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Tissue-specific differences in PPARγ control of macrophage function.

ABSTRACT: PPARγ is known for its anti-inflammatory actions in macrophages. However, which macrophage populations express PPARγ in vivo and how it regulates tissue homeostasis in the steady state and during inflammation is not completely understood. We show that lung and spleen macrophages constitutively expressed PPARγ, while other macrophage populations did not. Recruitment of monocytes to sites of inflammation was associated with induction of PPARγ as they differentiated to macrophages. Its absence in these macrophages led to failed resolution of inflammation, characterized by persistent, low-level recruitment of leukocytes. Conversely, PPARγ agonists supported an earlier cessation in leukocyte recruitment during resolution of acute inflammation and likewise suppressed monocyte recruitment to chronically inflamed atherosclerotic vessels. In the steady state, PPARγ deficiency in macrophages had no obvious impact in the spleen but profoundly altered cellular lipid homeostasis in lung macrophages. Reminiscent of pulmonary alveolar proteinosis, LysM-Cre x PPARγflox/flox mice displayed mild leukocytic inflammation in the steady-state lung and succumbed faster to mortality upon infection with S. pneumoniae. Surprisingly, this mortality was not due to overly exuberant inflammation, but instead to impaired bacterial clearance. Thus, in addition to its anti-inflammatory role in promoting resolution of inflammation, PPARγ sustains functionality in lung macrophages and thereby has a pivotal role in supporting pulmonary host defense.

ORGANISM(S): Mus musculus

PROVIDER: GSE32034 | GEO | 2012/10/03

SECONDARY ACCESSION(S): PRJNA147493

REPOSITORIES: GEO

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Project description:Inflammation is the natural defensive response of the immune system to an injury or infection and is regulated by small molecule mediators triggering different phases of the inflammatory process. In particular, lipid mediators (LM) and cytokines exhibit crucial regulatory functions in the progression and resolution of inflammation. Macrophages play a central role in this process and can adopt distinct phenotypes with specialized functions depending on their microenvironment: inflammatory M1 macrophages drive inflammation by the release of pro-inflammatory cytokines and LMs, like prostaglandins (PG) and leukotrienes (LT), while resolving M2 macrophages promote inflammation resolution and tissue regeneration by production of anti-inflammatory cytokines and specialized pro resolving mediators (SPM). Aging is associated with chronic and unresolved, low-grade inflammation (“inflammaging”) and aging-related dysfunction of macrophages in the resolution of inflammation and tissue maintenance has been reported. Yet, the underlying molecular mechanisms and functional consequences of latter processes remain poorly understood. Here, we show that polarization of peritoneal macrophages (PM) from geriatric mice towards an M2-like phenotype is impaired versus adult mice, resulting in aberrant LM formation and cytokine release. In PMs isolated from adult mice (PM-A) we observed a shift in LM formation from PGs and LTs to SPMs already after 4 h of polarization towards M2 with interleukin (IL) 4. In contrast, PMs from geriatric mice (PM-G) produced mainly LTs and PGs upon polarization. This pattern persists over the course of 48 h of polarization. Proteomic profiling revealed that polarization of PM A towards M2 yields a more distinct phenotype, clearly separated from M1, when compared to PM-G. We observed similar aging-related changes in the lipidome and cytokine profile of spleen, lung and liver tissue from mice. Hence, we hypothesize that during aging macrophage polarization towards M2 is impaired, which in turn drives chronic inflammation and disturbs tissue maintenance. By combining state-of-the art lipidomic and proteomic profiling we aim to uncover new molecular targets for pharmaceutical interventions to improve therapeutic strategies for elderly patients with chronic inflammatory diseases.

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Tissue-specific differences in PPARγ control of macrophage function.

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