The epidemic of obesity, insulin resistance and type 2 diabetes with associated co-morbidities is a daunting problem in public health. Metabolic disease involves a strong inflammatory component in which adipose tissue macrophages (M) play a critical pathogenic role. Glucocorticoids (GCs) remain the most common and cost-effective class of medications for managing inflammatory diseases however, GCs are themselves diabetogenic, which hampers their therapeutic utility. In M, GCs broadly inhibit the production of proinflammatory mediators; moreover, GCs promote the polarization of 'alternatively activated' M2 M that display an anti-inflammatory phenotype, and, paradoxically, increase insulin sensitivity. Untangling the dichotomy between the systemic vs. M2-specific metabolic actions of GCs is crucial for understanding the pathogenesis of insulin resistance and for creating anti-inflammatory drugs with improved therapeutic profiles. GCs signal through the GC receptor (GR) a ligand-regulated transcription factor of the nuclear receptor (NR) superfamily that recruits numerous coregulators to activate or repress transcription. Among those, the GR-interacting protein (GRIP) 1 is an established NR coactivator, that we have recently shown to serve as a key GR corepressor attenuating the inflammatory gene expression program in M. Unexpectedly, GRIP1 was also found to cooperate with non-receptor regulators in the immune system. What enables GRIP1 to selectively engage in antagonistic biological pathways or display opposite transcriptional activities is unknown. Recently, we found that GRIP1 undergoes GC-induced, GR interaction-dependent phosphorylation at multiple sites that was required for the induction of a subset of GR targets. The objective of this application is to dissect the role and mechanisms of GR-induced GRIP1 phosphorylation in M as a modulator of anti-inflammatory and metabolic effects of GCs. Our central hypothesis is that liganded GR modifies its own coregulator by enabling GC-response element (GRE)-specific recruitment of GRIP1 kinases, thus dictating GRIP1 function in distinct GR transcription complexes, or preferential utilization of its activating vs. repressing properties.
Our Specific Aims are to: 1) utilize our M-specific conditional GRIP1-deficient mice and GRIP1 phosphosite-specific antibodies to assess the genome-wide distribution of GRIP1 phospho-isoforms in M and determine the functionally relevant binding sites; 2) dissect the function of putative GRIP1 kinases, cyclin-dependent kinase-9 and casein kinase-2, as GRE-specific components of GR transcription complexes; 3) identify (by blocking phosphorylation of endogenous GRIP1 or integrating GRIP1 phosphosite mutants into GRIP1-null cells) the role of GRIP1 phosphorylation in the assembly and gene regulation by GR transcription complexes, as well as in M polarization and the M:adipocyte interactions. This work will yield detailed information on a novel mechanism contributing specificity to GR-mediated gene expression, which should reveal an important facet of metabolic control and new opportunities for the design of safer therapies for inflammatory diseases.
Obesity, metabolic syndrome and type 2 diabetes are on the rise around the world and represent a serious public health problem with an economic cost measured in billions of dollars. Although metabolic disease is characterized by chronic inflammation, one of the key classes of drugs for treating inflammatory diseases is glucocorticoids, which are themselves pro-diabetic. The proposed project aims to understand a newly discovered mechanism contributing to glucocorticoid actions which should help separate their anti-inflammatory effects from undesirable effects on metabolism, and generate safer glucocorticoid-based therapies.
