c-MYC Triggers Lipid Remodelling During Early Somatic Cell Reprogramming to Pluripotency.

Metabolic rewiring and mitochondrial dynamics remodelling are hallmarks of cell reprogramming, but the roles of the reprogramming factors in these changes are not fully understood. Here we show that c-MYC induces biosynthesis of fatty acids and increases the rate of pentose phosphate pathway. Time-course profiling of fatty acids and complex lipids during cell reprogramming using lipidomics revealed a profound remodelling of the lipid content, as well as the saturation and length of their acyl chains, in a c-MYC-dependent manner. Pluripotent cells displayed abundant cardiolipins and scarce phosphatidylcholines, with a prevalence of monounsaturated acyl chains. Cells undergoing cell reprogramming showed an increase in mitochondrial membrane potential that paralleled that of mitochondrial-specific cardiolipins. We conclude that c-MYC controls the rewiring of somatic cell metabolism early in cell reprogramming by orchestrating cell proliferation, synthesis of macromolecular components and lipid remodelling, all necessary processes for a successful phenotypic transition to pluripotency. c-MYC promotes anabolic metabolism, mitochondrial fitness and lipid remodelling early in cell reprogramming. A high rate of aerobic glycolysis is crucial to provide intermediaries for biosynthetic pathways. To ensure the availability of nucleotides, amino acids and lipids for cell proliferation, cells must provide with a constant flux of the elemental building blocks for macromolecule assembly and fulfil the anabolic demands to reach the critical cellular mass levels to satisfactorily undergo cell division. A high rate of aerobic glycolysis is induced by c-MYC, increasing the amounts of intracellular Glucose-6-phosphate (G6P), fructose-6-phosphate (F6P), and glyceraldehyde-3-phosphate (GA3P), which can all enter pentose phosphate pathway (PPP) to produce Ribose-5-Phosphate (R5P) and NADPH, which are necessary for the biosynthesis of biomolecules such as proteins, nucleic acids, or lipids. C-MYC-dependent activation of glucose-6-phosphate dehydrogenase (G6PD) may play a critical role in the shunting of G6P to PPP and generation of NADPH. High glycolytic flux increases the amounts of dihydroxyacetone phosphate (DHAP), which is crucial for biosynthesis of phospholipids and triacylglycerols, and pyruvate (Pyr), which can be converted to citrate (Cit) in the mitochondria and enter the biosynthesis of fatty acids (FA). During cell reprogramming, c-MYC-dependent lipid remodelling leads to Polyunsaturated Fatty Acid (PUFA) downregulation and Monounsaturated Fatty Acid (MUFA) upregulation, which may play critical roles in cytoarchitectural remodelling of cell membrane or non-canonical autophagy, respectively. Cardiolipin (pink dots) rise early in cell reprogramming correlates with an increase in mitochondrial fitness, suggesting that c-MYC may restore proper levels of cardiolipins and antioxidant proteins, such as UCP2, to guarantee an optimal mitochondrial function while upholding ROS levels, reinforcing the idea of cell rejuvenation early in cell reprogramming.

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