International Journal of Molecular Sciences Review Exosomes as Emerging Pro-Tumorigenic Mediators of the Senescence-Associated Secretory Phenotype Rekha Jakhar 1 and Karen Crasta 1,2,3,* 1 Lee Kong Chian School of Medicine, Nanyang Technological University Singapore, 59 Nanyang Drive, Singapore 636921, Singapore; [email protected] 2 School of Biological Sciences, Nanyang Technological University Singapore, 59 Nanyang Drive, Singapore 636921, Singapore 3 Agency of Science, Technology and Research, Institute of Molecular and Cell Biology, Singapore 636921, Singapore * Correspondence: [email protected]; Tel.: +65-65927870 Received: 17 April 2019; Accepted: 7 May 2019; Published: 24 May 2019 Abstract: Communication between cells is quintessential for biological function and cellular homeostasis. Membrane-bound extracellular vesicles known as exosomes play pivotal roles in mediating intercellular communication in tumor microenvironments. These vesicles and exosomes carry and transfer biomolecules such as proteins, lipids and nucleic acids. Here we focus on exosomes secreted from senescent cells. Cellular senescence can alter the microenvironment and influence neighbouring cells via the senescence-associated secretory phenotype (SASP), which consists of factors such as cytokines, chemokines, matrix proteases and growth factors. This review focuses on exosomes as emerging SASP components that can confer pro-tumorigenic effects in pre-malignant recipient cells. This is in addition to their role in carrying SASP factors. Transfer of such exosomal components may potentially lead to cell proliferation, inflammation and chromosomal instability, and consequently cancer initiation. Senescent cells are known to gather in various tissues with age; eliminating senescent cells or blocking the detrimental effects of the SASP has been shown to alleviate multiple age-related phenotypes. Hence, we speculate that a better understanding of the role of exosomes released from senescent cells in the context of cancer biology may have implications for elucidating mechanisms by which aging promotes cancer and other age-related diseases, and how therapeutic resistance is exacerbated with age. Keywords: senescence; SASP; pro-tumourigenic; exosomes; cancer 1. Introduction Cellular senescence is a cellular stress response that culminates in a state of stable cell cycle arrest [1]. As such, it has long been thought to function as an anti-proliferative mechanism against tumor formation in cancers. Senescence has also been strongly associated with age-associated diseases, and has been implicated in developmental processes and wound healing [2]. Cells undergo cellular senescence in response to stressful conditions such as DNA damage, oxidative stress, telomere attrition, oncogenic stress, irradiation and hypoxia. Importantly, the secretion of exosomes has been shown to increase under these conditions [3,4]. This is exciting as exosomes contain proteins, lipids, microRNAs, mRNA and DNA, and can act as “messengers” from one cell to another. This also implies a role for exosomes as senescence effectors. Under stressful conditions, exosomes could relay intercellular cell non-autonomous communication to neighbouring cells and thereby determine the appropriate cell fate response. Int. J. Mol. Sci. 2019, 20, 2547; doi:10.3390/ijms20102547 www.mdpi.com/journal/ijms Int. J. Mol. Sci. 2019, 20, 2547 2 of 17 Int. J. Mol.The Sci. main 2019, focus20, x FOR of PEER this REVIEW review aims to discuss the emerging cell non-autonomous role2 of of17 senescence-derived exosomes and its possible implications for tumorigenesis. We first take a look at 45 exosomal biogenesisbiogenesis andand their their functional functional roles roles upon upon uptake uptake in premalignantin premalignant and and cancer cancer cells. cells. We thenWe 46 thenhighlight highlight the role the of role exosomes of exosomes during during senescence, senescence, with awith key a focus key focus on exosomes on exosomes as constituents as constituents of the 47 ofsenescent the senescent secretome secretome known known as senescence-associated as senescence-associated secretory secretory phenotype phenotype (SASP). (SASP). Lastly, we Lastly, provide we 48 providean overall an perspective overall asperspective well as speculate as well on theas implicationsspeculate on of exosomesthe implications as pro-tumourigenic of exosomes SASP as 49 pro-tumourigenicin the “aging-cancer” SASP nexus. in the “aging-cancer” nexus. 50 2. Exosome Exosome Biogenesis, Composition and Uptake 51 Extracellular vesicles are membrane-bound vesicles released by multiple cell types that include 52 immune cells, prostate epithelialepithelial cells,cells, stemstem cells,cells, cancercancer cells,cells, andand neuronsneurons [5[5].]