Browsing by Author "Wang, Xiao-Fan"

During the course of breast cancer progression, normally dormant tumor-promoting effects of transforming growth factor-beta (TGF-beta) including migration, invasion, and metastasis are unmasked. Although this switch or gain of TGF-beta function has been modeled extensively in in-vivo and in-vitro breast cancer systems, the signaling mechanisms that control this TGF-beta switch are poorly understood. Indeed, the precise role of canonical TGF-beta signaling through the type I TGF-beta receptor, ALK5, and its intracellular effectors, Smad2 and Smad3, is still poorly understood. In an effort to identify mechanisms that regulate the ability of TGF-beta to stimulate mammary epithelial cell migration in-vitro, we found that TGF-beta stimulates the phosphorylation of Smad1 and Smad5, intracellular effectors that are typically associated with bone morphogenetic protein (BMP) signaling. As this phosphorylation response has not been reported extensively, little is known about the prevalance, mechanism, function, or pathological relevance of TGF-beta-stimulated Smad1/5 phosphorylation.

Herein, we use pharmacologic inhibition, RNA interference, and additional biochemical and cell-based approaches to identify a novel mechanism and function for non-canonical TGF-beta signaling through an ALK5-Smad1/5 axis. We show that TGF-beta stimulates Smad1/5 phosphorylation in an ALK5 dependent manner in cells of epithelial, endothelial, and embryonic origin. Mechanistically, this phosphorylation event requires the kinase activity and, unexpectedly, the L45 loop motif of ALK5. Functionally, this phosphorylation event is essential to the initiation and promotion of TGF-beta-stimulated migration in mammary epithelial cells. Interestingly, this phosphorylation event may promote migration by regulating TGF-beta target gene expression, as evidenced by the identification of putative Smad1/5-dependent TGF-beta target genes using microarray analysis. Finally, of particular relevance to mammary tumor progression, this phosphorylation event is preferentially detected in permissive environments such as those created by tumorigenic cells or HER2 oncogene activation.

Taken together, our data provides evidence that TGF-beta-stimulated Smad1/5 phosphorylation, which occurs through a non-canonical mechanism that challenges the notion of selective Smad phosphorylation by ALK5, mediates the pro-migratory TGF-beta switch in mammary epithelial cells.

Transforming growth factor–β (TGF–β) signaling regulates a range of processes in a variety of cell types. Consequently, TGF–β plays a complex role in the progression of several types of cancers; it acts as a tumor suppressor in normal cells and early in tumor progression, yet it can promote tumor progression in later stages of cancer.

Among the cancers that TGF–β has been implicated in is glioblastoma multiforme (GBM), the most common primary brain neoplasm and one of the most lethal types of cancer. Because of its high mortality rate and the lack of effective treatments, discovering the molecular mechanisms that underlie GBM formation and growth is of great clinical interest. To this end, we investigated the function of a TGF–β target gene — the putative tumor suppressor N‐Myc downstream‐regulated gene 4 (NDRG4) — in GBM cell viability, proliferation and tumor formation. Contrary to the established roles of other NDRG family members, we found that NDRG4 expression is elevated in GBM and that NDRG4 is required for the survival of established GBM cell lines and primary GBM xenograft cells enriched for highly tumorigenic GBM cancer stem cells. Knockdown of NDRG4 expression results in G₁ cell cycle arrest followed by apoptosis that is associated with a decrease in the expression of XIAP and survivin. Finally, knockdown of NDRG4 expression in established GBM cell lines and GBM cancer stem cells results in decreased tumorigenicity following intracranial implantation of these cells into immunocompromised mice. Collectively, these data indicate that NDRG4 does not function as a tumor suppressor like other NDRG family members, but rather it is essential for GBM tumorigenicity and may represent a potential therapeutic target for this devastating disease.

