Differentiation, reduction, and proliferation of pancreatic β-cells and their regulatory factors.

The prevalence of diabetes has increased rapidly in recent years, and many types of therapeutic agents have been developed. However, the main purpose of these drugs is to lower blood glucose levels, and they are not fundamental solutions. In contrast, our research has been aimed at stimulating and inducing β-cell proliferation in vivo and replenishing β-cells. We demonstrated that pancreatic ductal cells are a source of β-cells both after birth and during regeneration after partial duct ligation: cell lineage tracing showed that 39% of growing islets and 50% of adult islets during tissue regeneration contained β-cells differentiated from duct cells. We also examined the factors contributing to β-cell depletion. Insulin and cyclin A genes are tightly regulated by transcriptional activators and repressors, and we found that imbalanced and excessive levels of repressors result in a drastic reduction of insulin and β-cell numbers, leading to severe diabetes. Thus, we searched for factors that induce β-cell proliferation in vivo. In our transgenic (Tg) mice, there was a sex difference in the progression of diabetes and sex steroid hormones were shown to contribute to this. Surprisingly, in diabetic male Tg mice, modulation of sex steroid hormones under certain conditions resulted in a marked increase of β-cells. We identified Greb1 as a factor inducing β-cell proliferation in response to a rapid elevation of E2 levels. This series of studies has demonstrated that islet cells exhibit plasticity and indicates that changes of islet cell mass and function are dynamic and recoverable.