Genetic downregulation of receptor-interacting protein 140 uncovers the central role of Akt signalling in the regulation of fatty acid oxidation in skeletal muscle cells.

The role of the nuclear co-repressor receptor-interacting protein 140 (RIP140) in metabolic regulation, gene and protein expression and insulin signalling in skeletal muscle cells remains to be delineated. To study this question, L6 myotubes were treated with or without an RNA interference oligonucleotide sequence to downregulate RIP140 expression and incubated with or without insulin (1 μM). Downregulation of RIP140 increased (P < 0.05) basal palmitate uptake (by 20%) and decreased (P < 0.05) basal palmitate oxidation (by 38%). In control small interfering RNA-treated cells, insulin increased (P < 0.05) glucose (by 31%) and palmitate uptake (by 20%) and decreased (P < 0.05) palmitate oxidation (by 35%). However, in RIP140 small interfering RNA-treated cells, insulin did not affect (P > 0.05) palmitate uptake and increased (P < 0.05) palmitate oxidation (by 79%). In insulin-mediated conditions, downregulation of RIP140 decreased (P < 0.05) Akt(Ser473) and atypical protein kinase C-ζ(Thr403/410) phosphorylation. As expected, downregulation of RIP140 was accompanied by an increase (P < 0.05) in cytochrome c oxidase subunit 4 isoform 1 and peroxisome proliferator-activated receptor receptor γ coactivator-1α mRNA content. Downregulation of RIP140 increased (P < 0.05) fatty acid transport protein 1 mRNA content and carnitine palmitoyltransferase 1b protein content and decreased (P < 0.05) medium chain acyl-CoA dehydrogenase mRNA content in basal conditions. In insulin-mediated conditions, downregulation of RIP140 increased (P < 0.05) carnitine palmitoyltransferase 1b, fatty acid transport protein 1 and fibroblast growth factor 21 mRNA content and decreased (P < 0.05) medium chain acyl-CoA dehydrogenase mRNA content and plasma membrane fatty acid translocase/cluster of differentiation 36 protein content. Our data show that, in skeletal muscle cells, RIP140 expression significantly impacts palmitate uptake and oxidation and that alterations in gene expression and Akt-atypical protein kinaseC-ζ signalling can partly explain these changes.