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BackgroundA growing body of literature suggests the cell–intrinsic activity of Atf6α during ER stress responses has implications for tissue cell number during growth and development, as well as in adult biology and tumorigenesis [1]. This concept is important, linking the cellular processes of secretory protein synthesis and endoplasmic reticulum stress response with functional tissue capacity and organ size. However, the field contains conflicting observations, especially notable in secretory cell types like the pancreatic beta cell.Scope of reviewHere we summarize current knowledge of the basic biology of Atf6α, along with the pleiotropic roles Atf6α plays in cell life and death decisions and possible explanations for conflicting observations. We include studies investigating the roles of Atf6α in cell survival, death and proliferation using well-controlled methodology and specific validated outcome measures, with a focus on endocrine and metabolic tissues when information was available.Major conclusionsThe net outcome of Atf6α on cell survival and cell death depends on cell type and growth conditions, the presence and degree of ER stress, and the duration and intensity of Atf6α activation. It is unquestioned that Atf6α activity influences the cell fate decision between survival and death, although opposite directions of this outcome are reported in different contexts. Atf6α can also trigger cell cycle activity to expand tissue cell number through proliferation. Much work remains to be done to clarify the many gaps in understanding in this important emerging field.

Aging is associated with loss of proliferation of the insulin-secreting β-cell, a possible contributing factor to the increased prevalence of type 2 diabetes in the elderly. Our group previously discovered that moderate endoplasmic reticulum (ER) stress occurring during glucose exposure increases the adaptive β-cell proliferation response. Specifically, the ATF6α arm of the tripartite Unfolded Protein Response (UPR) promotes β-cell replication in glucose excess conditions. We hypothesized that β-cells from older mice have reduced proliferation due to aberrant UPR signaling or an impaired proliferative response to ER stress or ATF6α activation. To investigate, young and old mouse islet cells were exposed to high glucose with low-dose thapsigargin or activation of overexpressed ATF6α, and β-cell proliferation was quantified by BrdU incorporation. UPR pathway activation was compared by qPCR of target genes and semi-quantitative Xbp1 splicing assay. Intriguingly, although old β-cells had reduced proliferation in high glucose compared to young β-cells, UPR activation and induction of proliferation in response to low-dose thapsigargin or ATF6α activation in high glucose were largely similar between young and old. These results suggest that loss of UPR-led adaptive proliferation does not explain the reduced cell cycle entry in old β-cells, and raise the exciting possibility that future therapies that engage adaptive UPR could increase β-cell number through proliferation even in older individuals.

Most prostate cancers are immunologically “cold” with poor tumor immunogenicity and a lack of cytotoxic T cells, and as such are generally unresponsive to immune checkpoint blockade (ICB) therapies that can reactivate T cell immunity and lead to durable and curative responses in other treatment-refractory solid tumors. We recently demonstrated that therapeutic induction of a durable growth arrest phenotype known as cellular senescence in the setting of pancreatic cancer can be novel approach to make “cold” tumors “hot” and sensitize them to anti-PD-1 ICB (Ruscetti et al. Cell 2020). This senescence-driven immunological effect is mediated through induction of immunomodulatory cell surface molecules, including MHC-I expression required for antigen presentation to T cells, and activation of the senescence-associated secretory phenotype (SASP), a collection of pleiotropic cytokines, chemokines, and other growth factors that can remodel the surrounding tumor-immune landscape. While this acute SASP activation can promote cytotoxic CD8+ T and Natural Killer (NK) cell anti-tumor immunity in some contexts, chronic SASP can alternatively lead to immune suppression and tumor cell invasion and metastasis that contributes to relapse after therapy. Here we set out to characterize the senescence-inducing capacity of chemotherapies, DNA repair inhibitors, and cell cycle targeting agents implicated in cellular senescence and used in the treatment of castration-resistant prostate cancer (CRPC), and the impact of their subsequent SASPs on immune responses and therapy outcomes. Using a suite of genetically-defined murine prostate cancer cell lines and an in vitro pipeline to rapidly define senescence phenotypes following treatment, we found that many commonly used cancer therapeutics can induce cellular senescence, with chemotherapies, Aurora kinase inhibitors, and CDK inhibitors producing the most robust senescence-induced cell cycle arrest across conditions. Prostate tumor cells harboring compound tumor suppressor losses such as Pten, p53, and Rb1 were more susceptible to therapeutic senescence induction. Strikingly, only a subset of therapies that induce senescence-associated growth arrest produced an inflammatory SASP and induction of activating NK cell ligands and antigen presentation and processing genes necessary for NK and T cell immunity. Moreover, whereas most SASPs induced NK cell activity in vitro, they also polarized macrophages toward a pro-tumorigenic and immune suppressive M2 phenotype that may limit their anti-tumor efficacy. CDK inhibitors produced a SASP that could activate lymphocytes without inducing suppressive myeloid populations in MYC-driven tumors, suggesting their use may be a promising approach to target prostate cancers harboring the currently “undruggable” MYC oncogene. Collectively, our studies demonstrate that induction of senescence and its non-cell autonomous arm, the SASP, could be a unique approach to treat CRPC and create new immunological dependencies that can be potentially harnessed for prostate cancer immunotherapy. Citation Format: Lin Zhou, Katherine C. Murphy, Jarin Snyder, Kelly D. DeMarco, Boyang Ma, Marcus Ruscetti. Leveraging therapy-induced senescence for prostate cancer immunotherapy [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr B035.

