Phosphatidylinositol 3-kinase-dependent insulin regulation of long-chain fatty acid (LCFA) metabolism in L6 muscle cells: involvement of atypical protein kinase C-ζ in LCFA uptake but not oxidation

Abstract

Insulin is important in the regulation of muscle metabolism. However, its role in the regulation of muscle long-chain fatty acid (LCFA) metabolism, independent of glucose, is not clear. To determine whether insulin regulates LCFA metabolism independent of glucose and if so, via which signaling pathway, L6 myotubes were incubated, in the presence or absence of insulin (100 nM) and with either an inhibitor of phosphatidylinositol 3-kinase (PI3K) (wortmannin (W), 50 nM), protein kinase B (PKB)/Akt (A, 10 mM), or atypical protein kinase C-z (aPKC-z) (mP, 100 mM). LCFA kinetic parameters were measured via incubation with [1-14 C]palmitate. Basal LCFA uptake was found to increase linearly with time (1-60 min) and concentration (50-750 mM). LCFA uptake increased in the presence of insulin and was maximum at 10 nM (P!0 . 05). Wortmannin prevented the insulininduced increase in LCFA uptake and decrease in LCFA oxidation. While mP abolished the insulin-induced increase in LCFA uptake, it did not prevent the insulin-induced decrease in LCFA oxidation. None of the variables were affected by Akt inhibition. These results suggest a direct effect of insulin on LCFA metabolism in muscle cells, and that downstream of PI3K, aPKC-z, but not PKB/Akt mediates the effects of insulin on LCFA uptake but not oxidation.

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

“…Neither basal nor insulin‐mediated glucose and palmitate uptake and oxidation were different ( P > 0.05) between groups (data not shown). In line with published data (Dyck et al 2001; Luiken et al 2002; Kelly et al 2008), insulin significantly ( P < 0.05) increased glucose (by 30–40%) and palmitate uptake (by 20–30%) and significantly ( P < 0.05) decreased palmitate oxidation (by 35–43%) in both the control siRNA and untransfected control groups.…”

“…The rates of glucose and palmitate uptake and of palmitate oxidation were calculated as described in detail by Kelly et al (2008). All presented data are expressed as means ± SEM of three or more experiments and are expressed as a percentage of the control value, where control refers to cells that were not treated with any agent or insulin (see figure legends for specific details).…”

Permissive action of protein kinase C-ζ in insulin-induced CD36- and GLUT4 translocation in cardiac myocytes

“…Recently, inhibition of insulin-stimulated LCFA uptake by z-PS was also reported in the L6 muscle cell line (Kelly et al 2008). This inhibitory action of both PKC inhibitors cannot be attributed to inhibition of insulin-induced Akt/PKB activity because both compounds did not affect Ser473 phosphorylation (Fig.…”

“…However, beyond the involvement of PI3K, it has been hardly investigated whether this similarity extends to members of the PKC family. Only one recent study has reported the involvement of PKC-z in insulin-stimulated LCFA uptake in a cell line, but this study did not attempt to reveal mechanistical causes for this PKC-z action, such as translocation of CD36 (Kelly et al 2008).…”

Adipocyte lipolysis links obesity to breast cancer growth: adipocyte-derived fatty acids drive breast cancer cell proliferation and migration

“…The mechanisms by which breast cancer cells increase fatty acid uptake in a high lipid environment are not well described. Interestingly, adipocytes also secrete insulin-like growth factor 1 (IGF-1) alongside fatty acids [46] and insulin-stimulated glucose and fatty acid uptake is phosphoinositide 3-kinase dependent in skeletal muscle and adipocytes [47, 48]. Using radiometric tracing techniques, we mapped the intracellular fate of extracellular-derived fatty acids in breast cancer cells in the presence of adipocytes.…”