α- To β-cell Conversion After STZ-induced Extreme β-cell Injury

Diabetes is one of the leading health problems in the world. Deficiency of β-cell mass and failure of islet function are fundamental in both type 1 and type 2 diabetes. Therefore, endogenous β-cell mass restoration and islet function reconstruction hold great promise for diabetic therapy. The a-cell represents an appropriate candidate for endogenous β-cell regeneration. Mature a-and β-cells share a common ancestor and are functionally very close, with a similar machinery to metabolize glucose and secrete hormones. Moreover, a-cells have an amazing capacity for compensation (2% being sufficient) and therefore the shortage of candidate cell is no longer a problem. Inspiringly, after genetic ablation of almost all the β-cells in mice, a-cells spontaneously converted in to β-cells. However, the extreme injury model cannot be duplicated in human and thus determining the key events and signaling pathways regulating the process is essential for realizing a-to-β-cell conversion in diabetic patients. In our study, we induced extreme destruction of β-cells by a single high dose of STZ (Streptozotocin). At 16 hrs after injection, insulin-stained cells per islet accounted for less than 1% of the normal control. Then, robust β-cell regeneration occurred with the number of insulin-stained cells in each islet increased by a factor of 11 between 16 hrs and 24 hrs following STZ injection, and further increased at 48 hrs, corresponding on average to 14.1% of the normal control. Lineage tracing of α-cells before p-cell destruction showed that a-cells, which underwent EMT (Epithelial-mesenchymal Transition), turned into round-shaped mensenchymal cells, and subsequently reprogrammed to β-like cells, leading to robust β-cell regeneration. Mild hyperglycemia markedly promoted the proliferation rate of the regenerated p-cells, which resulted in β-cell mass and function restoration. Interestingly, blocking pancreatic macrophage infiltration by clodronate after STZ injection completely inhibited EMT of a-cells, therefore significantly suppressed β-cell regeneration. The number of M2 macrophages significantly increased in the islets during β-cell regeneration. In vitro co-cultrue experiments showed that M2 macrophages, but not M1 macrophages, induced EMT of hamster InR1G9 cells through TGFP signaling, which led to the up-regulation of pancreas progenitor marker Pdx1. Overall, our data presented here suggested that considerable amount of M2 macrophages were recruited in the islets after STZ-induced extreme β-cell inury. M2 macrophages mediated EMT of a-cells through TGFB signaling, which play key roles in the spontaneous α-to β-cell conversion in the model. The β-cells could response to glucose stimulation with an increase of replication rate and cell number, which efficiently maintained glucose homeostasis. Our research partly revealed the mechanism of a-to-β-cell conversion after extreme β-cell loss, providing important clues for clinical use.