Insulin-like growth factor binding protein-2: a novel regulator of skeletal gender differences?

Sex-Specific Control of Muscle Mass: Elevated IGFBP Proteolysis and Reductions of IGF-1 Levels Are Associated with Substantial Loss of Carcass Weight in Male DU6PxIGFBP-2 Transgenic Mice

In farmed animals, carcass weight represents an important economic trait. Since we had demonstrated that IGFBP-2 represents a potent inhibitor of muscle accretion in inbred mice, we wanted to quantify the inhibitory effects of IGFBP-2 under conditions of elevated protein mass in growth selected non-inbred mice (DU6P). Therefore, we crossed male DU6P mice with female IGFBP-2 transgenic mice. Male IGFBP-2 transgenic offspring (DU6P/IGFBP-2) were characterized by more than 20% reductions of carcass mass compared to male non-transgenic littermates. The carcass mass in males was also significantly lower (p < 0.001) than in transgenic female DU6P/IGFBP-2 mice, which showed a reduction of less than 10% (p < 0.05) compared to non-transgenic female DU6P/IGFBP-2 mice. Although transgene expression was elevated in the muscle of both sexes (p < 0.001), serum levels were normal in female, but significantly reduced in male transgenic DU6P/IGFBP-2 mice (p < 0.001). In this group, also IGFBP-3 and IGFBP-4 were significantly reduced in the circulation (p < 0.01). Particularly in male transgenic mice, we were able to identify proteolytic activity against recombinant IGFBP-2 included in diluted serum. IGFBP-proteolysis in males correlated with massive reductions of IGF-1 in serum samples and the presence of elevated levels of IGFBP-2 fragments. From our data, we conclude that elevated tissue expression of IGFBP-2 is an essential effector of muscle accretion and may block more than 20% of carcass mass. However, in the circulation, intact IGFBP-2 contained no reliable biomarker content. Notably, for the estimation of breeding values in meat-producing animal species, monitoring of IGFBP-2 expression in muscle appears to be supported by the present study in a model system.

Gender-specific effects on food intake but no inhibition of age-related fat accretion in transgenic mice overexpressing human IGFBP-2 lacking the Cardin-Weintraub sequence motif

IGFBP-2 affects growth and metabolism and is thought to impact on energy homeostasis and the accretion of body fat via its heparin binding domains (HBD). In order to assess the function of the HBD present in the linker domain (HBD1) we have generated transgenic mice overexpressing mutant human IGFBP-2 lacking the PKKLRP sequence and carrying a PNNLAP sequence instead. Transgenic mice expressed high amounts of human IGFBP-2, while endogenous IGFBP-2 or IGF-I serum concentrations were not affected. In both genders we performed a longitudinal analysis of growth and metabolism including at least 4 separate time points between the age of 10 and 52 weeks. Body composition was assessed by nuclear magnetic resonance (NMR) analysis. Food intake was recorded by an automated online-monitoring. We describe negative effects of mutant human IGFBP-2 on body weight, longitudinal growth and lean body mass (p < 0.05). Very clearly, negative effects of mutant IGFBP-2 were not observed for fat mass accretion throughout life. Instead, relative fat mass was increased in transgenic mice of both genders (p < 0.05). In male mice transgene expression significantly increased absolute mass of total body fat over all age groups (p < 0.05). Food intake was increased in female but decreased in male transgenic mice at an age of 11 weeks. Thus our study clearly provides gender- and time-specific effects of HBD1-deficient hIGFBP-2 (H1d-BP-2) on fat mass accretion and food intake. While our data are in principal agreement with current knowledge on the role of HB-domains for fat accretion we now may also speculate on a role of HBD1 for the control of eating behavior.