A Novel Partial Agonist of Peroxisome Proliferator-Activated Receptor-γ (PPARγ) Recruits PPARγ-Coactivator-1α, Prevents Triglyceride Accumulation, and Potentiates Insulin Signaling in Vitro

Abstract

Partial agonists of peroxisome proliferator-activated receptor-gamma (PPARgamma), also termed selective PPARgamma modulators, are expected to uncouple insulin sensitization from triglyceride (TG) storage in patients with type 2 diabetes mellitus. These agents shall thus avoid adverse effects, such as body weight gain, exerted by full agonists such as thiazolidinediones. In this context, we describe the identification and characterization of the isoquinoline derivative PA-082, a prototype of a novel class of non-thiazolidinedione partial PPARgamma ligands. In a cocrystal with PPARgamma it was bound within the ligand-binding pocket without direct contact to helix 12. The compound displayed partial agonism in biochemical and cell-based transactivation assays and caused preferential recruitment of PPARgamma-coactivator-1alpha (PGC1alpha) to the receptor, a feature shared with other selective PPARgamma modulators. It antagonized rosiglitazone-driven transactivation and TG accumulation during de novo adipogenic differentiation of murine C3H10T1/2 mesenchymal stem cells. The latter effect was mimicked by overexpression of wild-type PGC1alpha but not its LXXLL-deficient mutant. Despite failing to promote TG loading, PA-082 induced mRNAs of genes encoding components of insulin signaling and adipogenic differentiation pathways. It potentiated glucose uptake and inhibited the negative cross-talk of TNFalpha on protein kinase B (AKT) phosphorylation in mature adipocytes and HepG2 human hepatoma cells. PGC1alpha is a key regulator of energy expenditure and down-regulated in diabetics. We thus propose that selective recruitment of PGC1alpha to favorable PPARgamma-target genes provides a possible molecular mechanism whereby partial PPARgamma agonists dissociate TG accumulation from insulin signaling.

Nuclear Receptors and Their Selective Pharmacologic Modulators

“…A mammalian twohybrid assay using full-length cofactors showed that the coactivator SRC2 (TIF-2) interacts with rosiglitazonebound PPARg but not with F-L-Leu-bound PPARg, whereas both F-L-Leu and rosiglitazone have similar binding of SRC1 (Rocchi et al, 2001). These full-length data differ from peptide recruitment assay results that have demonstrated that SRC 1, 2, and 3 peptides are all less recruited to PPARg by F-L-Leu than by rosiglitazone (Rocchi et al, 2001;Burgermeister et al, 2006). The lack of SRC2 recruitment and the maintained SRC1 recruitment (compared with rosiglitazone) by F-L-Leubound PPARg may be at least partially responsible for the reduced weight gain in F-L-Leu-treated mice.…”

“…However, both rosiglitazone and GW0072 recruited far less NCoR to PPARg compared with vehicle in a mammalian two-hybrid assay (Oberfield et al, 1999). Additionally, GW0072 recruited the PGC1a peptide to a similar degree as rosiglitazone, despite recruiting far fewer SRC1, SRC2, and SRC3 peptides (Burgermeister et al, 2006). Recruitment of PGC1a likely promotes insulin sensitization, as it is upregulated with exercise and likely contributes to some of the observed benefits of exercise on metabolic syndrome, so a strong case for the benefits of PGC1a can be made (Handschin and Spiegelman, 2008).…”

“…A conceptual model of PPARg function is emerging that offers some insight into how SPPARgMs may tune PPARg to produce selective effects . The binding of a particular ligand (or no ligand) by PPARg produces a unique equlibria of conformations and dynamics that favors/disfavors interaction with a particular set of protein kinases, ubiquitin ligases, or coregulators, thus producing ligand-specific transcriptional effects (Oberfield et al, 1999;Rocchi et al, 2001;Fujimura et al, 2005;Pascual et al, 2005;Burgermeister et al, 2006;Kim et al, 2006c;Motani et al, 2009;Choi et al, 2010). Many of the SPPARgMs do show distinct cofactor binding from rosiglitazone or pioglitazone; however, the right combination of coregulator recruitment necessary for desired physical effects remains unclear.…”

Proteasomal degradation of retinoid X receptor α reprograms transcriptional activity of PPARγ in obese mice and humans

“…[54][55][56]. Our data suggest that PPARγ operates differently in normal and pathological tissues as a result of heterodimerization with distinct RXR isotypes.…”

Differential Expression and Function of Caveolin-1 in Human Gastric Cancer Progression

“…Primers covering exon 3 and the 5 ¶-untranslated region of the human caveolin-1 gene were Cav1-S, 5 ¶-TTGGAAGGC-CAGCTTCAC-3 ¶, and Cav1-AS, 5 ¶-CTATCCTTGAAATGTCA-3 ¶ (23). The results were normalized against h 2 -microglobulin (21) and S12 ribosomal protein (22) as previously published.…”