Potential Use of Senolytics for Pharmacological Targeting of Precancerous Lesions

Senescence is a cell state that contributes to several homeostatic and pathologic processes. In addition to being induced in somatic cells in response to replicative exhaustion (replicative senescence) as part of organismal aging, senescence can also be triggered prematurely by oncogene hyperactivation or tumor suppressor dysfunction [oncogene-induced senescence (OIS)]. Consequently, senescent cells comprise a major component of precancerous lesions of skin, oral mucosa, nasopharynx, prostate, gut, and lung. Unfortunately, invasive (or minimally invasive) interventions are currently the only available approach employed to eradicate premalignant lesions that carry the potential for cancer progression. Senolytics are a newly emerging drug class capable of selectively eliminating senescent cells. Although senolytics have been successfully demonstrated to mitigate a myriad of aging-related pathologies and to cull senescent cancer cells, there is a paucity of evidence for the potential use of senolytics as a novel approach to eliminate oncogene-induced senescent cells. This Emerging Concepts commentary will 1) summarize evidence in established models of OIS including B-Raf-induced nevi, transgenic lung cancer, and pancreatic adenocarcinoma models, as well as evidence from clinical precancerous lesions; 2) suggest that OIS is targetable; and 3) propose the utilization of senolytic agents as a revolutionary means to interfere with the ability of senescent premalignant cells to progress to cancer in vitro and in vivo If proven to be effective, senolytics will represent an emerging tool to pharmacologically treat precancerous lesions. SIGNIFICANCE STATEMENT: The treatment of premalignant lesions is largely based on the utilization of invasive (or minimally invasive) measures. Oncogene-induced senescence (OIS) is one form of senescence that occurs in response to oncogene overexpression in somatic cells and is present in precancerous lesions. Although the contribution of OIS to disease progression is undetermined, recent evidence suggests that senescent cells are permissive for malignant transformation. Accordingly, the pharmacological targeting of oncogene-induced senescent cells could potentially provide a novel, less invasive, means for the treatment of premalignant disease.

HIV-1 Tat and Morphine Differentially Disrupt Pyramidal Cell Structure and Function and Spatial Learning in Hippocampal Area CA1: Continuous versus Interrupted Morphine Exposure

About half the people infected with human immunodeficiency virus (HIV) have neurocognitive deficits that often include memory impairment and hippocampal deficits, which can be exacerbated by opioid abuse. To explore the effects of opioids and HIV on hippocampal CA1 pyramidal neuron structure and function, we induced HIV-1 transactivator of transcription (Tat) expression in transgenic mice for 14 d and co-administered time-release morphine or vehicle subcutaneous implants during the final 5 d (days 9-14) to establish steady-state morphine levels. Morphine was withheld from some ex vivo slices during recordings to begin to assess the initial pharmacokinetic consequences of opioid withdrawal. Tat expression reduced hippocampal CA1 pyramidal neuronal excitability at lower stimulating currents. Pyramidal cell firing rates were unaffected by continuous morphine exposure. Behaviorally, exposure to Tat or high dosages of morphine impaired spatial memory Exposure to Tat and steady-state levels of morphine appeared to have largely independent effects on pyramidal neuron structure and function, a response that is distinct from other vulnerable brain regions such as the striatum. By contrast, acutely withholding morphine (from morphine-tolerant ex vivo slices) revealed unique and selective neuroadaptive shifts in CA1 pyramidal neuronal excitability and dendritic plasticity, including some interactions with Tat. Collectively, the results show that opioid-HIV interactions in hippocampal area CA1 are more nuanced than previously assumed, and appear to vary depending on the outcome assessed and on the pharmacokinetics of morphine exposure.

Clearance of therapy-induced senescent tumor cells by the senolytic ABT-263 via interference with BCL-XL -BAX interaction

Tumor cells undergo senescence in response to both conventional and targeted cancer therapies. The induction of senescence in response to cancer therapy can contribute to unfavorable patient outcomes, potentially including disease relapse. This possibiliy is supported by our findings that tumor cells induced into senescence by doxorubicin or etoposide can give rise to viable tumors in vivo. We further demonstrate sensitivity of these senescent tumor cells to the senolytic ABT-263 (navitoclax), therefore providing a "two-hit" approach to eliminate senescent tumor cells that persist after exposure to chemotherapy or radiation. The sequential combination of therapy-induced senescence and ABT-263 could shift the response to therapy toward apoptosis by interfering with the interaction between BCL-XL and BAX. The administration of ABT-263 after either etoposide or doxorubicin also resulted in marked, prolonged tumor suppression in tumor-bearing animals. These findings support the premise that senolytic therapy following conventional cancer therapy may improve therapeutic outcomes and delay disease recurrence.

