Cell survival following direct executioner-caspase activation

Executioner-caspase activation has been considered a point-of-no-return in apoptosis. However, numerous studies report survival from caspase activation after treatment with drugs or radiation. An open question is whether cells can recover from direct caspase activation without pro-survival stress responses induced by drugs. To address this question, we engineered a HeLa cell line to express caspase-3 inducibly and combined it with a quantitative caspase activity reporter. While high caspase activity levels killed all cells and very low levels allowed all cells to live, doses of caspase activity sufficient to kill 15 to 30% of cells nevertheless allowed 70 to 85% to survive. At these doses, neither the rate, nor the peak level, nor the total amount of caspase activity could accurately predict cell death versus survival. Thus, cells can survive direct executioner-caspase activation, and variations in cellular state modify the outcome of potentially lethal caspase activity. Such heterogeneities may underlie incomplete tumor cell killing in response to apoptosis-inducing cancer treatments.

The functions of caspase in whitefly Bemisia tabaci apoptosis in response to ultraviolet irradiation

Whiteflies (Bemisia tabaci) are phloem feeders, and some invasive species are composed of cryptic species complexes that cause extensive crop damage, particularly via the direct transmission of plant viruses. Apoptosis is a type of programmed cell death essential for organismal development and tissue homeostasis. The caspases belong to a family of cysteine proteases that play a central role in the initiation of apoptosis in many organisms. Here, we employed a comprehensive genomics approach to identity caspases in B. tabaci Middle East Asia Minor 1 (MEAM1), an invasive whitefly that carries a cryptic species complex that is devastating to crops. Four caspase genes were identified, and their motif compositions were predicted. Structures were relatively conserved in both putative effector and initiator caspases. Expression patterns of caspase genes differed across insect developmental stages. Three caspase genes were induced immediately after ultraviolet (UV) treatment. Expression levels of Bt-caspase-1 and Bt-caspase-3b increased in the midgut and salivary glands during apoptosis induced by UV treatments, whereas silencing of both genes reduced UV-triggered apoptosis. Our study demonstrates that Bt-caspase-1 and Bt-caspase-3b, respectively, act as putative initiator and effector apoptotic caspases in the MEAM1 whitefly.