Glucocorticoid-regulated transcription networks in macrophage biology

Hospital For Special Surgery, New York NY

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Abstract

Every day, billions of cells in the body undergo apoptosis and are cleared by ‘continual efferocytosis’ - a highly efficient, immunologically silent process whereby professional efferocytes recognize, bind and ingest apoptotic cells (AC), degrade large cargo in the phagosome while also recycling the membrane. Impaired efferocytosis has been linked to many diseases including metabolic syndrome, diabetic ulcers and autoimmunity. A key cell type that gives rise to efferocytes is macrophages, MΦ – versatile immune cells that can produce inflammatory mediators to fight infection (M1) or, conversely, suppress inflammation, perform homeostatic functions and repair injury (M2). Homeostatic polarization can be elicited by disparate stimuli, including interleukin-4 (M2IL4) or glucocorticoids (M2GC). IL4 signals to transcription factor STAT6, which upregulates a critical driver of the M2 state, KLF4. GC act through their cognate receptor, GR, which activates and represses hundreds of genes utilizing various coregulators. Among them, NCoA2/GRIP1 has been implicated in both activities yet, how GR or GRIP1 contribute to MΦ polarization remained obscure. We discovered that M2IL4 and M2GC transcriptomes share a substantial overlap mirrored by a striking shift in chromatin landscape. This convergence occurred at the level of GR and KLF4 transcription complexes and was integrated, in a signal-specific manner, by GRIP1 which served as a cofactor for both and unexpectedly, enabled KLF4 to act as an effector of GC-induced polarization. These studies helped delineate M2 MΦ programming by ‘pure’ signals, however efferocytes are functionally distinct as: 1) homeostatic stimuli co-exist in vivo yielding novel regulatory patterns, 2) ingested ACs represent, uniquely, both signals to shape MΦ phenotype and a source of a large cargo to be metabolized and recycled. To date, unlike the well-dissected bioenergetics, trafficking and broad metabolic adaptations of an efferocyte, the epigenetic and transcriptional circuits that maintain these stable phenotypic changes are unknown. Our objective is to define the key transcription factors and epigenome transitions that impart to homeostatic MΦ a novel efferocytic state, adapt at continual AC clearance - in vitro and in tissue micro- environment. We hypothesize that AC ingestion by homeostatic MΦ upregulates distinct transcription factors which nucleate, in a feed-forward manner, novel enhancer-promoter interactions that establish efferocyte identity. We further posit that GRIP1, an emerging key player in the homeostatic MΦ programming, facilitates efferocytosis in vitro and in vivo. Our Specific Aims are to: 1) Identify AC-inducible drivers of efferocytosis in distinct homeostatic MΦ populations; 2) Dissect epigenomic programming that transforms a homeostatic MΦ into an efferocyte; 3) Decipher the role of GRIP1 in efferocytosis in vitro and in vivo. The successful completion of this project will reveal the conserved global regulatory hubs enacting the transformation of homeostatic MΦ into efferocytes by both discovering new players and delineating the role of GRIP1 in efferocytosis, from molecular mechanisms to complex cell-cell interactions in tissue, in a relevant model of human disease.