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Adipogenesis is an important process for the improvement of insulin resistance by peroxisome proliferator-activated receptor (PPAR) gamma agonists, such as rosiglitazone and pioglitazone. FK614 [3-(2,4-dichlorobenzyl)-2-methyl-N-(pentylsulfonyl)-3-Hbenzimidazole-5-carboxamide] is a structurally novel class of PPARgamma agonist that improves insulin sensitivity in animal models of type 2 diabetes. Herein, we characterize FK614, a selective PPARgamma modulator (SPPARM) with differential properties affecting the regulation of fat cell function. FK614 behaves as a partial agonist in inducing the interaction of PPARgamma with both transcriptional coactivators, cAMP response element-binding protein-binding protein and steroid receptor coactivator-1, but as a full agonist with both PPAR-binding protein and PPAR-interacting protein, which are required for PPARgamma-mediated adipogenesis. In the differentiating 3T3-L1 adipocytes, the levels of adipose fatty acid-binding protein (aP2) mRNA expression and triglyceride accumulation induced by FK614 were as efficacious as those of rosiglitazone and pioglitazone. In contrast, the effect of FK614 on aP2 gene expression in mature adipocytes was less than that of the other PPARgamma agonists. Furthermore, the long-term treatment of mature adipocytes with rosiglitazone and pioglitazone reduced the expression of phosphodiesterase 3B, the down-regulation of which has an important role in the development of insulin resistance; however, FK614 had no such effect in mature adipocytes. Thus, FK614 behaves as an SPPARM with differential effects on the activation of PPARgamma at each stage of adipocyte differentiation. The stage-dependent selectivity of FK614 may contribute to its enhanced insulin sensitization in differentiating adipocytes and to reduced insulin resistance at the stage of adipocyte hypertrophy.

FK614 is a structurally novel class of peroxisome proliferator-activated receptor gamma (PPARgamma) agonist, with the mechanism of its insulin-sensitizing action most likely due to activation of PPARgamma. In this study, properties of FK614 for PPARgamma binding, ability to induce conformational change, and coactivator recruitment were investigated. FK614, rosiglitazone, and pioglitazone competed specific binding of [3H]rosiglitazone to PPARgamma with Ki values of 11 nM, 47 nM, and 1.3 microM, respectively. Limited trypsin digestion of PPARgamma with FK614 or rosiglitazone produced distinct patterns of digested polypeptides, suggesting that FK614 directly binds to PPARgamma but induces specific alterations in receptor conformation. FK614 induced interaction of PPARgamma with nuclear receptor coactivator CBP but of lower magnitude than rosiglitazone and pioglitazone. The estimated Kd values of FK614-, rosiglitazone-, and pioglitazone-PPARgamma complex to CBP peptide were 1.8, 0.64, and 0.72 microM, respectively, indicating FK614-PPARgamma complex exhibits a lower affinity for CBP peptide compared to other agonist-PPARgamma complexes. When tested the effect of FK614 on CBP recruitment induced by 9(S)-hydroxyoctadecadienoic acid, an endogenous ligand, FK614 negatively modulated PPARgamma activation. The unique properties of FK614 may underlie the molecular basis of ligand-dependent transcriptional modulation mediated by PPARgamma.

Peroxisome proliferator-activated receptor gamma (PPARgamma) is a ligand-dependent transcriptional factor implicated in regulating adipogenesis, glucose homeostasis, and in mediating the action of the insulin sensitizing anti-diabetic thiazolidinedione (TZD) compounds. [3-(2,4-Dichlorobenzyl)-2-methyl-N-(pentylsulfonyl)-3-H-benzimidazole-5-carboxamide] (FK614) is a structurally novel PPARgamma agonist that demonstrates potent anti-diabetic activity in vivo. Herein, we describe that FK614 is a selective PPARgamma ligand with specific transactivation properties that are dependent upon the context of coactivators. FK614 dissociates the corepressors NCoR (nuclear receptor corepressor) and SMRT (silencing mediator of retinoid and thyroid hormone receptors) from PPARgamma as effectively as rosiglitazone and pioglitazone, but can also differentially induce a ligand specific interaction of PPARgamma with coactivators. The amount of CBP (CREB-binding protein) and SRC-1 (steroid receptor coactivator-1) recruited by FK614 was less than that induced by rosiglitazone and pioglitazone, but FK614 caused similar PGC-1alpha (PPARgamma coactivator-1alpha) recruitment as these compounds. As a consequence of these ligand-specific differences in the strength of ligand-type specific interactions of PPARgamma and coactivators, FK614 functions as a partial or full agonist for transcriptional activation depending upon the amount of specific coactivators in cells following overexpression. In conclusion, FK614 is a novel, non-TZD type, and selective PPARgamma modulator whose pharmacological properties are distinct from rosiglitazone and pioglitazone.