RISING STARS: Evidence for established and emerging forms of β-cell death

Abstract

β-Cell death contributes to β-cell loss and insulin insufficiency in type 1 diabetes (T1D), and this β-cell demise has been attributed to apoptosis and necrosis. Apoptosis has been viewed as the lone form of programmed β-cell death, and evidence indicates that β-cells also undergo necrosis, regarded as an unregulated or accidental form of cell demise. More recently, studies in non-islet cell types have identified and characterized novel forms of cell death that are biochemically and morphologically distinct from apoptosis and necrosis. Several of these mechanisms of cell death have been categorized as forms of regulated necrosis and linked to inflammation and disease pathogenesis. In this review, we revisit discoveries of β-cell death in humans with diabetes and describe studies characterizing β-cell apoptosis and necrosis. We explore literature on mechanisms of regulated necrosis including necroptosis, ferroptosis and pyroptosis, review emerging literature on the significance of these mechanisms in β-cells, and discuss experimental approaches to differentiate between various mechanisms of β-cell death. Our review of the literature leads us to conclude that more detailed experimental characterization of the mechanisms of β-cell death is warranted, along with studies to better understand the impact of various forms of β-cell demise on islet inflammation and β-cell autoimmunity in pathophysiologically relevant models. Such studies will provide insight into the mechanisms of β-cell loss in T1D and may shed light on new therapeutic approaches to protect β-cells in this disease.

Keywords: diabetes; ferroptosis; inflammation; necroptosis; pyroptosis; β-cell death.

Figure 1

Basic mechanisms of necroptosis signaling. The binding of TNFα to TNFR1 initiates the recruitment of TRAF2, TRADD, RIPK1, cIAP1/2, and LUBAC to form complex I at the receptor. Under appropriate stimulation, RIPK1 associates with pro-caspase 8, FADD, and RIPK3 to form complex IIb. When pro-caspase 8 is inactive, RIPK1 activates RIPK3 via phosphorylation, and activated RIPK3 recruits and phosphorylates MLKL in complex IIc (the necrosome). Phosphorylation of MLKL leads to its conformational change, oligomer formation, membrane translocation, and disruption of membrane integrity, resulting in cell lysis. Other receptors including IFNγR, TRAILR, FAS, and TLRs have also been found to contribute to necrosome formation and necroptosis.

Figure 2

Basic mechanisms of ferroptosis signaling. Ferroptosis is triggered by an imbalance of intracellular free iron and cell antioxidant capacity, eventually leading to excessive lipid peroxidation and cell lysis. Cystine is essential for glutathione (GSH) production, and GSH is a cofactor for glutathione peroxidase 4 (GPX4), a critical lipid peroxidase. Iron is taken up by the cell, and stable Fe³⁺ can be converted to free redox-active Fe²⁺. This transition facilitates the accumulation of reactive oxygen species (ROS) through the Fenton reaction with H₂O₂, and GPX4 is needed to counteract lipid peroxidation that arises through this process. Stimuli that increase intracellular free iron, reduce cellular cystine uptake, or decrease cellular antioxidant capacity can lead to ferroptosis.

Figure 3

Basic mechanisms of canonical pyroptosis signaling. In canonical pyroptosis, extracellular signals such as danger-associated molecular patterns (DAMPs) and pathogen-associated molecular patterns (PAMPs) elicit priming via upregulation of the NLRP3 inflammasome, pro-IL-1β, and pro-IL-18. During the activation phase, stimuli trigger the NLRP3 inflammasome leading to caspase 1 activation. Caspase 1 then cleaves gasdermin D (GSDMD), resulting in the formation of GSDMD-N, which forms pores in the cell membrane. Simultaneously, caspase 1 generates mature IL-1β and IL-18, which are released through GSDMD-N pores, culminating in pyroptosis.

Figure 4

Heterogeneity of β-cell death responses. A Sartorius Incucyte S3 live cell imaging and analysis instrument was used to monitor cell death (Cytotox Red, Sartorius, 250 nM) and caspase 3/7 activation in real-time (Caspase 3/7 Green, Sartorius, 5 µM). NIT-1 β-cells were plated and treated with vehicle (circles) or TNFα (40 µg/mL) + IFNγ (100 µg/mL) (squares). Cytotox Red−, Caspase 3/7 Green+ (caspase 3/7 activation), Cytotox Red+, Caspase 3/7 Green− (dead, non-apoptotic), and Cytotox Red+; Caspase 3/7 Green+ (dead, apoptotic) objects were monitored hourly over 6 h and quantified at 0, 2, 4, and 6 h post treatment. Total cell count was determined using AI-mediated cell-by-cell phase contrast analysis, and data is represented as a percent of total cells. (A) Percent live cells (dark gray: vehicle, light gray: TNFα+IFNγ), (B) percent non-apoptotic dead cells (Cytotox Red+, Caspase 3/7 Green−; gray: vehicle, red: TNFα+IFNγ), (C) percent apoptotic dead cells (Cytotox Red+; Caspase 3/7 Green+; gray: vehicle, yellow: TNFα+IFNγ), and (D) percent caspase 3/7 activated live cells (Cytotox Red−, Caspase 3/7 Green+; gray: vehicle, green: TNFα+IFNγ) were quantified. (E) Representative images of Cytotox Red and Caspase 3/7 Green-positive NIT-1 cells 0, 2, 4, and 6 h post TNFα+IFNγ treatment (10× magnification). Data are presented as mean ± s.e.m . and were analyzed by two-way ANOVA with Holm–Sidák multiple comparisons correction. ns, not significant; *P < 0.05; ****P < 0.0001, P > 0.05.