Carnitine palmitoyltransferase 1A prevents fatty acid-induced adipocyte dysfunction through suppression of c-Jun N-terminal kinase

Abstract

The adipocyte is the principal cell type for fat storage. CPT1 (carnitine palmitoyltransferase-1) is the rate-limiting enzyme for fatty acid β-oxidation, but the physiological role of CPT1 in adipocytes remains unclear. In the present study, we focused on the specific role of CPT1A in the normal functioning of adipocytes. Three 3T3-L1 adipocyte cell lines stably expressing hCPT1A (human CPT1A) cDNA, mouse CPT1A shRNA (short-hairpin RNA) or GFP (green fluorescent protein) were generated and the biological functions of these cell lines were characterized. Alteration in CPT1 activity, either by ectopic overexpression or pharmacological inhibition using etomoxir, did not affect adipocyte differentiation. However, overexpression of hCPT1A significantly reduced the content of intracellular NEFAs (non-esterified fatty acids) compared with the control cells when adipocytes were challenged with fatty acids. The changes were accompanied by an increase in fatty acid uptake and a decrease in fatty acid release. Interestingly, CPT1A protected against fatty acid-induced insulin resistance and expression of pro-inflammatory adipokines such as TNF-α (tumour necrosis factor-α) and IL-6 (interleukin-6) in adipocytes. Further studies demonstrated that JNK (c-Jun N terminal kinase) activity was substantially suppressed upon CPT1A overexpression, whereas knockdown or pharmacological inhibition of CPT1 caused a significant enhancement of JNK activity. The specific inhibitor of JNK SP600125 largely abolished the changes caused by the shRNA- and etomoxir-mediated decrease in CPT1 activity. Moreover, C2C12 myocytes co-cultured with adipocytes pre-treated with fatty acids displayed altered insulin sensitivity. Taken together, our findings have identified a favourable role for CPT1A in adipocytes to attenuate fatty acid-evoked insulin resistance and inflammation via suppression of JNK.