In the second portion of this dissertation, we examine the TGF–β cytostatic signaling pathway in B lymphocytes. TGF–β–induced growth inhibition is the most extensively studied biological response to a TGF–β signal. Although in most cell types this response is mediated by Smad3– dependent regulation of c–Myc, p15Ink4B, and p21Cip1 transcription, studies from Smad3 null mice suggest that TGF–β–induced growth inhibition in B lymphocytes occurs regardless of Smad3 status. We prove that this response does indeed occur independently of Smad3 in purified primary B lymphocytes and WEHI–231 cells. Consistent with this, p15Ink4B and p21Cip1 are not noticeably induced by TGF–β in these cells, whereas Id3 and cyclin G2 are induced in a Smad3–independent manner. Finally, unlike the MAPK pathways we tested, the BMP–specific Smads 1 and 5 are activated in response to TGF–β in these cells, and this activation is dependent on ALK5 kinase activity. Collectively, these data indicate that TGF–β induces growth inhibition in B lymphocytes through a novel signaling pathway, and Smads 1 and 5 may help mediate this response.

Abstract

The trefoil factor family of secreted proteins contains three members; trefoil factor 1 or TFF1, trefoil factor 2 or TFF2, and trefoil factor 3 or TFF3. These three proteins share a conserved 42-43 amino acid domain containing 6 cysteine residues resulting in three disulfide bonds that holds the protein in a characteristic three-loop or "trefoil structure" known as the P domain. TFF1 is primarily localized to the stomach and secreted by the gastric mucosa while TFF2 and TFF3 are primarily localized to the colon and duodenum and secreted by the goblet cells. All three of these proteins play a protective role in the gastrointestinal tract where they are normally localized and have been identified as possible tumor suppressors, however, these proteins are also upregulated in cancer within tissues where they are not normally expressed including the breast, pancreas, prostate, and liver. The mechanisms by which two of these factors, TFF1 and TFF3, promote tumorigenesis remain largely undefined. In this dissertation we will attempt to elucidate these mechanisms as well as the regulation of these two proteins in both pancreatic and prostate cancer. Many of the underlying genetic and molecular mechanisms involved in the development of both pancreatic and prostate cancer remain largely unknown and as a result, therapeutic and diagnostic tools for treating these diseases are not as effective as they could be. By deciphering the role of TFF1 and TFF3 in these cancers, they could potentially serve as new therapeutic targets or biomarkers for treating both diseases.

Chapter 2 of this dissertation will examine the functional role of TFF1 promoting tumorigenesis in pancreatic and prostate cancer. We will show that TFF1 expression is critical for the viability of both pancreatic and prostate cancer cells and that reduction of TFF1 expression in these cells results in decreased tumorigenicity when implanted in immunocompromised mice. It will also be demonstrated that TFF1's function in promoting tumorigenicity is its ability to assist tumor cells overcome the tumor suppressive barrier of senescence. Thirdly, we show that the form of senescence that TFF1 assists in allowing the cells overcome is oncogene-induced senescence (OIS). Lastly, a cell cycle array identifies the potential downstream target p21CIP, a cyclin-dependent kinase inhibitor and OIS marker, whose expression is induced by loss of TFF1 expression.

In Chapter 3 of this work, we examine the role of another trefoil factor family member, TFF3, and its role in promoting prostate tumorigenesis. Just as with TFF1, it appears that TFF3 3 expression is critical for prostate cancer cell viability and tumorigenicity using the same experimental techniques used in Chapter 2. Using a genetically defined model of prostate cancer, a PI3-kinase-dependent regulatory mechanism of TFF3 emerges in this prostate cancer context. Using this system we begin to see a divergence in both regulation and function of TFF1 and TFF3 in prostate cancer. Finally, a mouse model expressing TFF3 was developed to monitor the histopthological changes associated with expression of this protein. Initial characterization of this model suggests a hyperplastic phenotype coinciding with TFF3 expression in the prostate.

