Faculty Bibliography

INTRODUCTION/BACKGROUND:Cellular senescence is a stable form of cell-cycle exit. Though they no longer divide, senescent cells remain metabolically active and secrete a plethora of proteins collectively termed the senescence-associated secretory phenotype (SASP). Although senescence-associated cell-cycle exit likely evolved as an anti-tumor mechanism, the SASP contains both anti- and pro-tumorigenic potential. Areas covered: In this review, we briefly discuss the discovery of senescent cells and its relationship to cancer and aging. We also describe the initiation and regulation of the SASP upon senescence stimulus onset. We focus on both the pro- and anti-tumorigenic properties of the SASP. Finally, we speculate on the potential benefits of therapy-induced senescence combined with selective SASP inhibition for the treatment of cancer. Expert opinion: Further identification and characterization of the SASP factors that are pro-tumorigenic and those that are anti-tumorigenic in specific contexts will be crucial in order to develop personalized therapeutics for the successful treatment of cancer.

Cellular senescence has emerged as a potent tumor-suppressor mechanism in numerous human neoplasias. Senescent cells secrete a distinct set of factors collectively termed the senescence-associated secretory phenotype (SASP), which has been postulated to carry both pro- and anti-tumorigenic properties depending on tissue context. However, the in vivo effect of the SASP is poorly understood due to the difficulty of studying the SASP independently of other senescence-associated phenotypes. Here, we report that disruption of the IL-1 pathway completely uncouples the SASP from other senescence-associated phenotypes such as cell cycle exit. Transcriptome profiling of IL-1 receptor (IL-1R) - depleted senescent cells indicates that IL-1 controls the late arm of the senescence secretome, which consists of pro-inflammatory cytokines induced by NF-κB. Our data suggest that both interleukin (IL)-1α and IL-1β signal through IL-1R to upregulate the SASP in a cooperative manner. Finally, we show that IL-1α inactivation impairs tumor progression and immune cell infiltration without affecting cell cycle arrest in a mouse model of pancreatic cancer, highlighting the pro-tumorigenic property of the IL-1-dependent SASP in this context. These findings provide novel insight into the therapeutic potential of targeting the IL-1 pathway in inflammatory cancers.

Cellular senescence is a stable growth arrest that is implicated in tissue ageing and cancer. Senescent cells are characterized by an upregulation of proinflammatory cytokines, which is termed the senescence-associated secretory phenotype (SASP). NAD⁺ metabolism influences both tissue ageing and cancer. However, the role of NAD⁺ metabolism in regulating the SASP is poorly understood. Here, we show that nicotinamide phosphoribosyltransferase (NAMPT), the rate-limiting enzyme of the NAD⁺ salvage pathway, governs the proinflammatory SASP independent of senescence-associated growth arrest. NAMPT expression is regulated by high mobility group A (HMGA) proteins during senescence. The HMGA-NAMPT-NAD⁺ signalling axis promotes the proinflammatory SASP by enhancing glycolysis and mitochondrial respiration. HMGA proteins and NAMPT promote the proinflammatory SASP through NAD⁺-mediated suppression of AMPK kinase, which suppresses the p53-mediated inhibition of p38 MAPK to enhance NF-κB activity. We conclude that NAD⁺ metabolism governs the proinflammatory SASP. Given the tumour-promoting effects of the proinflammatory SASP, our results suggest that anti-ageing dietary NAD⁺ augmentation should be administered with precision.

Ubiquitously-expressed, prefoldin-like chaperone (UXT) also called Androgen Receptor Trapped clone-27 (ART-27) is widely expressed in human tissues. Our previous studies showed that UXT regulates transcription repression including androgen receptor (AR) signaling in prostate cancer. Here we analyzed a tissue microarray consisting of normal prostate, benign prostatic hyperplasia, high grade prostatic intraepithelial neoplasia (HGPIN) and primary prostate cancer cases for UXT protein expression. We found that HGPIN and malignant tumors have significantly decreased UXT expression compared to the normal prostate. Loss of UXT expression in primary prostate cancer is positively associated with high Gleason grade and poor relapse-free survival. We engineered prostate-specific Uxt

