Protein kinase B/AKT has three
isoforms (AKT1-3) and is renowned for its central role in the regulation of cell growth and proliferation, due to its constitutive activation in various
cancers. AKT2, which is highly expressed in
insulin-responsive tissues, has been identified as a primary regulator of
glucose metabolism as Akt2 knockout mice (Akt2(-/-)) are
glucose-intolerant and
insulin-resistant. However, the role of AKT1 in
glucose metabolism is not as clearly defined. We previously showed that mice with myristoylated Akt1 (AKT1(Myr)) expressed through a bicistronic Pdx1-TetA and TetO-MyrAkt1 system were susceptible to
islet cell carcinomas, and in this study we characterized an early onset, prediabetic phenotype. Beginning at weaning (3 weeks of age), the
glucose-intolerant AKT1(Myr) mice exhibited non-fasted
hyperglycemia, which progressed to fasted
hyperglycemia by 5 months of age. The
glucose intolerance was attributed to a fasted hyperglucagonemia, and hepatic
insulin resistance detectable by reduced phosphorylation of the
insulin receptor following
insulin injection into the inferior vena cava. In contrast, treatment with
doxycycline diet to turn off the transgene caused attenuation of the non-fasted and fasted
hyperglycemia, thus affirming AKT1 hyperactivation as the trigger. Collectively, this model highlights a novel
glucagon-mediated mechanism by which AKT1 hyperactivation affects
glucose homeostasis and provides an avenue to better delineate the molecular mechanisms responsible for
diabetes mellitus and the potential association with
pancreatic cancer.