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  • In terms of its protease activity MME has a


    In terms of its protease activity, MME has a broad range of substrates being able to target glucagon, bradykinin, GLP1, and several other tepp of circulating small molecules [25]. MME has been shown to target free insulin B-chain [32], although whether MME could target and degrade the insulin receptor is unknown. Our data suggest that MME is affecting some aspect of INSR trafficking such that the alpha subunit accumulates, whereas the beta subunit does not. Several factors can contribute to this observation. One is that the alpha subunit has a longer half-life than the beta subunit in the presence of insulin [66]. Another is that the glycosylation state of the receptor affects not only western blot detection but also the processing and degradation of the receptor [67]. Lastly, previous research has shown that glucose deprivation causes aberrant glycosylation of the insulin proreceptor that results in an accumulation of the beta subunit but not the alpha subunit [68]. MME may be part of a larger class of extracellular proteases important in regulating insulin signaling. For example, β-secretase 2 (BACE2), another extracellular protease that shares at least one common substrate with MME (amyloid precursor protein), has been shown to regulate insulin receptor (IRβ) internalization in pancreatic β-cells [69]. Furthermore, mice with functionally-inactive BACE2 show increased β-cell mass and increased circulating insulin levels [70]. Interestingly, the closely related homolog BACE1 does not affect insulin receptor internalization [69]. Thus, the MME-dependent regulation of insulin signaling may share a mechanism similar to BACE2, where proteolytic cleavage of INSR is modulated. The effects of MME on insulin signaling are potentially responsible for its documented effects on adipogenesis. For example, MME overexpression has been shown to potentiate adipogenesis by activating the AKT-PI3K pathway [35]. Preadipocytes sorted by MME expression predicts adipogenic capacity: preadipocytes with low MME expression have lower lipid accumulation after adipogenesis and vice versa [50]. Furthermore, depot-specific differences in MME expression are present in both the preadipocytes and adipocytes [50]. Mice with knockout of MME do not show defects in adipose tissue development, at least up to 20 weeks of age [30]. However, MMEKO mice on chow diet develop obesity, which is evident around 36 weeks of age [28]. In 20 week-old MMEKO, we observed that a reduction in MME expression increased basal insulin receptor phosphorylation and decreased insulin-stimulated receptor phosphorylation. It is possible that this altered insulin response already at 20 weeks of age contributed to the consequent development of obesity with age. Elevated circulating levels of soluble MME are associated with increased BMI and HOMA-IR [30]. Also, pharmacological treatment of rats with MME inhibitors has been associated with increased insulin sensitivity [71], consistent with our data in which the MMEKO mice are more insulin sensitive than their littermate controls as assessed by an ITT with a low insulin dose (0.1 mU/gBW). This may be due to increased circulating MME-degraded hormones such as bradykinin [71], [72]. MME has also been shown to be secreted on exosomes from adipose-derived stem cells [36]. Thomou et al. have shown that exosomes released from adipose tissue may regulate gene expression in distal tissues [73]; while this activity has been shown to be related to exosomal miRNAs, this may also be related to exosomal-bound MME. In effect, subcutaneous-adipose-derived circulating MME may have distal regulatory effects on peripheral tissue. Thus, MME may have a distinct cellular role to regulate insulin signaling at the level of the insulin receptor and a somewhat distinct systemic effect to regulate whole-body insulin sensitivity by degrading several small peptide hormones. Determining how to modify each of these hierarchical roles will be crucial in therapeutic interventions for adipose-associated diseases mediated through MME.