Studies on the function of the NGFIB subfamily of nuclear receptors in adipose tissue
The NGFIB family of nuclear receptors consists of three members: Nurrl, nur77 and NOR-1. Genes from this family are classified as immediate early genes in that they are rapidly and transiently induced by a variety of stimuli, including neurotransmitters, growth factors, cAMP and fatty acids. Although identified as having no putative ligand binding domain, regulation of these nuclear receptors may be at the level of phosphorylation or RXR heterodimerisation. Nurrl and nur77 are expressed in adipose tissue at levels greater than in liver, spleen, lung and kidney, in both human and mouse tissue. During adipogenesis in both 3T3-L1 cells and primary sub-cutaneous murine preadipocytes, there is induction of Nurrl and nur77 expression between 60 and 120 minutes after which expression subsides. Using different protocols to induce differentiation it was demonstrated that isobutylmethylxanthine (IBMX), a phosphodiesterase inhibitor which elevates intracellular cAMP, was a critical requirement for inducing the transient Nurrl expression as well as adipogenesis. In order to investigate the effect of Nurrl expression in adipogenesis, an adenovirus mediated over-expression system was employed. Over-expression of Nurrl resulted in a decrease in adipogenesis, as well as decreases in the expression of PPARy and C/EBPa. Examination of adipocyte morphology illustrated that cells over-expressing Nurrl had less lipid accumulation, as assessed by Oil red O and by confocal microscopy. Furthermore mRNA analysis of adipocyte markers showed reduced levels of Hormone Sensitive Lipase (HSL), CD36 and Perilipin expression. In addition, the effect on adipocyte function, namely lipolysis and glucose uptake, was also investigated. Results suggest that Nurrl does not significantly alter glucose uptake but does elevate lipolysis. Subsequent to the findings above, microarray analysis has been performed on control and Nurrl over-expressing 3T3-L1 cells at two time points (72hrs and 192hrs), post differentiation. Analysis of the microarray data confirmed the earlier findings on adipogenesis and has identified a number of additional pathways, in which Nurrl would appear to play a role, namely - NO pathway, steroidogenesis and cytokine/prostaglandin regulation. Preliminary work has been performed with siRNA to knock-down the gene expression of Nurrl in order to investigate potential consequences on adipogenesis. In conclusion, a supposed role for Nurrl has been demonstrated in adipogenesis and lipid accumulation and also adipocyte secretory and metabolic function, with implications for insulin resistance at the adipocyte level.