Fight Aging!

Senescent cells accumulate with age throughout the body, and cause considerable disruption to tissue structure and function via their pro-inflammatory secretions. Clearing senescent cells is an important approach to rejuvenation and reversal of age-related disease, based on the impressive results produced in mice to date. One of the challenges inherent in the destruction of senescent cells is the variation shown in their biochemistry, depending on how they become senescent and on which tissue they reside in. Different treatments exhibit widely varying outcomes for different varieties of senescent cell, and those varieties are far from fully or comprehensively catalogued.

In today's open access paper, researchers describe a novel approach to the selective destruction of senescent cells, focusing on characteristics of the dysfunctional iron metabolism exhibited by cells that become senescent in response to the signaling of other senescent cells, undergoing what is know as paracrine senescence. The researchers show that should be possible to produce an iron-activated prodrug, in which the active cell-killing drug substance is masked by a chemical addition that is only stripped in cells that exhibit the aberrant iron metabolism characteristic of senescent cells. It is worth noting that prodrugs based on the high levels of β-galactosidase in senescent cells have shown considerable promise to date, so we might expect analogous approaches to be similarly interesting.

Selective ablation of primary and paracrine senescent cells by targeting iron dyshomeostasis

The molecular biology of cellular senescence has opened the possibility of exploiting the differential vulnerabilities of senescent cells (SCs) compared with healthy cells for the development of a new class of longevity therapeutics against aging and age-related disorders. However, the significant heterogeneity among SCs based on cell type of origin or senescence induction method suggests the need to develop senolytics that either have a broader therapeutic efficacy or that can target recalcitrant SCs.

In this context, paracrine senescence (PS) is the least understood type of senescence. Even though there have been previous efforts to characterize PSs, the fact that only a subset of cells exposed to the senescence-associated secretory phenotype (SASP) factors become senescent means that previous experimental protocols were compromised, with mixed cell populations dominated by non-senescent cells labeled as PSs. We were able to circumvent this major methodological issue by isolating and enriching PSs using the previously characterized SC surface marker DPP4.

We discovered that DPP4+ paracrine SCs (PSDPP4+) engage prosurvival pathways that are distinct from those on which DPP4+ primary SCs (SDPP4+) rely and are also relatively resistant to killing by senolytic drugs previously identified to be effective against primary SCs. Given that SCs accumulate ferrous iron (Fe(II), also known as labile iron), we sought to test a Fe(II)-targeting strategy in which Fenton reaction of a prodrug was coupled to release of drug payload. Others previously showed that the tumor-activated prodrug TRX-CBI (comprising a trioxolane-based [TRX] sensor of Fe(II) conjugated to a cytotoxic cyclopropylbenzindoline [CBI] payload) demonstrated selective toxicity in Fe(II)-rich cancer cells.

Here, we used a form of TRX-CBI to target cytotoxic CBI to SCs. We demonstrated that treatment with TRX-CBI triggers significant senolysis of both PSDPP4+ and SDPP4+, with negligible cytotoxicity toward non-senescent cells. Based on our results, we propose Fe(II)-based targeting of SCs with ferroptosis inducers or iron-activated drug conjugates as broad-spectrum senolytic agents.