FoxO1 mediates insulin-dependent regulation of hepatic VLDL production in mice
Adama Kamagate, … , Marcia Meseck, H. Henry Dong
Adama Kamagate, … , Marcia Meseck, H. Henry Dong
Published May 22, 2008
Citation Information: J Clin Invest. 2008;118(6):2347-2364. https://doi.org/10.1172/JCI32914.
View: Text | PDF
Research Article

FoxO1 mediates insulin-dependent regulation of hepatic VLDL production in mice

Abstract

Excessive production of triglyceride-rich VLDL is attributable to hypertriglyceridemia. VLDL production is facilitated by microsomal triglyceride transfer protein (MTP) in a rate-limiting step that is regulated by insulin. To characterize the underlying mechanism, we studied hepatic MTP regulation by forkhead box O1 (FoxO1), a transcription factor that plays a key role in hepatic insulin signaling. In HepG2 cells, MTP expression was induced by FoxO1 and inhibited by exposure to insulin. This effect correlated with the ability of FoxO1 to bind and stimulate MTP promoter activity. Deletion or mutation of the FoxO1 target site within the MTP promoter disabled FoxO1 binding and resulted in abolition of insulin-dependent regulation of MTP expression. We generated mice that expressed a constitutively active FoxO1 transgene and found that increased FoxO1 activity was associated with enhanced MTP expression, augmented VLDL production, and elevated plasma triglyceride levels. In contrast, RNAi-mediated silencing of hepatic FoxO1 was associated with reduced MTP and VLDL production in adult mice. Furthermore, we found that hepatic FoxO1 abundance and MTP production were increased in mice with abnormal triglyceride metabolism. These data suggest that FoxO1 mediates insulin regulation of MTP production and that augmented MTP levels may be a causative factor for VLDL overproduction and hypertriglyceridemia in diabetes.

Authors

Adama Kamagate, Shen Qu, German Perdomo, Dongming Su, Dae Hyun Kim, Sandra Slusher, Marcia Meseck, H. Henry Dong

×

Figure 1

Effect of FoxO1 on MTP expression in HepG2 cells.

Options: View larger image (or click on image) Download as PowerPoint
Effect of FoxO1 on MTP expression in HepG2 cells. (A) Hepatic MTP protei...
(A) Hepatic MTP protein levels. (B) Hepatic MTTP mRNA levels. (C) Hepatic MTTP mRNA levels. HepG2 cells were transduced with control LacZ or FoxO1 vector or FoxO1 plus CA-Akt vector (MOI, 25 pfu/cell), followed by the determination of MTTP mRNA levels after 24-h incubation in the presence or absence of insulin (100 nM). (D) FoxO1 subcellular distribution. FoxO1 vector-transduced HepG2 cells were incubated with control or CA-Akt vector for 24 h, followed by immunoblot analysis of FoxO1 protein levels in cytosolic and nuclear fractions (E). (F) Hepatic MTTP mRNA levels. HepG2 cells were treated with PI3K activator IRS-1 (Y608) peptide (IRS-1, 1 μM) or inhibitor LY294002 (LY, 10 μM) in the presence and absence of insulin for 24 h. (G) FoxO1 induction of MTP promoter activity. HepG2 cells were cotransfected with 2 μg each pGH11 and pCMV-LacZ vectors in the presence of Adv–FoxO1-ADA vector at doses ranging from 50 to 400 pfu/cell, followed by luciferase activity assay after 24-h incubation. Likewise, HepG2 cells were cotransfected with pGH11 and pCMV-LacZ plasmids together with FoxO1 (H) or FoxO1-ADA (I) vector at a fixed dose (MOI, 100 pfu/cell) in the presence and absence of insulin for the determination of luciferase activity. (J) FoxO1 subcellular distribution. FoxO1 vector-treated HepG2 cells were incubated in the absence or presence of insulin for 30 min, followed by immunoblot analysis of FoxO1 in cytosolic and nuclear fractions (K). (L) Insulin inhibition of MTP promoter activity. FoxO1 vector-transduced HepG2 cells were transfected with MTP promoter–directed reporter system in the presence or absence of insulin. Cells were assayed for luciferase activity at different times. Data represent 3–5 experiments. *P < 0.05, #P < 0.001 versus controls.