β-Cell senescence in the pathogenesis of type 2 diabetes.

概要

The prevalence of type 2 diabetes increases with aging. Type 2 diabetes developing in the elderly is often accom- panied by a gradual impairment of b-cell function and reduced b-cell mass along with aging. However, the contribution of b-cell senescence to the pathogenesis of type 2 diabetes remains uncertain.

In a recent Cell Metabolism article, Aguayo-Mazzucato et al.1 described their efforts to identify the signatures of senes- cent b-cells, and explored role(s) of senescent b-cells in type 2 diabetes.

First, the authors sorted primary b-cells isolated from 7- to 8-month-old mice based on b-galactosidase (b-gal) activity, and compared gene expressions between b-gal-positive and -negative b-cells using ribonucleic acid (RNA) sequencing analyses. The b-gal-positive cells accounted for 8–10% of the b-cell populations in these mice. Using RNA sequencing analysis, the authors observed downregulation of key hallmark b-cell identity genes, such as Ins1, Mafa, Nkx6.1 and Pdx1, and upregulation of aging and senescence-related genes in b-gal-positive b-cells, as compared with b-gal-negative b-cells. Using these data, the authors gen- erated indices for assessment of b-cell identity, aging and senescence.

The authors then examined the expressions of senescence-associated secretory phenotype (SASP) genes in b-gal-positive b-cells. They found that SASP genes, such as TNF and CXCL1, were increased in b-gal-positive b-cells, as compared with b-gal-negative b-cells. In addition, conditioned media from cul- tured b-gal-positive b-cells upregulated expressions of p16Ink4a in isolated islets, showing that senescent b-cells secrete functional SASP factors (Figure 1).

Next, the authors explored whether insulin resistance promotes b-cell senes- cence in vivo. They generated two mouse models of insulin resistance by chroni- cally administering S961, an insulin receptor antagonist, using a mini-pump and by high-fat diet loading. In both models, b-gal-positive b-cells, as well as aging and SASP indices of b-cells, were significantly increased, indicating promo- tion of b-cell senescence. Therefore, these results suggest that insulin resistance is a driver of b-cell aging (Figure 1), which supports this group’s previous findings that b-cell aging marker appearance is accelerated by insulin resistance2. Inter- estingly, 2 weeks after the discontinua- tion of S961 administration, increased aging and SASP indices of b-cells returned to normal, along with the nor- malization of hyperglycemia. Accordingly, b-cell senescence is reversible at least under these experimental conditions.

Based on these findings, the authors administered senolytic therapy by elimi- nating senescent b-cells in insulin resis- tance-induced and in aged INK-apoptosis through targeted activation of caspase (INK-ATTAC) mice in which adminis- tration of B/B homodimerizer leads to deletion of cells expressing p16Ink4a. In aged female INK-ATTAC mice of 15– 18 months old, treatments with B/B homodimerizer improved b-cell aging and SASP indices. Glucose tolerance was unaffected by B/B homodimerizer admin- istration in aged INK-ATTAC mice. However, glucose-stimulated insulin secretion was augmented on glucose tol- erance testing. Interestingly, basal insulin levels were reduced after an overnight fast, suggesting hepatic insulin resistance to be improved by B/B homodimerizer administration. With the second model, insulin resistance was induced by chronic administration of S961 for 2 weeks in 9- to 14-month-old INK-ATTAC mice, and the effects of B/B homodimerizer were analyzed in these mice. In the second model, B/B homodimerizer administra- tion improved not only aging and SASP indices in b-cells, but also blood glucose levels. With the third model, insulin resis- tance was induced by high-fat diet feeding for 12 weeks to 9-month-old INK- ATTAC mice. In this model, improve- ments of glucose tolerance, as well as aging and identity indices in b-cells, were observed after B/B homodimerizer administration. These results suggest the significance of b-cell senescence in glu- cose homeostasis (Figure 1).

Anti-apoptotic pathways are known to be upregulated in senescent cells. The authors found that the B-cell lymphoma gene 2 (BCL2) pathway, one of the anti- apoptotic pathways, is upregulated in b- gal-positive b-cells. Therefore, the authors administered b-cell senolytic therapies with ABT263, which targets the BCL2 pathway. ABT263 effectively killed b-gal- positive islet cells in vitro. Then, the authors administered ABT263 to 6- to 9- month-old INK-ATTAC mice in which insulin resistance had been induced by chronically administering S961. Blood glucose levels of INK-ATTAC mice trea- ted with both S961 and ABT263 were reduced as compared with those in INK- ATTAC mice given only S961. The b- gal-positive islet cells were significantly decreased, by 25%, in INK-ATTAC mice treated with both S961 and ABT263. as compared with control INK-ATTAC mice. This decrease was accompanied by a significant decrement in the SASP index. Furthermore, the effects of ABT263 were also examined in INK- ATTAC mice in which insulin resistance had been induced by high-fat diet load- ing. This model yielded a significant decrease in b-gal-positive islet cells, as well as aging and SASP indices, although effects on blood glucose levels were mini- mal. These results showed the senolytic drug to partially reverse the adverse metabolic effects induced by insulin resis- tance (Figure 1).

