Pregnancy-associated plasma protein-A (PAPP-A) is a zinc metalloprotease that was discovered to function outside of pregnancy to enhance local insulin-like growth factor (IGF) bioavailability. Mice deficient in PAPP-A (gene knock-out) have a 30-40% increase in both median and maximum lifespan compared to wild-type littermates, without evidence of endocrine abnormalities or caloric restriction. However, the particular mechanisms underlying this longevity phenotype are poorly understood. In preliminary experiments, we observed that PAPP-A knock-out mice on a high fat diet were resistant to fat accumulation in the mesenteric fat depot (mouse equivalent to human visceral fat) where there was a significant reduction in cell size. Were these pre-adipocytes that were not being stimulated to differentiate due to decreased IGF-I bioavailability in the absence of PAPP-A? Or was there suppressed lipid accumulation by mature adipocytes? Moreover, why was the defect relatively specific for the mesenteric fat depot in the global PAPP-A knock-out model? Did it have anything to do with differential PAPP-A expression? Micro-array analyses of RNA from human and non-human primate pre-adipocytes may be relevant to the last question. In these experiments, PAPP-A was the most distinctive gene found to be overexpressed, with levels in pre-adipocytes from visceral fat depots greatly exceeding those in subcutaneous fat. Furthermore, the most potent stimulators of PAPP-A expression are pro- inflammatory, which are associated with both aging and obesity. Our primary hypothesis is that PAPP-A stimulates adipogenesis in an IGF-dependent manner and with depot-specific effects on pre-adipocyte proliferation and differentiation. The corollary is that inhibition of PAPP-A will moderate adipogenesis preferentially in visceral fat. This has potential importance since major clinical consequences occur with increased visceral fat relative to subcutaneous fat. Our secondary hypothesis is that inhibition of PAPP-A will prolong lifespan even in animals on a high fat diet.
Specific Aims : (1) Determine age-related changes in depot- specific adipogenesis and the role of PAPP-A in this regulation, (2) Determine the lifespan of PAPP-A knock- out and wild-type mice fed a high fat diet, and (3) Determine the effect of conditional PAPP-A gene knock-out in adult mice on depot-specific adipogenesis and lifespan. All expertise and model systems are in hand to conduct this research. Significance: These studies will provide new insight into regulation of fat distribution and function and elucidate mechanisms by which PAPP-A deficiency promotes longevity. Impact: Our findings could have clinical implications for novel strategies using PAPP-A as a preventive target for visceral fat accumulation and its life-shortening morbidities.
Increased visceral fat relative to subcutaneous fat is associated with increased risk of severe clinical complications. Thus, the proposed studies to better understand the factors regulating the different fat tissue depots, with particular focus on a novel enzyme, could have major implications for potential treatment and/or prevention strategies for obesity and obesity-related morbidities and the consequent extension of a healthy lifespan.
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