One of the more unexpected recent findings relating to cellular senescence is that it appears to be an important part of the mechanisms that lead to loss of the pancreatic β-cells responsible for insulin secretion in both type 1 diabetes and type 2 diabetes - which are very different conditions, despite the shared name. The authors of the brief open access commentary noted here discuss the present state of this research.
Age is one of the major risk factors for the development of type 2 diabetes mellitus (T2D). However, the understanding of how cellular aging contributes to diabetes pathogenesis is incomplete and as a result, current therapies do not target this aspect of the disease. In recent work we showed that insulin resistance induced the expression of aging markers, suggesting that β-cell aging could accelerate the progression toward diabetes. Therefore, reversing the hallmarks of cellular aging presents a potential avenue for novel T2D therapies; in particular, transcriptomic analysis of aged β-cells pointed us toward cellular senescence as a promising target.
Senescent cells enter a state of long-term growth inhibition and replicative arrest after exposure to environmental insults, including genomic damage, oncogene activation, and reactive oxygen species. The resulting changes in gene expression impair cell function and proliferation while modifying intercellular signaling through the senescence-associated secretory phenotype (SASP). The potential paracrine effects of senescent β-cells highlight the importance of the β-cell SASP in driving metabolic dysfunction.
Along these lines, we demonstrated that senescent β-cells downregulated hallmark identity genes, upregulated disallowed genes, and secreted proinflammatory cytokines. We established two models of insulin resistance in mice: one using the delivery of the insulin receptor antagonist S961, and the other using a more physiologically representative high fat diet. In both cases, the metabolic stress increased the number of senescent β-cells while impairing glucose tolerance. Aging and SASP genes were also upregulated, but after insulin resistance was stopped, gene expression returned to healthy levels. This suggests that there might be critical windows during which β-cell senescence may be reversible. These results were consistent with experiments on human β-cells, in which senescence increased with age, body mass index, and in the presence of T2D.
Additionally, we found that the targeted deletion of senescent cells, or senolysis, in mice improved β-cell function, reduced blood glucose levels, and restored healthy expression levels of aging and SASP genes. Our findings highlight the transformative therapeutic potential of senolytic drugs in restoring β-cell function among T2D patients. The partial reversibility of β-cell senescence suggests that, consistent with recent publications, this is a non-binary phenomenon. External insults may create subpopulations of aged β-cells activating distinct levels of the senescence-associated regulatory progression.
The progression of damaged β-cells through this regulatory cascade likely accelerates T2D; eventually, the accumulation of senescent β-cells may cross a threshold inducing long-term metabolic dysfunction through the permanent loss of β-cell mass and function. The deletion of senescent β-cells or the reversal of senescence in a targeted subpopulation of aged β-cells may inhibit this cascade of dysfunction. To advance these therapeutic strategies, it is imperative to characterize the distinct subpopulations of senescent β-cells and the temporal expression patterns of senescence genes.