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Microsomal triglyceride transfer protein (MTP) regulation in HepG2 cells: insulin negatively regulates MTP gene
M C Lin1, D Gordon, J R Wetterau
1Department of Metabolic Diseases, Bristol-Myers Squibb, Princeton, NJ 08543-4000, USA.
Abstract:
The microsomal triglyceride transfer protein (MTP) is a heterodimeric lipid transfer protein that is required for the assembly and secretion of apoB-containing lipoproteins. In this study, four factors that modulate lipid and lipoprotein metabolism were tested for their ability to regulate MTP levels in HepG2 cells. Of the factors tested, only insulin (> or = 10(-9) M), and high concentrations of glucose (> 30 mM) were found to decrease MTP large subunit mRNA levels. Oleate and glucagon had no effect on MTP mRNA levels. The insulin effect was dose- and time-dependent and was mediated through the insulin receptor. In addition, insulin also decreased protein disulfide isomerase (the small subunit of MTP) mRNA levels, although to a lesser extent. Due to the slow turnover rate of MTP (t1/2 = 4.4 days), short-term insulin treatment (24 h) did not change MTP activity levels, indicating that the regulation of MTP mRNA levels by insulin is unrelated to insulin's acute inhibition of apoB secretion in HepG2 cells. In summary, MTP mRNA levels are acutely regulated by insulin in HepG2 cells; however, sustained changes in MTP mRNA levels would be required to affect MTP protein levels.
Insights
Insulin and high glucose levels reduce microsomal triglyceride transfer protein (MTP) mRNA in liver cells. This regulation occurs via the insulin receptor but does not immediately impact MTP activity due to slow protein turnover.
Area of Science:
- Biochemistry
- Molecular Biology
- Cell Biology
Background:
- Microsomal triglyceride transfer protein (MTP) is crucial for assembling and secreting lipoproteins containing apolipoprotein B (apoB).
- Understanding MTP regulation is key to managing lipid and lipoprotein metabolism.
Purpose of the Study:
- To investigate how insulin, glucose, oleate, and glucagon affect MTP levels in HepG2 cells.
- To elucidate the mechanisms and implications of MTP regulation by these factors.
Main Methods:
- HepG2 cells were treated with varying concentrations of insulin, glucose, oleate, and glucagon.
- Messenger RNA (mRNA) levels of MTP large and small subunits were quantified.
- MTP activity and apoB secretion were assessed following insulin treatment.
Main Results:
- Insulin (≥ 10⁻⁹ M) and high glucose (> 30 mM) significantly decreased MTP large subunit mRNA levels.
- Insulin's effect was dose- and time-dependent, mediated via the insulin receptor, and also reduced protein disulfide isomerase (small subunit) mRNA.
- Oleate and glucagon did not affect MTP mRNA levels.
- Short-term insulin treatment (24 h) did not alter MTP activity due to MTP's slow turnover rate (t½ = 4.4 days).
Conclusions:
- Insulin acutely regulates MTP mRNA levels in HepG2 cells, independent of its immediate effect on apoB secretion.
- Sustained changes in MTP mRNA are necessary to influence MTP protein levels and activity.
- High glucose also plays a role in modulating MTP mRNA, suggesting a complex regulatory network in hepatic lipoprotein metabolism.