The two studies in this dissertation establish a role of TFF1 and TFF3 in both prostate and pancreatic tumorigenesis and demonstrate that ablation of expression of both proteins is a potent inhibitor of tumorigenesis. With this knowledge, it is possible that TFF1 and TFF3 may become a potential therapeutic target or diagnostic marker for better treatment of prostate and pancreatic cancer.

Cellular senescence is a fundamental cell fate playing significant and complex roles during tumorigenesis and natural aging process. However, the molecular determinants distinguishing senescence from other temporary and permanent cell-cycle arrest states such as quiescence and post-mitotic state and the specified mechanisms underlying cell-fate decisions towards senescence versus cell death in response to cellular stress stimuli remain less understood. In our studies, we aimed to employ multi-omics approaches to deepen our understanding of cellular senescence, in particular, regarding the specific molecular determinants distinguishing cellular senescence from other non-dividing cell fates.

Notably, one of the most prominent features of cellular senescence differing from other non-dividing cell fates is the increased expression of senescence-associated beta-galactosidase. Because 5-Dodecanoylaminofluorescein Di-β-D-Galactopyranoside (C12FDG) is known as the substrate catalyzed by beta-galactosidase for producing a green fluorescent product, we applied this compound to the cells undergoing G1 cell-cycle arrest (a mixture of senescent and quiescent cells). Employing fluorescence-activated cell sorting, we separated and collected senescent and quiescent cell populations based on green fluorescence intensity. As cellular senescence is more than just the non-dividing cell fate, we therefore systematically compared the gene expression between senescence and quiescence to provide insights into the specific features underlying senescence programming beyond cell cycle arrest. Following this strategy for the comparative gene expression analysis, we identified and characterized several genes critically involved in the program of cellular senescence, and one of the major findings was to identify IMMP2L, a nuclear-encoded mitochondrial intermembrane peptidase, can act as a molecular switch for determining the cell fates of healthy living, cell death, and senescence.

Inhibiting IMMP2L signaling through either the suicidal protease inhibitor SERPINB4 or transcriptional downregulation was sufficient to initiate cellular senescence by reprogramming the mitochondria functionality. Employing proteomics, we identified at least two mitochondrial target proteins processed by IMMP2L, including metabolic enzyme GPD2 and cell death regulator/electron transport chain complex I component AIF. Functional study suggests that, in healthy cells, the IMMP2L-GPD2 axis catalyzes redox reactions to produce phospholipid precursor Glycerol 3-phosphate; while under oxidative stress, IMMP2L cleaves AIF into its truncated pro-apoptotic form leading to cell death initiation to remove cells with irreparable damage. For cells programmed to senesce, the IMMP2L-GPD2 axis is switched off to block phospholipid biosynthesis leading to reduced availability of membrane building blocks for cell growth together with the disruption of mitochondrial localization of certain phospholipid-binding kinases, such as protein kinase C-δ (PKC-δ) and its downstream signaling. These alterations in mitochondria-associated metabolism and signaling network promote entry into a senescent state featuring high levels of reactive oxygen species (ROS). Simultaneously, blockage of pro-apoptotic AIF generation, which is due to the loss of IMMP2L, ensures the viability of senescent cells under ROS-mediated oxidative stress. Taken together, we have mechanistically uncovered IMMP2L-mediated signaling as a key regulatory pathway in the control of fates of healthy, apoptotic, and senescent cells.

In the physiological conditions, we observed that IMMP2L is downregulated in the muscle tissues and the blood samples of geriatric groups compared to that from young cohorts. Besides, centenarians display better genomic integrity at the IMMP2L locus when compared with the general population. Taken together, it suggests IMMP2L could also be an important player associated with the natural aging process.

