Abstract
The ability of pancreatic beta cells to proliferate is critical both for normal tissue maintenance and in conditions where there is an increased demand for insulin. Protein kinase B (Akt) plays a major role in promoting proliferation in many cell types, including the insulin-producing beta cells. We have previously reported that mice overexpressing a constitutively active form of Akt (caAkt
Tg
) show enhanced beta cell proliferation that is associated with increased protein levels of cyclin D1, cyclin D2 and cyclin-dependent kinase inhibitor 1A (p21Cip). In the present study, we sought to assess the mechanisms responsible for augmented p21Cip levels in caAkt
Tg
mice and test the role of p21Cip in the proliferative responses induced by activation of Akt signalling.To gain a greater understanding of the relationship between Akt and p21Cip, we evaluated the mechanisms involved in the modulation of p21Cip by Akt and the in vivo role of reduced p21Cip in proliferative responses induced by Akt.Our experiments showed that Akt signalling regulates p21
Cip
transcription and protein stability. caAkt
Tg
/p21
Cip+/−
mice exhibited fasting and fed hypoglycaemia as well as hyperinsulinaemia when compared with caAkt
Tg
mice. Glucose tolerance tests revealed improved glucose tolerance in caAkt
Tg
/p21
Cip+/−
mice compared with caAkt
Tg
. These changes resulted from increased proliferation, survival and beta cell mass in caAkt
Tg
/p21
Cip+/−
compared with caAkt
Tg
mice.Our data indicate that increased p21Cip levels in caAkt
Tg
mice act as a compensatory brake, protecting beta cells from unrestrained proliferation. These studies imply that p21Cip could play important roles in the adaptive responses of beta cells to proliferate in conditions such as in insulin resistance.
