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. 2009 Feb;9(2):165-76.
doi: 10.1016/j.cmet.2009.01.002.

Molecular mechanisms of hepatic steatosis and insulin resistance in the AGPAT2-deficient mouse model of congenital generalized lipodystrophy

Víctor A Cortés  1 , David E Curtis, Suja Sukumaran, Xinli Shao, Vinay Parameswara, Shirya Rashid, Amy R Smith, Jimin Ren, Victoria Esser, Robert E Hammer, Anil K Agarwal, Jay D Horton, Abhimanyu Garg
Affiliations

Molecular mechanisms of hepatic steatosis and insulin resistance in the AGPAT2-deficient mouse model of congenital generalized lipodystrophy

Víctor A Cortés et al. Cell Metab. 2009 Feb.

Abstract

Mutations in 1-acylglycerol-3-phosphate-O-acyltransferase 2 (AGPAT2) cause congenital generalized lipodystrophy. To understand the molecular mechanisms underlying the metabolic complications associated with AGPAT2 deficiency, Agpat2 null mice were generated. Agpat2(-/-) mice develop severe lipodystrophy affecting both white and brown adipose tissue, extreme insulin resistance, diabetes, and hepatic steatosis. The expression of lipogenic genes and rates of de novo fatty acid biosynthesis were increased approximately 4-fold in Agpat2(-/-) mouse livers. The mRNA and protein levels of monoacylglycerol acyltransferase isoform 1 were markedly increased in the livers of Agpat2(-/-) mice, suggesting that the alternative monoacylglycerol pathway for triglyceride biosynthesis is activated in the absence of AGPAT2. Feeding a fat-free diet reduced liver triglycerides by approximately 50% in Agpat2(-/-) mice. These observations suggest that both dietary fat and hepatic triglyceride biosynthesis via a monoacylglycerol pathway may contribute to hepatic steatosis in Agpat2(-/-) mice.

