Fibroblast activation protein (FAP) has over 50% sequence identity with dipeptidyl peptidase-4 (DPP-4). The therapeutic success of DPP-4 inhibitors for the treatment of type 2 diabetes has stimulated interest in the metabolic actions of FAP. Panaro and colleagues found that Fap-/- mice did not exhibit improved glucose control or resistance to weight gain after prolonged high fat feeding. Their findings indicate that murine FAP is not a critical determinant of islet function, glucose homeostasis, weight gain, and the metabolic response to high fat feeding in mice.
Fibroblast activation protein is dispensable for control of glucose homeostasis and body weight in mice
Objective: Fibroblast Activation Protein (FAP), an enzyme structurally related to dipeptidyl peptidase-4 (DPP-4), has garnered interest as a potential metabolic drug target due to its ability to cleave and inactivate FGF-21 as well as other peptide substrates. Here we investigated the metabolic importance of FAP for control of body weight and glucose homeostasis in regular chow-fed and high fat diet-fed mice.
Methods: FAP enzyme activity was transiently attenuated using a highly-specific inhibitor CPD60 and permanently ablated by genetic inactivation of the mouse Fap gene. We also assessed the FAP-dependence of CPD60 and talabostat (Val-boroPro), a chemical inhibitor reportedly targeting both FAP and dipeptidyl peptidase-4
Results: CPD60 robustly inhibited plasma FAP activity with no effect on DPP-4 activity. Fap gene disruption was confirmed by assessment of genomic DNA, and loss of FAP enzyme activity in plasma and tissues. CPD60 did not improve lipid tolerance but modestly improved acute oral and intraperitoneal glucose tolerance in a FAP-dependent manner. Genetic inactivation of Fap did not improve glucose or lipid tolerance nor confer resistance to weight gain in male or female Fap−/− mice fed regular chow or high-fat diets. Moreover, talabostat markedly improved glucose homeostasis in a FAP- and FGF-21-independent, DPP-4 dependent manner.
Conclusions: Although pharmacological FAP inhibition improves glucose tolerance, the absence of a metabolic phenotype in Fap−/−mice suggest that endogenous FAP is dispensable for the regulation of murine glucose homeostasis and body weight. These findings highlight the importance of characterizing the specificity and actions of FAP inhibitors in different species and raise important questions about the feasibility of mouse models for targeting FAP as a treatment for diabetes and related metabolic disorders.