PPARγ, an X-ceptor for Xs

Evidence from both human genetic studies and characterization of peroxisome proliferator-activated receptor gamma (PPARgamma) knockout mice suggested that the prime function of PPARgamma is fat formation and that its role in insulin sensitization might be secondary to this function. The thrifty function of PPARgamma was most likely evolutionary beneficial, but might in times of plenty contribute to the pathogenesis of disorders, such as obesity, insulin resistance, type 2 diabetes, and hyperlipidemia, often commonly referred to as syndrome X. This role of PPARgamma in these diseases also questions the eventual therapeutic benefits of pure PPARgamma activation, which is associated with an increase in adipose tissue mass. We characterized a new chemical class of PPARgamma agonists, that is, FMOC-l-leucine (FLL). FLL induces a different conformation of PPARgamma relative to classical PPARgamma ligands. Mass spectrometry indicates that two molecules of FLL bind to a single PPARgamma molecule, making its mode of receptor interaction distinctive. FLL recruits a different set of coactivators and activates PPARgamma with a lower potency, but a similar maximal efficacy, relative to known PPARgamma ligands. In contrast, FLL is a more effective insulin sensitizer than current PPARgamma agonists, an effect potentially linked to its weak adipogenic activity. These data make a strong point for potential therapeutic benefits of PPARgamma modulation rather than activation.