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Journal of Lipid Research, Vol. 45, 1324-1332, July 2004
Copyright © 2004 by American Society for Biochemistry and Molecular Biology

Quantifying carbon sources for de novo lipogenesis in wild-type and IRS-1 knockout brown adipocytes

Hyuntae Yoo^*, Gregory Stephanopoulos and Joanne K. Kelleher^1,

^* Departments of Chemistry, Massachusetts Institute of Technology, Cambridge, MA 02139
Chemical Engineering, Massachusetts Institute of Technology, Cambridge, MA 02139

¹ To whom correspondence should be addressed. e-mail: jkk{at}mit.edu

Studies were conducted to evaluate the flux of various carbon sources to lipogenesis during brown adipocyte differentiation. ¹³C labeling and isotopomer spectral analysis quantified the contribution of metabolites to de novo lipogenesis in wild-type (WT) and insulin receptor substrate-1 knockout (KO) brown adipocytes. Both glucose (Glc) and glutamine (Gln) provided substantial fractions of the lipogenic acetyl CoA for both WT and KO cells in standard media, together contributing 60%. Adding acetoacetate (AcAc; 10 mM) to the medium resulted in a large flux of AcAc to lipid, representing 70% of the lipogenic acetyl CoA and decreasing the contribution of Glc plus Gln to 30%. For WT cells, the fractional synthesis of new fatty acids during 4 days of differentiation was 80% of the total. Similarly, 80% of the lipidic glycerol was derived from Glc in the medium; Gln was not a precursor for glycerol. When Gln was removed from the medium, the contribution of Glc to fatty acid synthesis doubled, replacing most of the contribution of Gln and maintaining total lipogenesis. Conversely, removal of Glc dramatically decreased lipogenesis.

These results indicate that Glc's distinct role in lipid synthesis during differentiation cannot be replaced by other carbon sources, consistent with the role of Glc supplying NADPH and/or glycerol for triglyceride synthesis.

Abbreviations: AC, acetate; AcAc, acetoacetate; Dex, dexamethasone; Glc, glucose; Gln, glutamine; G3P, glycerol-3-phosphate; IBMX, isobutylmethylxanthine; IRS-1, insulin receptor substrate-1; ISA, isotopomer spectral analysis; KO, knockout; MTBSTFA, N-methyl-N-[tert-butyldimethylsilyl]trifluoroacetimide; PEPCK, phosphoenolpyruvate carboxykinase; SIM, selected ion monitoring; TBDMS, tert-butyldimethylsilyl; WT, wild-type

Supplementary key words insulin receptor substrate-1 • ¹³C • isotopomer • gas chromatography-mass spectrometry • adipogenesis • glucose • glutamine • acetoacetate • acetate

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