Gemcitabine radiosensitization primes irradiated malignant meningioma cells for senolytic elimination by navitoclax

Androgen deprivation-induced senescence confers sensitivity to a senolytic strategy in prostate cancer

We have previously demonstrated that androgen-dependent prostate cancer (PCa) cell lines enter a state of senescence following exposure to androgen deprivation therapies (ADT). ADT-induced senescence was found to be transient, as senescent cells develop castration resistance and re-emerge into a proliferative state even under continuous androgen deprivation in vitro. Moreover, the BCL-XL/BCL-2 inhibitor, ABT-263 (navitoclax), an established senolytic agent, promoted apoptosis of senescent PCa cells, suppressing proliferative recovery and subsequent tumor cell outgrowth. As this strategy has not previously been validated in vivo, we used a clinically relevant, syngeneic murine model of PCa, where mice were either castrated or castrated followed by the administration of ABT-263. Our results largely confirm the outcomes previously reported in vitro; specifically, castration alone results in a transient tumor growth suppression with characteristics of senescence, which is prolonged by exposure to ABT-263. Most critically, mice that underwent castration followed by ABT-263 experienced a statistically significant prolongation in survival, with an increase of 14.5 days in median survival time (56 days castration alone vs. 70.5 days castration + ABT-263). However, as is often the case in studies combining the promotion of senescence with a senolytic (the "one-two" punch approach), the suppression of tumor growth by the inclusion of the senolytic agent was transient, allowing for tumor regrowth once the drug treatment was terminated. Nevertheless, the results of this work suggest that the "one-two" punch senolytic strategy in PCa may effectively interfere with, diminish, or delay the development of the lethal castration-resistant phenotype.

The Combination of Radiation with PARP Inhibition Enhances Senescence and Sensitivity to the Senolytic, Navitoclax, in Triple Negative Breast Tumor Cells

Despite significant advances in the treatment of triple-negative breast cancer, this disease continues to pose a clinical challenge, with many patients ultimately suffering from relapse. Tumor cells that recover after entering into a state of senescence after chemotherapy or radiation have been shown to develop a more aggressive phenotype, and to contribute to disease recurrence. By combining the PARP inhibitor (PARPi), talazoparib, with radiation, senescence was enhanced in 4T1 and MDA-MB-231 triple-negative breast cancer cell lines (based on SA-β-gal upregulation, increased expression of CDKN1A and the senescence-associated secretory phenotype (SASP) marker, IL6). Subsequent treatment of the radiation- and talazoparib-induced senescent 4T1 and MDA-MB231 cells with navitoclax (ABT-263) resulted in significant apoptotic cell death. In immunocompetent tumor-bearing mice, navitoclax exerted a modest growth inhibitory effect when used alone, but dramatically interfered with the recovery of 4T1-derived tumors induced into senescence with ionizing radiation and talazoparib. These findings support the potential utility of a senolytic strategy in combination with the radiotherapy/PARPi combination to mitigate the risk of disease recurrence in triple-negative breast cancer.

Involvement of Bcl-2 Family Proteins in Tetraploidization-Related Senescence

The B-cell lymphoma 2 (Bcl-2) family of proteins is the main regulator of apoptosis. However, multiple emerging evidence has revealed that Bcl-2 family proteins are also involved in cellular senescence. On the one hand, the different expression of these proteins determines the entry into senescence. On the other hand, entry into senescence modulates the expression of these proteins, generally conferring resistance to apoptosis. With some exceptions, senescent cells are characterized by the upregulation of antiapoptotic proteins and downregulation of proapoptotic proteins. Under physiological conditions, freshly formed tetraploid cells die by apoptosis due to the tetraploidy checkpoint. However, suppression of Bcl-2 associated x protein (Bax), as well as overexpression of Bcl-2, favors the appearance and survival of tetraploid cells. Furthermore, it is noteworthy that our laboratory has shown that the joint absence of Bax and Bcl-2 antagonist/killer (Bak) favors the entry into senescence of tetraploid cells. Certain microtubule inhibitory chemotherapies, such as taxanes and vinca alkaloids, induce the generation of tetraploid cells. Moreover, the combined use of inhibitors of antiapoptotic proteins of the Bcl-2 family with microtubule inhibitors increases their efficacy. In this review, we aim to shed light on the involvement of the Bcl-2 family of proteins in the senescence program activated after tetraploidization and the possibility of using this knowledge to create a new therapeutic strategy targeting cancer cells.

From the divergence of senescent cell fates to mechanisms and selectivity of senolytic drugs

Senescence is a cellular stress response that involves prolonged cell survival, a quasi-irreversible proliferative arrest and a modification of the transcriptome that sometimes includes inflammatory gene expression. Senescent cells are resistant to apoptosis, and if not eliminated by the immune system they may accumulate and lead to chronic inflammation and tissue dysfunction. Senolytics are drugs that selectively induce cell death in senescent cells, but not in proliferative or quiescent cells, and they have proved a viable therapeutic approach in multiple mouse models of pathologies in which senescence is implicated. As the catalogue of senolytic compounds is expanding, novel survival strategies of senescent cells are uncovered, and variations in sensitivity to senolysis between different types of senescent cells emerge. We propose herein a mechanistic classification of senolytic drugs, based on the level at which they target senescent cells: directly disrupting BH3 protein networks that are reorganized upon senescence induction; downregulating survival-associated pathways essential to senescent cells; or modulating homeostatic processes whose regulation is challenged in senescence. With this approach, we highlight the important diversity of senescent cells in terms of physiology and pathways of apoptosis suppression, and we describe possible avenues for the development of more selective senolytics.

