Peroxisome proliferator activated receptor y (PPARy) is poised at the apex of a regulatory network that controls bone physiology, yet it remains unclear how activation of PPARy in the bone marrow may alter the microenvironment that supports life-long B cell development. This is an important problem, as a growing number of environmental contaminants, including Superfund chemicals such as phthalates and organotins, are being recognized for their ability to activate PPARy and its heterodimerization partners the retinoid X receptors (RXR). Our long-term goal is to understand the molecular mechanisms by which individual and complex mixtures of Superfund chemicals impair development in the mammalian immune system, a system that requires ongoing development in the face of continuing pathogen exposures. The objective here is to determine the role of PPARy activation in phthalate- and organotin-induced alteration of bone marrow physiology. We hypothesize that environmental PPAR/RXR ligands suppress B lymphopoiesis by two mechanisms, directly by inducing apoptosis in early B cells and indirectly by altering the bone marrow microenvironment that supports lymphopoiesis, resulting in aging-like suppression of immune responses. We will investigate this hypothesis by pursuing three Specific Aims: 1) Determine the relationship between PPAR and RXR activation and the functional consequences for multipotent mesenchymal stromal cell differentiation by determining changes in the osteogenic transcriptome induced by a phthalate, an organotin, and contaminant mixtures, 2) Determine the mechanisms by which environmental PPAR/RXR agonists damage B lymphopoiesis, both directly and indirectly by defining mechanisms of toxicant-induced apoptosis and by testing contaminant-altered bone marrow environments for the ability to support B cell development, and 3) Determine mechanisms by which in vivo exposure to environmental PPAR/RXR agonists negatively affects bone physiology, lymphopoiesis and immune responses by examining organotin-induced defects in bone integrity, B cell development and B cell function. Critical knowledge will be gained to refine human risk assessment and to improve prevention of both bone loss and immune compromise.
Aging is associated with an impaired immune response to infection, and exposure to environmental obesogens may mimic this phenomenon. Our data suggest that contaminant-induced fat formation in bone results not only in loss of bone quality but also in compromise ofthe microenvironment required to support immune cell development, similar to aging. Results from these studies will provide fundamental information on the interaction of the bone marrow environment and immune cell development and targets for intervention.
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