Alpha cell dysfunction in type 1 diabetes is independent of a senescence program

Abstract

Type 1 Diabetes (T1D) is caused by insulin deficiency, due to progressive autoimmune destruction of pancreatic β cells. Glucagon-secreting α cells become dysfunctional in T1D and contribute to pathophysiology, however, the mechanisms involved are unclear. While the majority of β cells are destroyed in T1D, some β cells escape this fate and become senescent but whether α cell dysfunction involves a senescence program has not been explored. Here we addressed the question of whether α cells become senescent during the natural history of T1D in the non-obese diabetic (NOD) mouse model and humans. NOD mice had several distinct subpopulations of α cells, but none were defined by markers of senescence at the transcriptional or protein level. Similarly, α cells of human T1D donors did not express senescence markers. Despite the lack of senescence in α cells in vivo, using a human islet culture model, we observed that DNA damage-induced senescence led to alterations in islet glucagon secretion, which could be rescued by inhibiting the senescence-associated secretory phenotype (SASP). Together our results suggest that α cell dysfunction in T1D is not due to activation of a senescence program, however, senescent β cell accumulation in the islet microenvironment may have a negative effect on α cell function.

Pancreatic β-cell senescence in diabetes: mechanisms, markers and therapies

“…p16 Ink4a expression was already quite high even in the youngest pediatric donors examined in that study (aged 12-14), consistent with the age-related β-cell maturation and cell cycle exit. Replicating this observation in a culture model, DNA damage-induced senescence in islets from healthy donors leads to stable activation of p21 Cip1 but not p16 Ink4a ( 1 , 34 , 35 ) and more recently p21 Cip1 expression was confirmed in β-cells in this model ( 36 ). Additional evidence for distinctions between p21 Cip1 and p16 Ink4a in senescent β-cells was recently shown with the INK-ATTAC mouse model, which revealed that ablation of p16 Ink4a -expressing β-cells left a remaining senescent β-cell population expressing SASP factor genes ( 96 ).…”

“…Conditioned media transfer experiments show that SASP factors secreted by β-cells are capable of triggering phenotypic changes in naïve recipient cells ( 1 – 3 , 35 ). Moreover, induction of SASP in human islets in culture leads to altered glucagon secretion from α-cells, suggesting that SASP from β-cells can affect α-cell function ( 36 ). BET inhibitor iBET-762, which reduces SASP, can partially rescue the glucagon secretion phenotype ( 36 ), lending further evidence implicating SASP in impaired α-cell function.…”

“…Moreover, induction of SASP in human islets in culture leads to altered glucagon secretion from α-cells, suggesting that SASP from β-cells can affect α-cell function ( 36 ). BET inhibitor iBET-762, which reduces SASP, can partially rescue the glucagon secretion phenotype ( 36 ), lending further evidence implicating SASP in impaired α-cell function. Given recent work implicating α-cell dysfunction in T1D ( 139 – 141 ), further investigation into how the accumulation of senescent β-cells in the islet may affect α-cell function is warranted.…”

Exploring Transcriptional Regulation of Beta Cell SASP by Brd4-Associated Proteins and Cell Cycle Control Protein p21

“…Immunohistochemistry was performed as previously described [ 18 ]. Formalin fixed paraffin embedded pancreas tissue sections from 6-, 14-, and 17-week-old euglycemic NOD/ShiLtJ female mice (gift from A. Bhushan, UCSF) were rehydrated with xylene–ethanol washes, and antigen retrieval was performed with citrate buffer pH 6.0 (Vector Labs, Newark, NJ, USA) in a lab-grade pressure cooker (BioSB Tinto Retrieval system).…”

“…NIT-1 beta cells respond similarly to primary NOD mouse islet cells to inflammatory stimuli such as cytokines [ 17 ] and have been used extensively in the field to model molecular mechanisms of beta cell stress in T1D. Although these cells are transformed with Simian Virus 40 large T antigen, following sub-lethal DNA double-strand break damage, they undergo a growth arrest, activate a canonical DNA damage response, and stably upregulate p21 [ 16 ], recapitulating features of senescent primary beta cells in NOD mice [ 8 ] and human islet beta cells in a DNA damage-induced senescence model [ 18 ]. Senescent NIT-1 cells also produce a SASP involving insulin-like growth factor binding protein 3 (Igfbp3) and plasminogen activator inhibitor (Pai1) [ 16 ], two factors in common with the SASP of primary NOD beta cells [ 8 ].…”