Abstract
Background: Adipose tissue is a heterogeneous endocrine organ with tremendous capability for expansion. The antithetical pathogenicity of visceral adipose tissue (VAT), compared to subcutaneous adipose tissue (SAT), has been linked to the metabolic stress of enlarging mature adipocytes and a limited ability to recruit new adipocytes. One of the major distinguishing features of VAT preadipocytes is the high expression of Pregnancy Associated Plasma Protein–A (PAPP-A) when compared to SAT. PAPP-A is a zinc metalloprotease that is secreted, and can associate with the cell surface in an autocrine or paracrine fashion. It is the only known physiological IGFBP-4 (Insulin-like Growth Factor Binding Protein) protease. It cleaves the IGF/IGFBP-4 complex, releasing IGF, making it more bio-available for receptor engagement and downstream signaling. The role of IGFs in adipogenic differentiation is well established. While there is quantitative depot-specific variability in PAPP-A expression among preadipocytes, mature adipocytes do not express any PAPP-A. These findings suggest that there may be a relationship between PAPP-A inhibition and adipogenic differentiation and maturation. Similar to human VAT, PAPP-A expression is highest in visceral fat in murine models. The PAPP-A KO mice, when fed a high fat diet, showed restrained visceral adiposity and decreased visceral adipocyte size, suggesting that PAPP-A could regulate adipogenesis locally in tissues that express high PAPP-A.
Hypothesis: PAPP-A inhibition is a novel anti-obesity treatment strategy. Methods/Results: We fed 20 male and 20 female wild type mice 42% high fat diet (HFD) starting at 10 weeks of age. Concomitantly, we treated 10 mice in each group with either mAb-PA1/41 (a PAPP-A neutralizing monoclonal antibody) or IgG2a (control isotope), intraperitoneally at a dose of 30 mg/kg weekly for the duration of the HFD. At the end of 15 weeks, the mice were sacrificed and the adipose tissue, serum and solid organs were harvested.
Compared to the control (IgG2a) mice, the mAb-PA1/41 treated male and female mice gained 40% less weight (P = 0.03) and had smaller visceral fat depots (mesenteric and pericardial). Also, when we looked at individual adipocyte size, the drug treated mice had 45% smaller mesenteric adipocytes (P = 0.002) and 44% smaller pericardial adipocytes (P= 0.003). Also, the visceral depots in the drug treated mice had 30% more cells (P = 0.006). In both groups, there was decreased liver lipid content (P=0.005). The mAb-PA1/41 treatment had no significant effect on subcutaneous fat depots.
Conclusion: Pharmacologic inhibition of PAPP-A decreased weight gain, visceral fat depot weight, visceral adipocyte size, hepatic lipid deposition and increased visceral adipocyte cell number in both male and female mice that were fed a high fat diet.
Glycerolipid biosynthesis in rat adipose tissue. Influence of adipocyte size