Peroxisome proliferator-activated receptors (PPAR) are ligand-activated transcription factors containing three isoforms (α, β/δ, γ) belonging to the nuclear hormone receptor superfamily. PPARα is a specific pharmacological target for dyslipidemia in humans, but prolonged administration of PPARα agonists causes peroxisome proliferation and hepatocarcinogenesis in rodents. Proliferation and cancer is not observed in humans suggesting divergence between mouse and human PPARα activation. The central hypothesis of this work is that PPARα is differentially regulated by LXXLL motif-containing coactivators and the forkhead box O subclass of proteins. The interaction of LXXLL motifs and PPARs was assessed utilizing fusion proteins containing the core motif in a mammalian expression vector. These experiments demonstrated the ability of fusion proteins to differentially block transcriptional activation of mouse PPAR isotypes and the human PPARα. Species specific PPARα activation was also caused by co-activators. Real time RT-PCR analysis of target genes with PPARα-null, human PPARα transgenic, and wild type mice suggested that these receptors differentially regulate target genes. Thus, PPARα activation can be traced to inherent species differences in the receptor dependent on both LXXLL motif recruitment and specific ligand activation.

The forkhead box protein subclass O (FOXO) is a transcription factor implicated in crosstalk with nuclear receptors. FOX proteins are transcription factors containing a conserved 100 amino acid DNA binding domain that interacts with the insulin responsive sequence as a monomer. FOXO1A is potentially involved in crosstalk with PPARs due to the regulation of gluconeogenesis, and the present set of experiments examines this crosstalk. PPARs and FOXO1A were co-expressed revealing mutual co-repression of PPARs and FOXO1A in a reporter system. One-hybrid analysis of FOXO and PPARα or RXR revealed that the FOXO1A-mediated repression of PPARα is independent of heterodimerization. Immunoprecipitation of FOXO1A and PPARα failed to demonstrate direct interaction between these proteins. Repression of PPARα mRNA message cells was able to prevent PPARα ligand treatment from repressing endogenous FOXO activation of a specific target gene. These two nutritionally-regulated pathways are involved in crosstalk suggesting the activation of one pathway may repress the transcriptional activation of the other. In conclusion, mouse and human PPARα functionally differ.