ECM includes BM, fibers & soluble components(“Stroma” is derived from the Greek word meaning something you spread on the ground to sit or lie on.)
Antigens present in early neoplastic tissues are transported to lymphoid organs activating adaptive immune responses resulting in chronic activation of innate immune cells in neoplastic tissues.Activated immune cells promote tumor development via modulation of gene expression programs in initiated neoplastic cells “altered cell cycle progression and enhanced survival”.Inﬂammatory cells positively inﬂuence tissue remodeling and angiogenesis by production of proangiogenic mediators and production of extracellular proteases. Tissues in which these pathways are chronically engaged exhibit enhanced risk of tumor development.
Many malignancies arise from areas of infection and inﬂammation simply as part of the normal host responseMost notably, those patients with chronic H. pylori infection exhibit a 75% increased risk for gastric cancer, the 2nd most common type of cancer globally
| Cancer cells in primary tumours are surrounded by a complex microenvironment comprising numerous cells including endothelial cells of the blood and lymphatic circulation, stromal fibroblasts and a variety of bone marrow-derived cells (BMDCs) including macrophages, myeloid-derived suppressor cells (MDSCs), TIE2-expressing monocytes (TEMs) and mesenchymal stem cells (MSCs). b | Invasive human breast cancer stained with haematoxylin and eosin, in which a prominent infiltration of leukocytes (indicated by white arrows) is evident at the invasive margin. c | Macrophages at the invasive edge of pancreatic islet cancers express cathepsin B (green), which is associated with loss of epithelial cadherin (red) on the neighbouring cancer cells. Cell nuclei are visualized by DAPI (blue).
Macrophages are inherently plastic cells, and this adaptability may be exploited by the tumour to elicit distinct functions at different stages of tumour progression
Tumour-associated macrophages (green) can be visualized in mammary tumours in living animals, in proximity to blood vessels (red), as indicated by arrows, and migrating along collagen fibres (blue, visualized by second harmonic resonance) as indicated by arrowheads. Thus it is possible that 'invasive niches' exist within the primary tumour, in which the proximity of cancer cells, macrophages and the endothelium establishes paracrinesignalling loops that lead to enhanced intravasation and dissemination of cancer cells
As the predominant cell in stroma, the fibroblast is responsible for the elaboration of most connective tissue components in the ECM, including collagens and structural proteoglycans, as well as various classes of proteolytic enzymes, their inhibitors and various growth factors.the ECM provides cues to the cells, thereby regulating their proliferation and morphology. The dense meshwork of ECM can be a hurdle to cell migration, especially for the basement membrane, which is impermeable for cells.The third way of communication between tumor and stromal cells is supported by proteases. The proteases are involved in modifying tumor — stroma interaction through activation or inactivation of various cytokines and growth factors and modification of adhesion molecules favoring tumor growth.
Models of tumor-stromainteractions driven by the (A) mutated epithelial cells (prevailing model),(B) mutated fibroblasts (emergingmodel), and (C) altered ECM/TME(newmodel). Matriptase, a typeIItransmembranetrypsin-like serineprotease, is expressed by epithelialcells and is overexpressed in a varietyof human cancers. Arrows, factors secretedby the respective cells that act in bothan autocrine and a paracrine manner.
