Alterations in Beta Cell Identity in Type 1 and Type 2 Diabetes.

PURPOSE OF REVIEW: To discuss the current understanding of "ß cell identity" and factors underlying altered identity of pancreatic ß cells in diabetes, especially in humans. RECENT FINDINGS: Altered identity of ß cells due to dedifferentiation and/or transdifferentiation has been proposed as a mechanism of loss of ß cells in diabetes. In dedifferentiation, ß cells do not undergo apoptosis; rather, they lose their identity and function. Dedifferentiation is well characterized by the decrease in expression of key ß cell markers such as genes encoding major transcription factors, e.g., MafA, NeuroD1, Nkx6.1, and Foxo1, and an increase in atypical or "disallowed" genes for ß cells such as lactate dehydrogenase, monocarboxylate transporter MCT1, or progenitor cell genes (Neurog3, Pax4, or Sox9). Moreover, altered identity of mature ß cells in diabetes also involves transdifferentiation of ß cells into other islet hormone producing cells. For example, overexpression of α cell specific transcription factor Arx or ablation of Pdx1 resulted in an increase of α cell numbers and a decrease in ß cell numbers in rodents. The frequency of α-ß double-positive cells was also prominent in human subjects with T2D. These altered identities of ß cells likely serve as a compensatory response to enhance function/expand cell numbers and may also camouflage/protect cells from ongoing stress. However, it is equally likely that this may be a reflection of new cell formation as a frank regenerative response to ongoing tissue injury. Physiologically, all these responses are complementary. In diabetes, (1) endocrine identity recapitulates the less mature/less-differentiated fetal/neonatal cell type, possibly representing an adaptive mechanism; (2) residual ß cells may be altered in their subtype proportions or other molecular features; (3) in humans, "altered identity" is a preferable term to dedifferentiation as their cellular fate (differentiated cells losing identity or progenitors becoming more differentiated) is unclear as yet.