Prostate cancer is the most common malignancy and the second leading cause of cancer-related death in American men. Although many disease cases remain latent for decades, others progress to malignant prostate cancer with high rates of mortality. The molecular underpinnings driving malignancy are still poorly understood, and there is an urgent need to identify mechanisms and informative biomarkers of disease progression. Using the Pb-Cre+; Ptenfl/fl (CP) mouse model, we and others found that Pten genomic loss, a common occurrence in human prostate cancer, leads to induction of cellular senescence, senescence-associated inflammation, and immune suppression during tumor onset. Similarly, senescence markers have also been identified in benign and early prostate cancer lesions in humans. This suggests that senescence may play a yet uncharacterized functional role in prostate tumorigenesis. Cellular senescence is a damage-induced pathway leading to durable growth arrest and has long been considered a potent tumor suppressive mechanism that limits the ability of pre-malignant cells to proliferate and develop into malignant tumors. However, recent findings suggest that the accumulation of senescent cells can paradoxically lead to chronic inflammation that fosters an environment for eventual tumor development and progression through the induction of growth and inflammatory factors collectively known as the senescence-associated secretory phenotype (SASP). Thus, it remains unclear whether senescence or specific senescent cell populations block or rather promote prostate tumor development. Leveraging the CP prostate cancer mouse model and genetic tools to track senescent cells, we observed that markers of senescence (SA-β-gal, p16, Bcl2) and the inflammatory SASP (NF-𝜅B, IL-1b, CXCL1, CSF-1) were not only present in early prostate intraepithelial neoplasia (PIN) lesions but remained elevated following adenocarcinoma development and progression, and could be found in both epithelial cells as well as other immune and stromal populations. Expression of the canonical senescence marker p16 also correlated with disease progression in human prostate cancer samples. Moreover, senescent cell accumulation was associated with a reduction in cytotoxic lymphocytes and an influx of suppressive myeloid populations that we and others have previously shown contribute to prostate cancer progression. Senolytic approaches to eliminate these senescent cell populations reduced progression to adenocarcinoma and remodeled the immune system to reverse myeloid-mediated immune suppression and activate cytotoxic lymphocyte immunity. Together, our preliminary results suggest that senescence might contribute to prostate cancer initiation and progression, and that removal of senescent cells could delay tumor onset and reverse prostate cancer immune suppression. Citation Format: Lin Zhou, Jarin Snyder, Katherine C. Murphy, Kelly D. DeMarco, Sachliv Chana, Karl Simin, Zhong Jiang, Marcus Ruscetti. Dissecting the role of cellular senescence in prostate cancer initiation and immune suppression [abstract]. In: Proceedings of the AACR Special Conference: Advances in Prostate Cancer Research; 2023 Mar 15-18; Denver, Colorado. Philadelphia (PA): AACR; Cancer Res 2023;83(11 Suppl):Abstract nr A028.