of CD36 affect lipid and glucose utilization and insulin sensitivity in different tissues. Abnormal regulation of Cd36 expression might therefore contribute to the onset or severity of several metabolic diseases.

CD36 abundance is regulated at different levels, including gene expression, mRNA stability and protein expression in a cell- and tissue-specific manner. Different physiological conditions, where the nutritional and/or hormonal status of the individual is affected, have been shown to impact on CD36 levels in the plasma membrane. Regulation at the level of mRNA expression in skeletal muscle has been reported to include starvation, refeeding [7] and exercise [8], but only a few studies report the molecular mechanisms behind these effects. Although PPARalpha, PPARgamma [9,10], PXR [11], NR4A (Nu77) [12] and Foxa2 (HNF3¦Â) [13] have been shown to affect CD36 expression, no direct interaction of these transcription factors in the Cd36 promoter have been described. Furthermore, recent analyses of the Cd36 gene has revealed a complicated promoter structure with alternative transcription start sites [14,15]. Alternative promoter usage has been shown to contribute to tissue-specific regulation of CD36 expression in both mice and humans [14,15]. Although the alternative promoters are mapped in the human and murine Cd36 genes, less information is available for the rat gene.

Hepatic CD36 has not been considered physiologically important, due to low level of expression in the liver and the fact that hepatic lipid uptake is independent of CD36. Perhaps as a consequence of this, little is known about hepatic Cd36 gene regulation. However, we have recently shown that CD36 expression is female-predominant in rat liver [16]. It was also shown that the female pattern of growth hormone (GH) secretion induces CD36 mRNA levels in male liver, suggesting that there might be conditions where increased or decreased activities of hepatic CD36 is needed and that these conditions might be slightly different in male and female livers. The aim with the present study was to extend our knowledge about regulatory mechanisms behind hepatic CD36 expression. Since alternative CD36 transcripts have been described in mouse and human tissues, we investigated whether alternative transcription start sites might be involved in differentially regulating CD36 mRNA abundance in rat liver in different hormonal and nutritional states.

[...]

Nutritional regulation of CD36 expression

The nutritional status of the individual has been shown to affect CD36 expression in skeletal muscle, including starvation-mediated up-regulation [7]. Both starvation and increased secretion of GH will lead to activation of lipolysis in the adipose tissue, increased levels of circulating fatty acids, as well as increased expression of CD36 in muscle (starvation) or liver (GH). To determine whether the level of CD36 expression and thus ability to import LCFA is coordinated between muscle and liver, we compared these tissues regarding the different CD36 transcripts in animals deprived of food for either 4 or 12 hours. Animals that have been without food for 12 hours (fasting) should have an increased lipolysis in comparison to 4 hours. As demonstrated in Figure 5A, 12 h of food deprivation led to significantly reduced CD36 mRNA expression in livers from females, without any differences in alternative exon 1 transcripts. No effect was observed in male rats. As a consequence, the sex difference in CD36 expression was greatly reduced in the fasted rats. In contrast, the expression of CD36 in skeletal muscle was increased in both males and females (Figure 5B). This stimulatory effect of food withdrawal was highest for exon 1a-3, suggesting that fasting might induce CD36 expression preferentially through the exon 1a promoter. These observations confirms previous reports on Cd36 regulation in skeletal muscle [7], and shows that CD36 expression is differently regulated in muscle and liver. As demonstrated in Figure 5C, the starvation-mediated down-regulation of CD36 in female livers could also be observed at the protein level, indicating that animals in this nutritional state might prioritize muscular uptake of LCFA at the expense of hepatic.

[...]

The results obtained in this study confirm and extend our previous observation that GH is an important regulator of hepatic CD36, and depending on the mode of treatment (continuous or intermittent) the gene might be either induced or repressed. We therefore suggest that the effects of continuous GH secretion in females (which is stimulatory) and intermittent GH secretion in males (which is inhibitory) explains the sex-different expression of this gene.

Categories: 2007, CD36-FAT, Fatty acid metabolism, Lipolysis, Dyslipidemia, Free fatty acids, Glycoproteins, Growth hormone physiology, Estrogen, Endocrine, Hypoglycemia, Hyperglycemia, Hypoinsulinemia, Hyperinsulinemia, Insulin resistance, Diabetes, Liver, Malnutrition, NR4A, PPARalpha, PPARgamma, Foxa2, PXR