The essential branched-chain amino acids (BCAAs) leucine, isoleucine, and valine play critical roles in protein synthesis and energy metabolism. Despite their widespread use as nutritional supplements, the comprehensive impacts of BCAAs on mammalian physiological aging process remain uncertain due to the complexities of BCAA metabolic regulation. Cellular senescence is a phenotype of stable cell cycle arrest that contributes to aging and age-related diseases. Senescent cells accumulate in aged tissues and drive age-related disorders through the senescence-associated secretory phenotype (SASP), a process whereby a spectrum of proteins including inflammatory cytokines are secreted into the extracellular space. However, the molecular mechanisms controlling SASP induction remain incompletely understood. On a mechanistic front, while the activity of the mammalian target of rapamycin complex 1 (mTORC1) signaling has links to cellular senescence, the exact events leading to mTORC1 activation are not clear. Here we report that alterations in BCAA metabolism, which are mediated by the BCAA transporters Solute Carrier Family 6 Members 14 and 15 (SLC6A14 and SLC6A15) and the enzyme BCAA transaminase 1 (BCAT1), trigger the SASP program during senescence. Increased expression of SLC6A14/15, with a concomitant reduction in BCAT1, contributes to elevated intracellular BCAAs in senescent cells; this in turn activates mTORC1 signaling to establish the full SASP program. Transgenic Drosophila models further indicate that orthologous of mammalian SLC6A15 are involved in the induction of cellular senescence and age-related phenotypes in flies, suggesting evolutionary conservation of this metabolic pathway during the aging process. Finally, experimentally blocking BCAA accumulation attenuates the inflammatory response in a mouse model, highlighting the therapeutic potential of modulating BCAA metabolism for the treatment of age-related and inflammatory diseases.

Ionizing radiation causes DNA damage that activates signal transduction pathways and transcriptional programs, which alters cell fate, tissue integrity, and even animal survival. Potential risks of radiation disasters and the common side effects of radiation therapy to cancer patients provide the rationale to better understand the mechanisms regulating radiation-induced injury. This dissertation interrogates the functions of p53 transactivation domains (TADs) in different contexts of radiation-induced tissue injury, including cardiac injury following whole heart irradiation (WHI) and radiation-induced gastrointestinal (GI) syndrome following sub-total body irradiation (SBI).Following our previous study showing the role of p53 in endothelial cells functions to protect against WHI-induced cardiac injury, here we used vascular endothelial cadherin-Cre (VECre) mice to delete p53 or express p53 TAD mutants in cardiac endothelial cells. Abrogation of full p53 transactivation capacity (both TAD1 and TAD2) led to the development of profound myocardial necrosis and mice succumbed to radiation-induced cardiovascular disease. Compromised p53 TAD1 function alone also sensitized mice to radiation-induced cardiac injury, although these mice showed less prominent radiation-induced myocardial necrosis. Taken together, our work highlights the importance of p53 TAD1-mediated DNA damage response in cardiac endothelial cells in preventing radiation-induced late effects to the heart. In addition to studying the roles of p53 transactivation in modulating late sequelae of radiation exposure to the heart, we investigated p53-mediated transcriptional networks that orchestrate cellular reprogramming following radiation injury in the intestine. Using villin 1-Cre (VillinCre) to modify p53 status in intestinal epithelial cells, we showed that p53 TAD1 is critical for p53 to prevent the radiation-induced GI syndrome. Moreover, we showed that p53 is essential for radiation-induced transient expansion of the clusterin (Clu)-positive revival stem cell population that facilitates reconstitution of the injured intestinal epithelium. Expression of p53 protein in the intestine preceded and coincided with the emergence and expansion of Clu+ cells. Remarkably, single-cell transcriptomic analysis revealed high enrichment of the p53 transcriptional program in Clu+ cells. Moreover, genetic deletion of p53 specifically in Clu+ cells increased the sensitivity of mice to the radiation-induced GI syndrome, though this difference was not statistically significant. Single-cell transcriptomic analysis indicated that decreased cell cycle arrest, higher levels of ferroptosis and apoptosis, and aberrantly upregulated YAP signature expression occurred in p53-deficient intestinal epithelium, all of which might collectively contribute to the inability of intestinal crypts to induce Clu+ cells to promote survival following SBI. In summary, this dissertation demonstrates the importance of p53 TAD1 in endothelial cells in controlling radiation-induced late effects in the heart and the essential role of p53 TAD1 in GI epithelial cells in protecting against radiation-induced intestinal injury. This work dissected the mechanism of radiation-induced intestinal regeneration by establishing that p53 signaling is necessary for the transient expansion of Clu+ revival stem cells. The work delineates tissue-specific and cell type-dependent functions of p53 transactivation, which has led to a new understanding of how p53 protects tissues from radiation injury. Ultimately, this work may promote the development of useful radiation countermeasures that can be used in a radiation emergency or to help prevent radiation injury to cancer patients.