. These include 53 exosomes, epididymosomes, prostasomes, ectosomes, apoptotic bodies, microvesicles, and more 54 recently oncosomes (Figure 11).). EvenEven thoughthough perplexityperplexity existsexists betweenbetween thethe termterm “exosome”“exosome” andand 55 “microvesicles”,“microvesicles”, these can be distinguished on the basis of their sizes, functional properties and 56 biogenesis (Figure1 1).). 57 58 Figure 1. Origin and biogenesis of different different groups of extracellularextracellular vesicles.vesicles. EVs are arranged by 59 increasing size from left toto right.right. Left- Left- Exosomes, Exosomes, are are secreted secreted by by a a variety variety of of cell types and are 60 formed in MVBs via thethe endocyticendocytic pathway.pathway. Epididymosomes and and Prostasomes are are EVs found in 61 seminal fluid. fluid. Epididymosomes Epididymosomes are are secreted secreted by by cells cells in in the epididymis through budding from the 62 plasma membranemembrane and and prostasomes prostasomes are are secreted secreted by epithelial by epithelial cells of cells the prostateof the glandprostate via gland endosome via 63 endosomeformation andformation release and into release the prostatic into the fluid. prostatic Ectosomes, fluid. Ectosomes, like exosomes, like exosomes, are secreted are by secreted a variety by of a 64 varietycell types of butcell unliketypes but exosomes, unlike exosomes, they are formed they are via formed budding via from budding the plasma from the membrane. plasma membrane. Apoptotic 65 Apoptoticbodies are thebodies results are of the blebs results arising of from blebs disassembly arising from of apoptotic disassembly cells. of They apoptotic are subdivided cells. They into twoare 66 subdividedgroups, depending into two on groups, their contents: depending nuclear on their (DNA contents: carrying) nuclear apoptotic (DNA bodes carrying) (NABs) apoptotic and cytoplasmic bodes 67 (NABs)apoptotic and bodies cytoplasmic (CABs). Microvesiclesapoptotic bodies are larger(CABs). in Microvesicles size and are also are secretedlarger in by size a variety and are of cells.also 68 Theysecreted are alsoby a generated variety of by cells. outward They budding are also from generated the plasma by membrane. outward budding Oncosomes from are muchthe plasma larger 69 membrane.than most extracellular Oncosomes vesicles are much are secretedlarger than by variousmost extracellular cancer cells viavesicles membrane are secreted shedding. by various 70 cancer cells via membrane shedding. Exosomes are basically nano-sized (ranging from 40–100 nm) intercellular communication shuttles. 71 SinceExosomes the discovery are basically of exosomes nano-sized in 1983, (ranging it has become from evident40–100 thatnm) exosomesintercellular contribute communication to many 72 shuttles.aspects of Since physiology the discovery and disease, of exosomes mainly in via1983, cell-to-cell it has become communication evident that [ 5exosomes]. We highlight contribute a few to 73 manyexciting aspects milestones of physiology in the biology and disease, of exosome mainly research via cell-to-cell in Figure2 communication. [5]. We highlight a 74 few exciting milestones in the biology of exosome research in Figure 2. Int. J. Mol. Sci. 2019, 20, 2547 3 of 17 Int. J. Mol. Sci. 2019, 20, x FOR PEER REVIEW 3 of 17 Int. J. Mol. Sci. 2019, 20, x FOR PEER REVIEW 3 of 17 75 Figure 2. Historic landmarks for the discovery and application of exosomes. Chronological summary 76 Figure 2. Historic landmarks for the discovery and application of exosomes. Chronological summary 77 of75of the the key key events events that that led led to to the the discovery discovery andand applicationapplication of exosomes exosomes and and EVs EVs from from 1983 1983 to to2016. 2016. Exosome76 Figure biogenesis 2. Historic begins landmarks with for inwardthe discovery budding and application of the of cellular exosomes. plasma Chronological membrane summary to form early 78 77Exosome of thebiogenesis key events thatbegins led to thewith discovery inward and buddingapplication ofof exosomes the cellular and EVs plasma from 1983 membrane to 2016. to form 79 endosomes.early endosomes. Inward Inward budding budding of this endosomalof this endosomal membrane membrane then forms then late forms endosomes late endosomes containing 78 80 intraluminalcontaining intraluminal vesiclesExosome (Figurebiogenesis vesicles3). begins These(Figure with endosomes 3). inward These budding endosomes are referredof the are cellular toreferred as plasma multivesicular to asmembrane multivesicular to bodies form bodies (MVBs) 79 early endosomes. Inward budding of this endosomal membrane then forms late endosomes 81 and(MVBs) contain and multiple containvesicles