Glucose increases hepatic lipase expression in HepG2 liver cells through upregulation of upstream stimulatory factors 1 and 2

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Abstract
Aims/hypothesis Elevated hepatic lipase (HL, also known as LIPC) expression is a key factor in the development of the atherogenic lipid profile in type 2 diabetes and insulin resistance. Recently, genetic screens revealed a possible association of type 2 diabetes and familial combined hyperlipidaemia with the USF1 gene. Therefore, we investigated the role of upstream stimulatory factors (USFs) in the regulation of HL. Methods Levels of USF1, USF2 and HL were measured in HepG2 cells cultured in normal- or high-glucose medium (4.5 and 22.5 mmol/l, respectively) and in livers of streptozotocin-treated rats. Results Nuclear extracts of cells cultured in high glucose contained 2.5 ± 0.5-fold more USF1 and 1.4 ± 0.2-fold more USF2 protein than cells cultured in normal glucose (mean ± SD, n = 3). This coincided with higher DNA binding of nuclear proteins to the USF consensus DNA binding site. Secretion of HL (2.9 ± 0.5-fold), abundance of HL mRNA (1.5 ± 0.2-fold) and HL (–685/+13) promoter activity (1.8 ± 0.3-fold) increased in parallel. In chromatin immunoprecipitation assays, the proximal HL promoter region was immunoprecipitated with anti-USF1 and anti-USF2 antibodies. Co-transfection with USF1 or USF2 cDNA stimulated HL promoter activity 6- to 16-fold. USF and glucose responsiveness were significantly reduced by removal of the −310E-box from the HL promoter. Silencing of the USF1 gene by RNA interference reduced glucose responsiveness of the HL (−685/+13) promoter region by 50%. The hyperglycaemia in streptozotocin-treated rats was associated with similar increases in USF abundance in rat liver nuclei, but not with increased binding of USF to the rat Hl promoter region. Conclusions/interpretation Glucose increases HL expression in HepG2 cells via elevation of USF1 and USF2. This mechanism may contribute to the development of the dyslipidaemia that is typical of type 2 diabetes.