Cellular senescence and the tumour microenvironment

The senescence-associated secretory phenotype (SASP), where senescent cells produce a variety of secreted proteins including inflammatory cytokines, chemokines, matrix remodelling factors, growth factors and so on, plays pivotal but varying roles in the tumour microenvironment. The effects of SASP on the surrounding microenvironment depend on the cell type and process of cellular senescence induction, which is often associated with innate immunity. Via SASP-mediated paracrine effects, senescent cells can remodel the surrounding tissues by modulating the character of adjacent cells, such as stromal, immune cells, as well as cancer cells. The SASP is associated with both tumour-suppressive and tumour-promoting effects, as observed in senescence surveillance effects (tumour-suppressive) and suppression of anti-tumour immunity in most senescent cancer-associated fibroblasts and senescent T cells (tumour-promoting). In this review, we discuss the features and roles of senescent cells in tumour microenvironment with emphasis on their context-dependency that determines whether they promote or suppress cancer development. Potential usage of recently developed drugs that suppress the SASP (senomorphics) or selectively kill senescence cells (senolytics) in cancer therapy are also discussed.

Gemcitabine Cooperates with Everolimus to Inhibit the Growth of and Sensitize Malignant Meningioma Cells to Apoptosis Induced by Navitoclax, an Inhibitor of Anti-Apoptotic BCL-2 Family Proteins

Simple Summary

Meningioma is the most common intracranial neoplasm derived from the arachnoid cap cells of the leptomeninges. Malignant meningioma is generally more aggressive than other meningioma and frequently recurs even after surgery and radiation therapy. Clinical trials have been performed on candidate drugs, including everolimus, an inhibitor of mammalian target of rapamycin. However, an effective standard systemic therapy has not yet been established and the prognosis of patients with malignant meningioma is still poor. We recently reported the radiosensitization effects of gemcitabine in malignant meningioma cells, which suggests its potential to enhance the efficacy of candidate drugs for meningioma. In the present study, we demonstrated that gemcitabine enhanced the therapeutic effects of everolimus in malignant meningioma cells, and these effects were further augmented by navitoclax, an inhibitor of anti-apoptotic BCL-2 family proteins, both in vitro and in vivo. The present results provide support for the clinical application of gemcitabine and navitoclax in combination with everolimus to the treatment of patients with malignant meningioma.

Abstract

Despite several clinical trials with encouraging findings, effective standard systemic therapies have yet to be established for malignant meningioma and the prognosis of these patients remains poor. Accumulating preclinical and clinical evidence suggests that gemcitabine is effective against malignant meningioma. To identify drugs with therapeutic effects that may be enhanced in combination with gemcitabine, we screened drugs that have been tested in preclinical and clinical trials for meningioma. In IOMM-Lee and HKBMM malignant meningioma cells, gemcitabine enhanced the growth inhibitory effects of the mTOR inhibitor everolimus, the clinical benefits of which have been demonstrated in patients with meningioma. The synergistic growth inhibitory effects of this combination were accompanied by cellular senescence characterized by an increase in senescence-associated β-galactosidase activity. To enhance the effects of this combination, we screened senolytic drugs that selectively kill senescent cells, and found that navitoclax, an inhibitor of anti-apoptotic BCL-2 family proteins, effectively reduced the number of viable malignant meningioma cells in combination with everolimus and gemcitabine by inducing apoptotic cell death. The suppression of tumor growth in vivo by the combination of everolimus with gemcitabine was significantly stronger than that by either treatment alone. Moreover, navitoclax, in combination with everolimus and gemcitabine, significantly reduced tumor sizes with an increase in the number of cleaved caspase-3-positive apoptotic cells. The present results suggest that the addition of gemcitabine with or without navitoclax to everolimus is a promising strategy that warrants further evaluation in future clinical trials for malignant meningioma.

Reactive Oxygen Species in the Adverse Outcome Pathway Framework: Toward Creation of Harmonized Consensus Key Events

Reactive oxygen species (ROS) and reactive nitrogen species (RNS) are formed as a result of natural cellular processes, intracellular signaling, or as adverse responses associated with diseases or exposure to oxidizing chemical and non-chemical stressors. The action of ROS and RNS, collectively referred to as reactive oxygen and nitrogen species (RONS), has recently become highly relevant in a number of adverse outcome pathways (AOPs) that capture, organize, evaluate and portray causal relationships pertinent to adversity or disease progression. RONS can potentially act as a key event (KE) in the cascade of responses leading to an adverse outcome (AO) within such AOPs, but are also known to modulate responses of events along the AOP continuum without being an AOP event itself. A substantial discussion has therefore been undertaken in a series of workshops named "Mystery or ROS" to elucidate the role of RONS in disease and adverse effects associated with exposure to stressors such as nanoparticles, chemical, and ionizing and non-ionizing radiation. This review introduces the background for RONS production, reflects on the direct and indirect effects of RONS, addresses the diversity of terminology used in different fields of research, and provides guidance for developing a harmonized approach for defining a common event terminology within the AOP developer community.