The determinants of delta cell mass are not known. As part of studies showing interruption of glucagon signaling (IGS) stimulates alpha cell proliferation in an amino acid (AA) -dependent manner, we noted an increase in delta cells. To investigate, we examined delta cell proliferation and mass in 3 mouse models and a human islet xenograft model. Adult mice with deletion of glucagon (Gcg-/-) had a 4.7-fold increase in Ki67-positive delta cells (1.50±0.14% vs. 0.32±0.12%, P<0.001, n=8) and a 3.1-fold increase in delta cell mass (0.49±0.vs. 0.16±0.mg, P=0.003, n=10) . Treatment of adult C57BL/6J mice with a Gcg receptor antibody (GCGR Ab) elicited a 6.3-fold increase in delta cell proliferation (2.01±0.4% vs. 0.32±0.07%, P=0.002, n=10) and a 2.6-fold increase in delta cell mass (0.20±0.03 vs. 0.078±0.mg, P=0.001, n=10) , respectively, compared to IgG-treated controls. Since hyperaminoacidemia is necessary for alpha cell proliferation in response to IGS, we investigated the requirement for Slc7a2, a cationic AA transporter, in delta cell growth. Indeed, Slc7a2+/+mice, but not Slc7a2-/-mice, treated with GCGR Ab had a 3.8-fold increase in delta cell proliferation (3.29±0.14% vs. 0.86±0.05%, P<0.001) and 1.6-fold increase in delta cell mass (0.20±0.02 vs. 0.126±0.mg, P=0.016, n=8) . Given that mTOR is a known intracellular mediator of AA-regulated cellular growth, we evaluated phospho-ribosomal protein S6 (pS6) expression in situ and found a marked increase in pS6 labeling of delta cells in Gcg-/-, but not Gcg+/+, mice. We also observed a 3.6-fold increase in delta cell proliferation (1.75±0.25% vs. 0.49±0.06%, P=0.004, n=3 adult normal donors) in human islets transplanted into immunodeficient mice treated with GCGR Ab. Taken together, these data indicate that IGS promotes delta cell proliferation and mass through mechanisms involving Slc7a2 and the mTOR signaling pathway and that AAs may be a determinant of delta cell mass.
C.Dai: None. A.Bradley: None. E.Spears: None. A.K.Singh: None. F.O.Oladipupo: None. R.Jenkins: None. K.C.Coate: None. A.C.Powers: None.