A functional peroxisome proliferator-activated receptor-gamma ligand-binding domain is not required for adipogenesis

The nuclear hormone receptor peroxisome proliferator-activated receptor-gamma (PPARgamma) is the central regulator of adipogenesis. Although it is the target for several drugs that function as agonist activators, a high affinity endogenous ligand for this receptor that is involved in regulating adipogenesis has yet to be identified. Here, we investigated the requirement for ligand activation of PPARgamma in fat cell differentiation, taking advantage of a natural mutant of this receptor that does not bind or become activated by any known natural or synthetic ligand. When ectopically expressed in PPARgamma-null fibroblasts, this Q286P allele was able to strongly promote morphological adipogenesis, without any significant difference compared with wild-type PPARgamma. In addition, no significant differences were found in the expression of several adipogenic genes between the wild-type and Q286P mutant alleles. To extend our studies to an in vivo setting, we performed subcutaneous injections of PPARgamma-expressing fibroblasts into nude mice. We found that both wild-type and Q286P mutant-expressing fibroblasts were able to generate fat pads in the mice. These results suggest that the binding and activation of PPARgamma by agonist ligands may not be required for adipogenesis under physiological conditions.     

Nuclear cytoplasmic shuttling by thyroid hormone receptors. multiple protein interactions are required for nuclear retention

In this report, we have studied the intracellular dynamics and distribution of the thyroid hormone receptor-beta (TRbeta) in living cells, utilizing fusions to the green fluorescent protein. Wild-type TRbeta was mostly nuclear in both the absence and presence of triiodothyronine; however, triiodothyronine induced a nuclear reorganization of TRbeta. By mutating defined regions of TRbeta, we found that both nuclear corepressor and retinoid X receptor are involved in maintaining the unliganded receptor within the nucleus. A TRbeta mutant defective in DNA binding had only a slightly altered nuclear/cytoplasmic distribution compared with wild-type TRbeta; thus, site-specific DNA binding is not essential for maintaining TRbeta within the nucleus. Both ATP depletion studies and heterokaryon analysis demonstrated that TRbeta rapidly shuttles between the nuclear and the cytoplasmic compartments. Cotransfection of nuclear corepressor and retinoid X receptor markedly decreased the shuttling by maintaining unliganded TRbeta within the nucleus. In summary, our findings demonstrate that TRbeta rapidly shuttles between the nucleus and the cytoplasm and that protein-protein interactions of TRbeta with various cofactors, rather than specific DNA interactions, play the predominant role in determining the intracellular distribution of the receptor.