The TSP family currently comprises five members, of which TSP-1 and TSP-2 (subgroup A) are trimeric matricellular proteins that influence cell function by modulating cell—matrix interactions (18). TSP-2 binds to both jagged1 and Notch3 ectodomains, potentially bridging two essential extracellular components of Notch signaling. TSP-2 and LRP1 (low-density lipoprotein receptor-related protein 1) stimulate Notch activity by driving trans-endocytosis of the Notch ectodomain into the signal-sending cell (19,20).
TSP-1 and TSP-2 are expressed in murine adipose tissues (3). Whereas TSP-1 deficiency did not affect development of obesity in murine models (21), no information is available on a potential functional role of TSP-2. We found that TSP-2−/− mice when kept on SFD or HFD developed adipose tissue to a comparable extent as their wild-type littermates, although a trend to reduced GON fat was observed in TSP-2−/− mice on HFD. We have only studied male mice, as it has been suggested previously that TSP-2 may alter adipose tissue regulation by female sex hormones (22). Two-way repeated measures ANOVA indicated a significantly lower body weight over the 15-week experimental period for TSP-2−/− mice on HFD but not on SFD. However, after overnight fasting at the end of the experiment this difference was no longer observed, probably because TSP-2+/+ mice lost more weight. The composition of SC and GON adipose tissues of both genotypes in terms of adipocyte size and density was very similar. In the mixed genetic background of the mice used in this study, weight gain upon HFD feeding for 15 weeks was less pronounced as compared to e.g., a pure C57Bl/6 genetic background (12). Nevertheless, our data allow to conclude that TSP-2 deficiency in mice has no significant effect on adipose tissue, food intake or physical activity. Furthermore, no significant differences were observed in adipose tissue associated angiogenesis between TSP-2−/− and TSP-2+/+ mice kept on SFD or HFD, as revealed by comparable size and density of blood vessels and comparable size distribution. This appears to be in contrast with the previously reported anti-angiogenic effect of TSP-2 in settings of wound healing and neovascularization (6,7,8,9,10), and the higher tissue density of medium and small blood vessels observed in TSP-2−/− mice (13). The absence of an effect of TSP-2 deficiency could not be explained by compensation through enhanced TSP-1 levels, which are comparable for both genotypes on SFD and somewhat lower for TSP-2−/− mice on HFD. The lack of TSP-2 may lead to aberrant extracellular matrix remodeling, increased neovascularization and reduced contraction, due in part to elevated levels of MMP-2 and MMP-9 (10,23). It was also reported that in TSP-2 deficient mice age-related cardiomyopathy was accompanied by increased MMP-2 activity, paralleled by reduced activity of tissue transglutaminase-2, which impairs collagen crosslinking and thus may contribute to cardiac dilation and dysfunction (11). We have previously reported that deficiency of MMP-2, but not MMP-9, impairs adipose tissue development in mice by contributing to adipocyte hypotrophy (17,24). Because of the suggested link between TSP-2 deficiency and MMP-2 and −9 levels, we have monitored expression and activity of gelatinases in the adipose tissues in this study. However, we did not find an effect of TSP-2 deficiency on MMP-2 mRNA levels in SC or GON adipose tissues, whereas MMP-9 was actually downregulated upon HFD feeding. Zymography with protein extracts of SC and GON adipose tissues of both genotypes did, however, not reveal significant differences in MMP-9 or MMP-2 activity. Thus, the functional interaction between TSP-2, angiogenesis and gelatinase expression/activity previously observed in wound healing and neovascularization does not play a relevant role in adipose tissue development. Interestingly, statistical analysis (two-way ANOVA) indicated an effect of the genotype on fasting plasma glucose levels. However, glucose and insulin tolerance tests did not reveal significant differences between both genotypes suggesting that glucose homeostasis is not markedly affected. It is unclear at present why TSP-2 deficiency is associated with enhanced basal glucose levels, both on SFD and HFD.
TSP-2 was also shown to play a role in inflammatory processes (25). Increased macrophage accumulation, associated with increased expression of proinflammatory compounds, was observed in adipose tissues of obese subjects due to an influx of bone marrow—derived macrophages (26,27,28). In our model, however, absence of TSP-2 had no effect on adipose tissue development. Furthermore, analysis of the content and expression of macrophage markers in SC and GON adipose tissues did not reveal significant differences between the two genotypes, except for a twofold downregulation of interleukin-1 mRNA in TSP-2−/− mice on HFD. Overall, this suggests that macrophage infiltration in the adipose tissues is not markedly affected by TSP-2 deficiency. This suggests that recruitment of precursor cells (from bone marrow) to adipose tissue, if any in this model, is not affected by TSP-2 deficiency.