Selective intranuclear redistribution of PPAR isoforms by RXRα

The intracellular localization of transcriptionally active green fluorescent protein (GFP) chimeras linked to PPARs for human PPARalpha (GFP-PPARhalpha) and mouse PPARalpha, beta, and gamma1 (GFP-PPARmalpha, GFPPPARmbeta, and GFP-PPARmgamma, respectively) was examined in the mouse hepatoma cell line, Hepa-1, using fluorescence microscopy. A predominantly nuclear and diffuse distribution of each isoform was found in both the presence and absence of specific ligands for each receptor. GFP-PPARmalpha-G (containing a Glu282Gly substitution of PPARmalpha) and a phosphorylation mutant, GFP-PPARmgamma-A (containing a Ser82Ala substitution of PPARmgamma), exhibited altered transcriptional activities, but displayed similar intracellular localization patterns compared with their respective wild-type receptors. Coexpression of nuclear receptor corepressor suppressed, whereas steroid receptor coactivator-1 enhanced the transcriptional activity of each of the GFP-PPAR isoforms, but did not discernibly alter their intracellular distributions, both in the presence and absence of PPAR ligands. Interestingly, coexpression of the obligate heterodimeric partner of PPARs, RXRalpha, resulted in an intranuclear redistribution of the GFP-PPARmgamma isoform characterized by a reticulated pattern of the green fluorescent label for PPARgamma within the nucleus, but not in nucleoli, and a heightened concentration of the fluorescent label surrounding nucleolar structures and at the nuclear membrane. Conversely, coexpression of yellow fluorescent protein-RXRalpha and native PPARmgamma resulted in a similar distribution of the yellow fluorescent tag. This localization pattern was not discernibly altered by PPARgamma or RXRalpha-specific ligands. These results implicate RXRalpha in the nuclear reorganization of PPARgamma and suggest that PPARgamma colocalizes with RXRalpha at specific locations within the nucleus independent of added ligand.