Cancer cells produce a line of growth factors, such as transforming growth factor beta (TGF-β), platelet-derived growth factor (PDGF), basic fibroblast growth factor (bFGF), the epidermal growth factor receptor (EGFR) ligand and vascular endothelial growth factor (VEGF).affect fibroblasts, and epithelial and endothelial cells.The endothelial cells produce connective tissue growth factor (CTGF), which together with TGF-β and PDGF not only attract stromal cells to the tumor site but also induce them to provide a supportive environment for the cancer cells.Under the influence of TGF-β and PDGF stromal fibroblasts develop into myofibroblasts. Tumor-induced stromal growth is accompanied by the production of additional growth factors by these myofibroblasts, such as insulin-like growth factor-1 (IGF-1), hepatocyte growth factor (HGF), CTGF and VEGF, which further enhance stromal growth.--------------------------------------------------E-cadherinFound in epithelial cells E-cadherin-containing cell-to-cell junctions often adjacent to actin-containing filaments of the cytoskeleton E-cadherin is a tumour suppressor. Loss of function/expression implicated in cancer progression and metastasis. Downregulation decreases strength of cellular adhesion within a tissue Increase in cellular motility. May allow cancer cells to cross the basement membrane and invade surrounding tissues Proteolytic cleavage of the extracellular domain of E-cadherin by MMP3 and MMP7 disrupts its ability to promote cellular interactions. Disruption of E-cadherin mediated cell-cell adhesion results in detachment of tumour cells from the epithelial cell layer. This affects signalling pathways Cadherin SwitchE-cadherin switches to N-cadherin. N-cadherin mediates cell-cell interactions Enables tumour cells to direct host responses Regulates fibroblast growth factor receptors and triggers downstream signals ## Phospholipase C-γ. ## Phosphatidylinositol 3 kinase ## Mitogen activated protein kinaseN-cadherinExtracellular domain of N-cadherin susceptible to proteolytic cleavage by MMPs Block cell-cell adhesion Stimulate fibroblast-growth-factor receptor signalling in paracrine fashion IntegrinsLarge family of adhesion receptors Alpha and beta transmembrane chain Recognize distinct ECM ligandsHelp form focal adhesions or ECM contacts Organize and remodel cytoskeleton of the cell Adhesive and migratory interactions with the ECM Impart polarity Control proliferation and survival alpha-v-beta-3 and alpha-6-beta-4 are upregulated: promote migration, invasion and proliferation. ----------Matrix-MetalloproteinasesSecreted MMPs tethered to the surface of tumour/stromal cells Activate relevant growth factors Promote angiogenesis Disrupt ECM barriers to invading cells Fibroblasts and Soluble RegulatorsCarcinoma-associated fibroblasts Platelet-derived Growth Factors and TGF-Beta alter fibroblast phenotype to one reminiscent of myofibroblasts. Source of proteolytic enzymes including MMP and cathepsinsMonocyteChemotactic Protein - 1 Regulate inflammatory response to tumour invasion Secrete Stromal Cell derived factor-1 that recruits bone marrow-derived endothelial cell precursor recruitment ------TGF-BetaTumour-host cross talk Inhibits proliferation of normal epithelial cells and early carcinoma cells Stimulates fibroblast growth ECM secretion Promotes late-stage carcinoma invasiveness Induce EMT Activation of normal fibroblasts
Tissue TropismIn humans, certain tumours metastasize to preferred organ sites. For example, breast cancers metastasize to lung, liver, bone and brain; melanoma to liver, brain and skin; prostate cancer to bone; and lung cancer to bone, liver and brain ( Box 3 ). By contrast, some sites such as muscle are rarely if ever sites of metastasisThe tropism of metastatic cells for specific organs may be mediated by chemokines, the local expression of these chemoattractants might guide cognate chemokine receptor-expressing tumour cells to specific destinations, as a result of locally induced chemotaxis and invasion of tumour cellsSDF1 the predominant sites of metastases simply reflect the first pass of the cells in the circulation and their entrapment in local capillaries. Thus, breast cancer cells spread predominantly to lung, lymph nodes and bone, whereas colon cancer cells travel to the liver through the hepatic-portal circulation. Platelets are also a potent source of SDF1 that may affect the migration of CXCR4-expressing tumour cells and the recruitment of BMDCs to sites of metastasis
Tumor induced stroma
• Stephen Paget in 1889 proposed the “Seed & Soil” Hypothesis:“Tumor cells like seeds are carried in all directions; but they can only live and grow if they fall on congenial soil”• Currently, the molecular characteristics of the seeds are much understood than those of the soil!• A tumor is much more than clusters of transformed cells standing alone.