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Figures

Figure 1
Figure 1. Strategy to disrupt Agpat2 in mice. Agpat2−/− mice have markedly reduced total hepatic AGPAT activity, complete lack of adipose tissue and abnormal energy metabolism
(A) The map of the wild type allele is shown for exons 1–6 of Agpat2. The genomic sequence spanning a portion of the promoter and exons 1–4 were replaced with the neomycin-resistance cassette (NEO). Abbreviation: Herpes simplex virus thymidine kinase, HSC-TK (B) Total AGPAT activity in the liver homogenates of the Agpat2−/− mice revealed significantly reduced activity (10% of that measured in wild type mouse livers). Livers from 6 Agpat2−/− mice and 6 wild type mice were studied. (C) Magnetic resonance imaging of the male and female wild type and Agpat2−/− mice. Coronal sections were obtained on the whole body mid-section for 3 males and 3 females Agpat2−/− and 3 male and 3 female wild type mice. The wild type mice show adipose tissue in the subcutaneous and intra-abdominal regions as areas of increased signal intensity on T-1 weighted images. The Agpat2−/− mice in contrast have no adipose tissue. (D) energy intake, (E) water intake, (F) O2 consumption (G) CO2 production, (H) Respiratory Exchange Ratio (RER) and (I) RER time course in wild type (unfilled bars and hatched bars for the day and night data, respectively) and Agpat2−/− mice (filled gray and black bars for the day and night data, respectively). The data are normalized to lean body mass, which was 5.7% greater in Agpat2−/− mice. Bars represent the mean and the whiskers represent SEM. * indicates p<0.05 in the comparison between the two genotypes for the same period (day or night).
Figure 2
Figure 2. Immunoblot analysis of insulin receptor subunit β (IR), IRS-1, IRS-2 and MGAT1 and models for cellular TG biosynthetic pathways in the wild type and Agpat2−/− mice
(A) Whole cell extracts were prepared from the livers of four wild type and Agpat2−/− female mice and equal aliquots (30 μg) were individually transferred to nitrocellulose membranes, incubated with the corresponding primary and IRDye800-conjugated secondary antibodies and scanned as described in “Supplementary Experimental Procedures”. Receptor associated protein (RAP) was used as the loading control. (B) The absolute signal intensity of IR, IRS-1 and IRS-2 was normalized to the corresponding signal of RAP and the mean values of the ratios were compared. Bars represent the mean ± S.E.M. * indicates p< 0.05, Student’s t test. (C) Pathways for hepatic TG accumulation in the wild type and Agpat2−/− mice. The left panel shows that hepatocytes can synthesize TG from either the glycerol-3-phosphate (G-3-P) or the monoacylglycerol (MAG) pathway and accumulate TG from receptor-mediated uptake of chylomicron remnants. G-3-P is the initial substrate for acylation at the sn-1 position by G-3-P acyltransferases (GPATs), to form 1-acylglycerol-3-phosphate or lysophosphatidic acid (LPA). LPA is further acylated at the sn-2 position by 1-acylglycerol-3-phosphate acyltransferases (AGPATs) to form phosphatidic acid (PA). In the next step, the phosphate group (Pi) is removed by type 1 phosphatidate phosphohydrolases (PAPs) to produce diacylglycerol (DAG). DAG is further acylated at the sn-3 position by DAG acyltransferases (DGATs) to produce triacylglycerol (TG). In addition, TG can also be synthesized via the acylation of 1- or 2 MAG by the enzymes MAG-acyltransferases (MGATs). Normally, most of TG biosynthesis occurs via G-3-P pathway. In Agpat2−/− mice (Right Panel), the suppression of AGPAT activity interrupts the classical G-3-P pathway which might result in accumulation of intracellular LPA. The concomitant overexpression of lipid phosphate phosphatases (LPPs) and MGAT1 observed in livers of Agpat2−/− mice could divert LPA to 1-MAG and DAG through an alternate pathway, which would allow the assembly of TGs by DGATs-mediated acylation. Additionally, the marked reduction in the hepatic steatosis observed in Agpat2−/− mice upon feeding fat-free diet suggests that dietary fat also contributes to the accumulation of TG in the liver. (D) Total membranes were prepared from the livers of the wild type and Agpat2−/− mice described in Table 2. Individual aliquots (30 μg) of membrane proteins were transferred into nitrocellulose membranes, incubated with actin or MGAT1 primary antibodies and IRDye680 or IRDy800-conjugated secondary antibodies, and quantified as described in “Supplementary Experimental Procedures”. Each specific MGAT1 signal was normalized to the respective actin loading control and then normalized to the MGAT1/Actin average signal in male wild type mice. The mean of each group is represented ± SEM * denotes P < 0.05.
Figure 3
Figure 3. Agpat2−/− mice have no change in liver nuclear SREBPs and ChREBP protein and mitochondrial fatty acid oxidation but have increased levels of lipogenic proteins and rates of fatty acids and TG synthesis
(A) Immunoblot analysis of SREBP-1, SREBP-2, and ChREBP. Nuclear proteins were prepared from individual livers of 5 male wild type and Agpat2−/− mice. Equal aliquots from each liver were pooled and 20 μg of nuclear extract protein was subjected to SDS-PAGE and immunoblotting using anti-mouse SREBP-1, SREBP-2, and ChREBP antibodies. The cAMP response element-binding (CREB) protein was used as the loading control. (B) Immunoblot analysis of ACC1 and FAS. Total cell lysate protein was isolated from individual livers of 4 male wild type and Agpat2−/− mice. Equal aliquots were pooled and 30 μg of protein was subjected to 8% SDS-PAGE for immunoblot analysis. RAP was used as a control for loading. (C) In vivo rates of hepatic fatty acid and sterol synthesis in 10 wild type and 8 Agpat2−/− mice using [3H]2O. Each bar represents the mean ± SEM. * indicates p<0.05 between the two genotypes using Student’s t-test. (D) TG synthesis rates were determined in primary hepatocytes from four wild type and Agpat2−/− female mice by measuring the incorporation of [14C]glycerol into newly synthesized TGs. Bars represent the mean ± SEM. * indicates p<0.05 between the two genotypes using Student’s t-test. (E) Hepatic mitochondrial β-oxidation was measured as production of acid soluble products in wild type and Agpat2−/− mice of both genders (n=4 each). Bars represent mean ± SEM.
Figure 4
Figure 4. Sexual dimorphism in the hepatic lipid composition of Agpat2−/− mice
(A) and (B) Hepatic concentrations of phosphatidyl ethanolamine (PE), phosphatidyl choline (PC), phosphatidic acid (PA), phosphatidyl serine (PS) and phosphatidyl glycerol (PG) in mol per g of liver tissue in male and female wild type and Agpat2−/− mice. (C)–(F) Concentrations of monoacylglycerol (MAG) and diacylglycerol (DAG) in the liver homogenates from the male and female wild type and Agpat2−/− mice. No statistically significant differences were found between the female wild type and Agpat2−/− mice among the various phospholipid, MAG, and DAG species measured. * indicates p< 0.05 for the comparison between the male wild type and Agpat2−/− mice.
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References

    1. Agarwal AK, Arioglu E, de Almeida S, Akkoc N, Taylor SI, Bowcock AM, Barnes RI, Garg A. AGPAT2 is mutated in congenital generalized lipodystrophy linked to chromosome 9q34. Nat Genet. 2002;31:21–23. - PubMed
    1. Agarwal AK, Barnes RI, Garg A. Functional characterization of human 1-acylglycerol-3-phosphate acyltransferase isoform 8: cloning, tissue distribution, gene structure and enzymatic activity. Arch Biochem Biophys. 2006;449:64–76. - PubMed
    1. Agarwal AK, Garg A. Congenital generalized lipodystrophy: significance of triglyceride biosynthetic pathways. Trends Endocrinol Metab. 2003;14:214–221. - PubMed
    1. Agarwal AK, Garg A. Genetic disorders of adipose tissue development, differentiation, and death. Annu Rev Genomics Hum Genet. 2006;7:175–199. - PubMed
    1. Agarwal AK, Simha V, Oral EA, Moran SA, Gorden P, O’Rahilly S, Zaidi Z, Gurakan F, Arslanian SA, Klar A, Ricker A, White NHLB, Herbst K, Kennel K, Patel SB, Al-Gazali L, Garg A. Phenotypic and genetic heterogeneity in congenital generalized lipodystrophy. J Clin Endocrinol Metab. 2003;88:4840–4847. - PubMed

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