The human epidermal growth factor receptor 2, or HER2, is overexpressed in 20-30% breast cancer patients and is associated with aggressive disease. Therapies targeting HER2, including monoclonal antibodies (trastuzumab and pertuzumab), a small molecule kinase inhibitor (lapatinib) and an antibody-drug conjugate (trastuzumab emtansine), have significantly prolonged the overall survival of HER2-positive breast cancer patients. However, almost all patients develop resistance either from the beginning of therapy or with prolonged treatment in two years.

Previous studies to unveil the resistance mechanisms were mainly focused on acquired resistance, culturing cells with HER2 inhibitors and making comparisons to their parental cells. In order to study the mechanism mediating intrinsic resistance, we conducted a loss-of-function genetic screen using a HER2-amplified cell line that is intrinsically resistant to HER2 inhibitors with the purpose to identify synthetic lethal targets. TALDO1, a gene encoding a metabolic enzyme in the non-oxidative pentose phosphate pathway was identified from the screen. Metabolic profiling with isotope-labeled glucose was used to understand the mechanism. The profiling results indicated that TALDO1 was necessary for cellular NADPH generation to combat increased cellular ROS and support synthesis of lipids as a result of HER2 inhibition.

Importantly, the higher expression of TALDO1 is associated with poor response to HER2-targeted therapy in a small cohort of HER2-positive breast cancer patients, suggesting it could potentially serve as a biomarker to predict patient response.

Together our study explained a novel mechanism mediating intrinsic resistance to HER2 inhibition with significant clinical value. Combined inhibition of HER2 signaling and the pentose phosphate pathway may result in a better clinical outcome.

The eukaryotic genome integrity is safeguarded by the DNA damage response, which is composed of a network of signal transduction pathways that upon genotoxic stresses, arrest cell cycle progression, motivate repair processes, or induce apoptosis or senescence when cells incur irreparable DNA damage. During this process, DNA damage-induced post-translational modifications, most notably protein phosphorylation, of a variety of DNA damage-responsive proteins has been shown to mediate the initiation, transduction and reception of the DNA damage signals, resulting in alterations of their stability, activities or subcellular localizations, ultimately leading to activation of various downstream effector pathways.

While a lot has been elucidated on the downstream events of the DNA damage response, little is known about how DNA damage is detected. Two still ongoing studies of this dissertation attempt to address this question. Our preliminary work on ATM indicates that serine 2546 is critical for its kinase activity. Substitution of this residue with phosphomimetic aspartate, but not nonphosphorylable alanine, abrogates the kinase activity of ATM and fails to rescue the checkpoint-deficient phenotype exhibited by the ATM-deficient cells, suggesting that removal of an inhibitory phospho group at S2546 might be required for the activation of ATM. In another study, we identified a novel DNA-damage responsive threonine residue (T622) in Rad17, which undergoes ATM/ATR-dependent phosphorylation in vitro and in vivo. Ectopic expression of a phosphodeficient mutant (T622A) of Rad17, but not its wild-type control, shows a pronounced defect in sustaining Chk1 phosphorylation and the corresponding G2/M checkpoint upon DNA damage, suggesting that phosphorylation at T622 might complement that on the two previously reported phosphorylation sites, S635 and S645, to mediate G2/M checkpoint activation while the latter is primarily responsible for intra-S phase checkpoint.