Senescence and castration resistance in prostate cancer: A review of experimental evidence and clinical implications

The development of Castration-Resistant Prostate Cancer (CRPC) remains a major challenge in the treatment of this disease. While Androgen Deprivation Therapy (ADT) can result in tumor shrinkage, a primary response of Prostate Cancer (PCa) cells to ADT is a senescent growth arrest. As a response to cancer therapies, senescence has often been considered as a beneficial outcome due to its association with stable growth abrogation, as well as the potential for immune system activation via the Senescence-Associated Secretory Phenotype (SASP). However, there is increasing evidence that not only can senescent cells regain proliferative capacity, but that senescence contributes to deleterious effects of cancer chemotherapy, including disease recurrence. Notably, the preponderance of work investigating the consequences of therapy-induced senescence on tumor progression has been performed in non-PCa models. Here, we summarize the evidence that ADT promotes a senescent response in PCa and postulate mechanisms by which senescence may contribute to the development of castration-resistance. Primarily, we suggest that ADT-induced senescence may support CRPC development via escape from senescence, by cell autonomous-reprogramming, and by the formation of a pro-tumorigenic SASP. However, due to the scarcity of direct evidence from PCa models, the consequences of ADT-induced senescence outlined here remain speculative until the relationship between senescence and CRPC can be experimentally defined.

Therapy-Induced Senescence: An "Old" Friend Becomes the Enemy

For the past two decades, cellular senescence has been recognized as a central component of the tumor cell response to chemotherapy and radiation. Traditionally, this form of senescence, termed Therapy-Induced Senescence (TIS), was linked to extensive nuclear damage precipitated by classical genotoxic chemotherapy. However, a number of other forms of therapy have also been shown to induce senescence in tumor cells independently of direct genomic damage. This review attempts to provide a comprehensive summary of both conventional and targeted anticancer therapeutics that have been shown to induce senescence in vitro and in vivo. Still, the utility of promoting senescence as a therapeutic endpoint remains under debate. Since senescence represents a durable form of growth arrest, it might be argued that senescence is a desirable outcome of cancer therapy. However, accumulating evidence suggesting that cells have the capacity to escape from TIS would support an alternative conclusion, that senescence provides an avenue whereby tumor cells can evade the potentially lethal action of anticancer drugs, allowing the cells to enter a temporary state of dormancy that eventually facilitates disease recurrence, often in a more aggressive state. Furthermore, TIS is now strongly connected to tumor cell remodeling, potentially to tumor dormancy, acquiring more ominous malignant phenotypes and accounts for several untoward adverse effects of cancer therapy. Here, we argue that senescence represents a barrier to effective anticancer treatment, and discuss the emerging efforts to identify and exploit agents with senolytic properties as a strategy for elimination of the persistent residual surviving tumor cell population, with the goal of mitigating the tumor-promoting influence of the senescent cells and to thereby reduce the likelihood of cancer relapse.

Abstract 902: The antiandrogen bicalutamide induces senescence in LNCaP cells and quiescence in Myc CaP cells

The survival benefits in prostate cancer (PCa) conferred by androgen deprivation therapies (ADT) and antiandrogens are often blunted by the development of castration resistant disease (CRPC). There is increasing evidence that the primary cellular response to ADT is senescence rather than apoptosis, and this may influence progression to CRPC. However, the majority of in vitro studies employ the charcoal stripped serum model, and very few studies use antiandrogens, despite the fact that antiandrogens are currently one of the primary modes of therapy for prostate cancer. In an effort to enhance our understanding of antiandrogen effects in PCa, we evaluated the cellular response to the clinically relevant antiandrogen, bicalutamide, in the human LNCaP and murine Myc-CaP cell lines. Acute treatment (48 hours) with bicalutamide resulted in a growth arrest followed by proliferative recovery in both cell lines. However, while the growth arrest was prolonged (lasting at least 6 days) in the LnCaP cells, growth arrest was exceedingly brief (lasting approximately 24 hours) in the Myc-CaP cells. Consistent with these observations, we were able to detect increased beta galactosidase staining (a hallmark of senescence) in the LnCaP treated cells but not the Myc-CaP treated cells. Despite the fact that profound morphological changes were evident in the Myc-Cap cells, including an enlarged and elongated cell body, the highly transient nature of the growth arrest suggests that the Myc-Cap cells undergo quiescence rather than senescence. The absence of senescence induction in the Myc-CaP cells may be a consequence of the androgen receptor copy number gain. However, androgen duplication is highly uncommon in human prostate cancers prior to development of CRPC. Thus, it is likely that antiandrogens can induce senescence in treatment-naïve patients, as seen in the LNCaP cell line. The ability of bicalutamide to induce senescence, albeit perhaps conditionally, suggests that some prostate cancers treated with ADT and/or antiandrogens may be susceptible to drugs that target senescent cells (senolytics). We therefore propose that elimination of senescent cells prior to recovery could attenuate or prevent the development of CRPC.

Citation Format: Valerie J. Carpenter, Tareq Saleh, David A. Gewirtz. The antiandrogen bicalutamide induces senescence in LNCaP cells and quiescence in Myc CaP cells [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 902.