B cell development is a highly regulated process that requires stepwise rearrangement of immunoglobulin genes to generate a functional B cell receptor (BCR). The polycomb group protein BMI1 is required for B cell development, but its function in developing B cells remains poorly defined. We demonstrate that BMI1 functions in a cell-autonomous manner at two stages during early B cell development. First, loss of BMI1 results in a differentiation block at the pro-B cell to pre-B cell transition due to the inability of BMI1-deficient cells to transcribe newly rearranged Igh genes. Accordingly, introduction of a pre-rearranged Igh allele partially restored B cell development in Bmi1^-/- mice. In addition, BMI1 is required to prevent premature p53 signaling, and as a consequence, Bmi1^-/- large pre-B cells fail to properly proliferate. Altogether, our results clarify the role of BMI1 in early B cell development and uncover an unexpected function of BMI1 during VDJ recombination.

The mammalian DREAM complex is responsible for the transcriptional repression of hundreds of cell-cycle-related genes in quiescence. How the DREAM complex recruits chromatin-modifying entities to aid in its repression remains unknown. Using unbiased proteomics analysis, we have uncovered a robust association between the chromatin-associated Sin3B protein and the DREAM complex. We have determined that genetic inactivation of Sin3B results in the de-repression of DREAM target genes during quiescence but is insufficient to allow quiescent cells to resume proliferation. However, inactivation of APC/C^CDH1 was sufficient for Sin3B^-/- cells, but not parental cells, to re-enter the cell cycle. These studies identify Sin3B as a transcriptional corepressor associated with the DREAM complex in quiescence and reveals a functional cooperation between E2F target repression and APC/C^CDH1 in the negative regulation of cell-cycle progression.

INTRODUCTION: Sin3B serves as a scaffold for chromatin-modifying complexes that repress gene transcription to regulate distinct biological processes. Sin3B-containing complexes are critical for cell cycle withdrawal, and abrogation of Sin3B-dependent cell cycle exit impacts tumor progression. Areas covered: In this review, we discuss the biochemical characteristics of Sin3B-containing complexes and explore how these complexes regulate gene transcription. We focus on how Sin3B-containing complexes, through the association of the Rb family of proteins, repress the expression of E2F target genes during quiescence, differentiation, and senescence. Finally, we speculate on the potential benefits of the inhibition of Sin3B-containing complexes for the treatment of cancer. Expert opinion: Further identification and characterization of specific Sin3B-containing complexes provide a unique opportunity to prevent the pro-tumorigenic effects of the senescence-associated secretory phenotype, and to abrogate cancer stem cell quiescence and the associated resistance to therapy.

Distinguishing between indolent and aggressive prostate adenocarcinoma (PCa) remains a priority to accurately identify patients who need therapeutic intervention. SIN3B has been implicated in the initiation of senescence in vitro Here we show that in a mouse model of prostate cancer, SIN3B provides a barrier to malignant progression. SIN3B was required for PTEN-induced cellular senescence and prevented progression to invasive PCa. Furthermore, SIN3B was downregulated in human PCa correlating with upregulation of its target genes. Our results suggest a tumor suppressor function for SIN3B that limits PCa progression, with potential implications for the use of SIN3B and its target genes as candidate diagnostic markers to distinguish indolent from aggressive disease.

Mechanisms that regulate tissue-specific progenitors for maintenance and differentiation during development are poorly understood. Here, we demonstrate that the co-repressor protein Sin3a is crucial for lung endoderm development. Loss of Sin3a in mouse early foregut endoderm led to a specific and profound defect in lung development with lung buds failing to undergo branching morphogenesis and progressive atrophy of the proximal lung endoderm with complete epithelial loss at later stages of development. Consequently, neonatal pups died at birth due to respiratory insufficiency. Further analysis revealed that loss of Sin3a resulted in embryonic lung epithelial progenitor cells adopting a senescence-like state with permanent cell cycle arrest in G1 phase. This was mediated at least partially through upregulation of the cell cycle inhibitors Cdkn1a and Cdkn2c. At the same time, loss of endodermal Sin3a also disrupted cell differentiation of the mesoderm, suggesting aberrant epithelial-mesenchymal signaling. Together, these findings reveal that Sin3a is an essential regulator for early lung endoderm specification and differentiation.