Although a large amount of knowledge has been accumulated about the initiation and activation process of the DNA damage response, how cells recover, the equally important flip side of the response, has remained poorly understood. We have found that in cells recovering from replication stress, RPA32 phosphorylation at ATM/ATR-responsive sites T21 and S33, which reportedly suppresses DNA replication and recruiting other checkpoint and repair proteins to the DNA lesions, is reversed by the serine/threonine protein phosphatase 2A (PP2A). Cells with a RPA32 persistent-phosphorylation mimic (T21D/S33D) exhibit normal checkpoint activation and re-enter the cell cycle normally after recovery, but display a pronounced defect in the repair of DNA breaks. These data indicate that PP2A-mediated RPA32 dephosphorylation may be a required event during the repair process in the DNA damage response.

In summary, these studies in this dissertation highlight the importance of reversible phosphorylation and dephosphorylation in the modulation of the DNA damage response. What's more, they also extend our knowledge and deepen our understanding of this process by revealing that dephosphorylation may positively regulate the activation of cell cycle checkpoints, which is seemingly dominated by protein phosphorylation upon DNA damage, that phosphorylation of certain checkpoint proteins at different sites may result in distinct consequences, and that dephosphorylation of some activated checkpoint/repair proteins may function as an important mechanism for cells to recover from the DNA damage response.

Epigenetic regulators play pivotal roles in many fundamental biological processes, including cell proliferation, cell death, and cell fate specification. As epigenetic abnormalities are frequently associated with human diseases, it is of great importance to understand the key genes involved in disease etiology. Ubiquitin Like with PHD and Ring Finger Domains 1 (UHRF1) is a multi-domain epigenetic regulator. By working coordinately with DNA methyltransferase 1 (DNMT1), UHRF1 plays a crucial role in maintaining epigenome integrity, especially in heterochromatin regions and retrotransposon elements. Knockout of Uhrf1 in mice is embryonic lethal and thus conditional Uhrf1 knockout mice have been used to delineate the role of UHRF1 in diverse biological contexts. These studies have revealed the essential functions for UHRF1 in regulating the proliferation, survival, and differentiation of colonic regulatory T cells, invariant natural killer T (iNKT) cells, neural stem cells, and hematopoietic stem cells. However, the function of UHRF1 in regulating self-renewal and differentiation in epithelial stem and progenitor cells has not been investigated. Additionally, although UHRF1 expression is upregulated in several types of cancer, most studies have focused on how UHRF1 affects cancer cell-autonomous processes instead of how this molecule impacts the tumor microenvironment in vivo.

The airway is lined by a pseudostratified mucociliary epithelium, which is composed of multiple cell types. Airway basal cells function as stem cells to replenish the mucociliary epithelium in response to airway injury. By analyzing a published dataset, we found that Uhrf1 was among the genes most highly upregulated in response to sulfur dioxide-induced airway injury. However, the functional significance of UHRF1 induction for the airway repair process has not been explored. Here we show that UHRF1 expression was mainly restricted to proliferating basal and progenitor cells during the regeneration of the mucociliary epithelium in vivo. In three-dimensional organoid culture, we also found that UHRF1 was expressed in proliferating human bronchial epithelial (HBE) cells. Reduction of UHRF1 expression by shRNAs in HBE cells reduced cell proliferation, an effect that is partially mediated by the cyclin dependent kinase inhibitor p15. To further evaluate the function of UHRF1 in basal cells in vivo, we generated a basal cell-specific Uhrf1-knockout mouse model and found that Uhrf1-null basal cells did not proliferate after airway injury. This proliferation defect and the G1 cell cycle arrest were linked to an inability of cells to enter S phase and form DNA replication complex, as evident by loss of PCNA puncta. Perturbation of DNA methylation is unlikely to contribute to the proliferation defect, since Uhrf1-null basal cells halt cell cycle progression and thus cannot passively loss genome methylation without DNA replication. Our results implicate a new paradigm for UHRF1 in regulating adult epithelial stem and progenitor cells.

