Abstract
There are marked variations in the activity of lipoprotein lipase (LPL) among adipose depots. The aim of this study was to compare the mechanisms of 24 h of fasting on LPL regulation between epididymal (EPI) adipocytes and mesenteric (MES) adipocytes in rats. 1-Day fasting consistently decreased activities of heparin-releasable LPL, total extractable LPL and cellular LPL markedly in both EPI and MES fat pads. LPL activity in MES fat pads was relatively lower than in the EPI fat pads. Consistent with data on LPL activity, the levels of expression of LPL mRNA in both nutritional states were lower in MES than EPI adipose tissue and isolated adipocytes. The decreased LPL activity after 1 day of fasting in MES adipocytes was explained mainly by a 50% decrease in the relative abundance of LPL mRNA level and a parallel 50% decrease in relative rate of LPL synthesis. In contrast, fasting of 1 day in EPI adipocytes decreased total LPL activity by 47% but did not affect LPL mRNA level or relative rate of LPL synthesis. A decrease in overall protein synthesis contributed to the decreased LPL activity after 1 day fasting both in EPI and MES adipocytes. In MES adipocytes the decrease in LPL activity, LPL mRNA and LPL synthesis were comparable, but in EPI adipocytes the changes in LPL activity were substantially larger than the changes in LPL mRNA level and LPL synthesis. Therefore, fasting decreased fat cell size, LPL activity, LPL mRNA level and relative rate of LPL synthesis in rats, and these effects were more marked in the MES adipocytes. These results clearly demonstrate the regional variations in the metabolic response of adipose tissue and LPL functions to fasting.
Highlights
Activity of the adipocyte enzyme lipoprotein lipase (LPL; EC 3.1.1.34) is an important determinant of the amount of triacylglycerol stored in adipose tissue (Eckel, 1989; Liu et al, 2006; Choi et al, 2007)
Scanning laser densitometry of fluorographys was used to quantitate the alterations in LPL biosynthesis
To quantitate relative rates of LPL synthesis, the LPL band or the corresponding location in a control immunoprecipitation was cut out of the dried gels, digested in 30% H2O2 (v/v) for 24 h at 60°C and radioactivity was determined by scintillation counting
Summary
Activity of the adipocyte enzyme lipoprotein lipase (LPL; EC 3.1.1.34) is an important determinant of the amount of triacylglycerol stored in adipose tissue (Eckel, 1989; Liu et al, 2006; Choi et al, 2007). LPL activity in adipose tissue increases several-fold within a few hours of ingestion of a meal (Eckel, 1989; Cruz and Williamson, 1992) and decreases progressively with fasting (Cryer, 1981; Lien and Jan, 2003; Wu et al, 2003). Other studies observe a tight correlation between abundance of LPL mRNA and activity of the enzyme in adipose tissue from rats fasted for 1 to 6 days (Ladu et al, 1991). The decrease in adipose tissue LPL activity during fasting may be due to pre- and/or posttranslational mechanisms (Doolittle et al, 1990; Ladu et al, 1991; Oliver and Rogers, 1993; Bergö et al, 1996)
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