Molecular pathways involved in metabolic control of CCL5 in adipocytes

Obesity is a chronic disorder characterized by a tonic low-gradeactivation of the innate immune system that affects steady-state measures ofmetabolic homeostasis over time. In addition, obesity is often accompanied byelevations in tissue and circulating FFA concentrations. Systemic levels ofFFAs can induce inflammatory cascades in adipocytes and macrophagesthrough TLR4-dependent effect. Signaling through TLR4 activates a broadrange of intracellular cascades that include stimulation of IKK-, NF-kB, JNKand AP1. Indeed, in addition to store excess calories in the form of lipid,adipose tissue produces classical cytokines and chemokines such as MCP-1,IL-8 and CCL5. CCL5, as other chemokines, participates in mediatingleukocyte infiltration of adipose tissue. Moreover circulating CCL5concentrations are elevated in obesity, impaired glucose tolerance (IGT) andtype 2 diabetes. In this study I have investigated the molecular mechanismsinvolved in the metabolic control of CCL5 expression in adipocytes.Cytokine/growth factor screening of conditioned media from 3T3-L1 preadipocytesand adipocytes revealed that adipocytes secreted higher amount ofCCL5 compared to their undifferentiated precursors. Higher concentrations ofglucose and fatty acids (oleate and palmitate) increased CCL5 secretion by3T3-L1 adipocytes. Moreover, both oleate and palmitate enhanced CCL5mRNA levels and induced an activation of JNK, NF-kB, MAPK andPI3K/AKT pathways. In cells treated with JSH23, a NF-kB inhibitor, theeffect of FFAs on CCL5 mRNA levels was reduced thus indicating a directinvolvement of NF-kB. Treatment of the cells with SP600125, a JNKinhibitor, also significantly reduced the stimulatory effect of oleate andpalmitate on CCL5 mRNA and interestingly prevented FFA-induced NF-kBbinding to CCL5 promoter. I have also obtained evidence that insulin exertedan inhibitory effect on CCL5 mRNA and counteracted fatty acid-inducedstimulation. Both PD98059 and LY294002, inhibitors of MAPK and PI3K,respectively, increased CCL5 expression levels reverted anti-inflammatoryeffect of insulin in presence of fatty acids. Consistently, insulin exposurereduced NF-kB recruitment onto CCL5 promoter, and almost completelyprevented fatty acid effect. In conclusion, oleate and palmitate induce CCL5mRNA, possibly via JNK and NF-kB pathways. Fatty acid effect on CCL5 islargely prevented by insulin and may involve PI3K/AKT and MAPK. [edited by Author]