Finally, the authors explored the clinical significance of b-cell senescence in patients with type 2 diabetes. Islets iso- lated from donors of different ages with and without type 2 diabetes were ana- lyzed, and the population of b-gal-positive islet cells was found to be increased in older donor-derived isolated islets. Fur- thermore, the population of b-gal-positive islet cells appeared to be further increased in islets from donors with type 2 diabetes. In addition, expressions of p16Ink4a,as well as SASP factors, such as CCL4 and IL6, were significantly increased in b-gal- positive human islet cells. The authors previously identified insulin-like growth factor 1 receptor (IGF1R) as a novel senescent marker of b-cells2. Therefore, pancreatic sections from different aged human donors with or without type 2 dia- betes were stained for IGF1R. The authors found that, in donors aged <40 years, the intensity of IGF1R was higher in speci- mens from donors with type 2 diabetes, suggesting early b-cell senescence in type 2 diabetes. Furthermore, staining for tumor protein p53 binding protein 1 (p53BP1), a well-known cell senescence marker, showed the intensity of p53BP1 to be increased in pancreatic islets from donors with type 2 diabetes whose body mass index values were <33. Collectively, these results raise the possibility of human b-cells being a potential target of senolytic therapies for type 2 diabetes.

Senolytic therapies improved glucose metabolism in aged mice, as well as in insulin-resistant model mice, suggesting that accelerated b-cell senescence con- tributes to the progression of glucose metabolism impairments induced by insulin resistance. The authors speculate that impairment of glucose metabolism might be due mainly to a decline in b-cell function. Downregulation of b-cell identity genes in senescent cells is one possible explanation for the decline in b-cell function. However, as just 8–10% of b-cells were b-gal-positive in aged mice, it is conceivable that SASP secreted from b-gal-positive cells might affect sur- rounding b-gal-negative b-cells in a para- crine fashion, thereby impairing insulin secretion. Another intriguing finding was that B/B treatment appeared to improve hepatic insulin resistance in aged INK- ATTAC mice. As the authors noted that B/B treatment had no significant effects on the expression of p16Ink4a in periph- eral tissues important for glucose home- ostasis, such as the liver, adipose tissues and muscles, a direct hepatic effect of B/B is unlikely. It would be interesting to explore how the reduction of senescent b-cells improves insulin resistance in the liver.

It is important to clarify the involve- ment of b-cell senescence in restricting b-cell proliferation in aged animals. Ter- minally differentiated b-cells retain prolif- erative potential. Self-replication of pre- existing b-cells is a primary mechanism of islet expansion during the neonatal stage or b-cell mass maintenance in adult animals. In addition, in insulin-resistant states, b-cells adaptively proliferate and secrete more insulin to meet the increased systemic demand for this hormone, thereby maintaining glucose homeostasis at the whole-body level. Therefore, these responses appear to be an endogenous mechanism acting to pre- vent diabetes development. Several recent studies have shown that signals from the liver mediated through neuronal path- ways3,4 or by humoral factors5 regulate adaptive b-cell proliferation. Proliferation of cells in aged animals is recognized as being severely impaired. Regarding b-cells as well, the proliferative capacity declines with aging in animal models. Considering the high prevalence of dia- betes in the elderly, impairment of adap- tive b-cell proliferation is likely to be involved in the pathogenesis of type 2 diabetes in the elderly. This assumption is based on aging often being accompa- nied by insulin resistance. In this regard, elucidating the mechanism of decline in adaptive b-cell proliferation in aged ani- mals is critical for overcoming type 2 diabetes affecting the elderly. However, the importance of either intrinsic factors involving senescent b-cells or extrinsic factors in the aged systemic environment, such as the signals from other organs, for impaired adaptive b-cell proliferation has yet to be clarified. Exploring the effects of senolytic therapies on adaptive b-cell proliferation in aged mice is an attractive avenue of future research.

This novel study appears to provide evidence that b-cell senescence con- tributes to the pathogenesis of type 2 dia- betes. The findings of this study might open a new window for developing inno- vative therapeutic strategies for a broad range of type 2 diabetes with insulin resistance, including that in the elderly.