Metabolic Regulation by Lipid Activated Receptors
Obesity and related metabolic disorders have reached epidemic levels with dire public health consequences. Efforts to stem the tide focus on behavioral and pharmacological interventions. Several hypolipidemic pharmaceutical agents target endogenous lipid receptors, including the peroxisomal proliferator activated receptor alpha (PPAR alpha) and cannabinoid receptor 1 (CB1). To further the understanding of these clinically relevant receptors, we elucidated the biochemical basis of PPAR alpha activation by lipoprotein lipolysis products and the metabolic and transcriptional responses to elevated endocannabinoid signaling.
PPAR alpha is activated by fatty acids and their derivatives in vitro. While several specific pathways have been implicated in the generation of PPAR alpha ligands, we focused on lipoprotein lipase mediated lipolysis of triglyceride rich lipoproteins. Fatty acids activated PPAR alpha similarly to VLDL lipolytic products. Unbound fatty acid concentration determined the extent of PPAR alpha activation. Lipolysis of VLDL, but not physiological unbound fatty acid concentrations, created the fatty acid uptake necessary to stimulate PPAR alpha. Consistent with a role for vascular lipases in the activation of PPAR alpha, administration of a lipase inhibitor (p-407) prevented PPAR alpha dependent induction of target genes in fasted mice. Apolipoprotein CIII, an endogenous inhibitor of lipoprotein lipase, regulated access to the lipoprotein pool of PPAR alpha ligands. Our results support a role for the local generation of PPAR alpha ligands by lipase activity.
The endocannabinoid system regulates diverse physiological functions, including energy balance. While loss of CB1 signaling has more pronounced effects on lipid parameters than expected based on weight loss alone, direct demonstration of endocannabinoid regulation of lipid metabolism is lacking. To test the effects of endogenously produced cannabinoids on lipid metabolism, independent of alterations in food intake, we analyzed tissues from mice treated with IDFP, an organophosphorus inhibitor of endocannabinoid breakdown. IDFP administration inhibited hepatic monoacylglycerol lipase leading to elevated levels of 2-arachidonylglycerol. We found that IDFP administration caused accumulation of apoE depleted VLDL. HDL particles accumulated apoE and failed to transfer it to VLDL in vitro. Importantly, these effects were prevented by pharmacological inhibition of CB1 and absent in plasma from CB1 mice. IDFP also caused a CB1-dependent increase in hepatic triglycerides. Thus, endocannabinoids inhibit the transfer of apoE from HDL to VLDL leading to apoE depletion of triglyceride rich lipoproteins.
Microarray analysis allowed us to determine the effects of IDFP on expression of genes involved in lipid metabolism and to discover novel cannabinoid responsive genes. IDFP increased expression of lipogenic and SREBP2 target genes in a CB1-dependent fashion. On a global scale, pre-administration of a CB1 antagonist prevented many of the IDFP induced alterations in gene expression. IDFP decreased expression of genes involved in amino acid metabolism and inflammation. PCR analysis of selected mRNAs confirmed several of the key array findings. Our work indicates that endocannabinoids exert a large influence on hepatic lipid metabolism independent of food intake and suggest that peripherally restricted CB1 antagonists may be of therapeutic value.
Overall, these findings shed light on the endogenous mechanisms of PPAR alpha activation and the hepatic responses to Cb1 activation. This information may help guide the continuing effort to develop treatments for metabolic disease.