We next sought to explore the function of UHRF1 in cancer development. Consistent with prior reports, we found that in non-small cell lung cancer (NSCLC) patients, the mRNA level of UHRF1 was markedly upregulated in tumor regions compared to adjacent non-tumor lung tissues. Immunostaining analysis showed that UHRF1 was barely detected in normal lung tissue, whereas its expression was easily detected in the papillary adenocarcinoma in a KRAS G12V-driven spontaneous lung adenocarcinoma mouse model. To investigate the role of UHRF1 in regulating lung tumor growth, we conducted experiments using a syngeneic NSCLC mouse model termed Lewis lung carcinoma (LLC). Knockdown of UHRF1 did not affect cancer cell growth in vitro. Surprisingly, UHRF1 downregulation in tumor cells strongly suppresses tumor growth in immune-competent, but not immune-compromised mice. Higher percentages of tumor-infiltrating CD4+ and CD8+ T cells were observed in UHRF1 knockdown tumors, and the tumor-infiltrating T cells had enhanced proliferative capacity as well as cytotoxicity. Further experimental results described here indicate that the loss of UHRF1 exerts anti-tumor immunity potentially mediated by downregulation of the immune checkpoint protein PD-L1.

Taken together, our work provides new insights into the functionality of UHRF1 in controlling the complex process underlying NSCLC tumor growth and emphasizes the necessity for immunocompetent mouse models to study the reciprocal interactions between tumor cells and the microenvironment.

Cellular senescence is a unique cell fate characterized by stable cell cycle arrest and the extensive production and secretion of various cytokines, chemokines, proteases, and growth factors, a phenomenon known as the senescence-associated secretory phenotype (SASP). Although secreted factors are known to have important biological effects on both senescent and non-senescent cells in the contexts of normal aging and disease, the precise molecular mechanisms responsible for generating a SASP in response to senescent stimuli have remained largely obscure. To identify the major initiator, we used an unbiased profiling strategy and discovered a multi-ligand scavenger receptor CD36 is rapidly upregulated in multiple cell types in response to replicative, oncogenic and chemical senescent stimuli. Moreover, ectopic CD36 expression in dividing mammalian cells is sufficient to initiate the production of a large subset of known components of the SASP via activation of the canonical Src-NFκB pathway, resulting in the subsequent onset of a full senescent state. The CD36-mediated secretome is further shown to be ligand-dependent, as fibroblast cultures lacking the CD36 ligand amyloid beta (Aβ) are unresponsive to CD36 upregulation but can be driven to senesce by the addition of exogenous ligand. Finally, loss-of-function experiments revealed a strict requirement for CD36 in secretory molecule production during conventional senescence reprogramming. These results uncover the Aβ-CD36-NFκB signaling axis as an important regulator of the senescent cell fate via induction of the SASP.

To further explore the possible implication of Aβ-CD36-NFκB-SASP signaling, we found that the CD36 expression is significantly down-regulated in the context of lung malignant tissues, specifically in cancer cells. Subsequent explorations revealed CD36 as a strong tumor suppressor by secreting pro-inflammatory cytokines and recruiting cytotoxic T. For the CD36 ligand - Aβ, we observed a major accumulation in the tumor region which might serve as the tumor-suppressing signaling initiation cue once CD36 is introduced. The findings indicate a possible tumor suppressive signaling lead by Aβ-CD36.

Taken together, we discovered a novel signaling of Aβ-CD36-NFκB in regulating SASP during the process of lung aging and the progression of lung malignancy.