Tumor Cells Stroma(harboring genetic mutations) Cells and connective tissue that provides contextual framework for an organ or tissue-Epithelial 1) Cells (activated and/or recruited to the local microenvironment)-Mesenchymal -Fibroblasts/Myofibroblasts-Hematopoietic -Endothelial Cells -Myoepithelial Cells -Innate & adaptive immune cells 2) ECM
Tightly controlled balance of cell proliferationand death, is achieved and maintained throughintercellular communication Important regulator of normal cell behaviourand tissue homeostasis is the surrounding ECM(mediated by tight junction proteins and CAMssuch as β1 integrins & epithelial (E)-cadherin) Studies indicated that the embryonic micro-environment is potent in its ability to reprogrammevarious cancer cells, including metastatic cells, to aless aggressive phenotype Co-culture experiments showed that normalfibroblasts prevented the growth of initiatedprostatic epithelial cells
During early tumor development, however, the protective constraints of the microenvironment are overridden by conditions such as chronic inflammation, and the local tissue microenvironment shifts to a growth-promoting state Rous was the ﬁrst to recognize thatcancers develop from “sub-thresholdneoplastic states” caused byinherited mutations or followingsomatic mutation of critical genesfollowing viral or chemicalcarcinogen exposure. “Initiation” is irreversible andpersists in otherwise normal tissueuntil nonspeciﬁc stimulation“promotion” occurs, typicallyfollowing exposure of initiated cellsto chemical irritants or fromexposure to factors released at sitesof chronic inﬂammation.
Among the stromal cell typesthat have been implicated intumor promotion:• Endothelial cells• Fibroblasts• Pericytes• BMDCs “macrophages, neutrophils, mast cells, mesenchymal stemcells MSC …etc”
The inﬂammatory component of a developing neoplasm includes a diverse populationof leukocytes (tumor cells modify certain inflammatory cell types to render them tumorpromoting rather than tumor suppressive)All of which are loaded with an assorted array of chemokines, cytokines, cytotoxicmediators including reactive oxygen species, metalloproteases and soluble mediatorsof cell killing and cell proliferation, such as tumor necrosis factor(TNF), interleukins, and interferonsTogether they create a microenvironment favoring cell proliferation, genomicinstability, and expansion of cell populations into ectopic tissuemicroenvironments, i.e., malignant conversion and cancer developmentTAMs may subvert normal macrophage-associated developmental processes and aid inthe invasion of neoplastic cells into surrounding stroma or in the direct remodeling ofstroma angiogenic and/or lymphangiogenic responses
• TAMs secrete VEGF and interluekin-8 (IL-8) which are angiogenic, and epidermal growth factor (EGF) which is mitogenic for tumor cells expressing EGF receptor• TAMs secrete matrix metalloproteinases, which help to carve out room for tumor expansion, and release depots of growth factors from the ECM
One of the mechanisms involved in TAM-enhancement of cancercell invasion:involves a paracrine loop in which epidermal growth factor (EGF) produced by TAMsincreases the invasiveness and migration of neighboring cancer cells that express theEGF receptor (EGFR)Cancer cells in turn express CSF1, apotent chemo-attractant andchemokinetic molecule for CSF1R-expressing TAMsAdditional paracrine loops existbetween cancer cells expressingCXC chemokine receptor 4 (CXCR4)and stromal cells, such asfibroblasts and pericytes, producingthe cognate ligand stromal cell-derived factor 1 (SDF1, also knownas CXCL12), which contribute todirectional cancer cell migration
In tumors, under the influence of TGF-β & PDGF, normal fibroblasts are transformedinto myofibroblasts (peritumoral fibroblasts, reactive stromal cells, carcinoma-associated fibroblasts (CAFs)) Higher proliferative index (vs normal), expressing α-SMA, surrounded by denseaccumulation of fibrillar collagens “desmoplasia” and is associated with the recruitmentof inﬂammatory cells and activation of angiogenic programs CAFs send signals that eitherinitiate abnormal epithelialgrowth or enhance theprogression of nontumorigeniccells to tumorigenic states CAFs release increased levelsof SDF-1
(a)Normal mammary gland duct system, lined by epithelial cells and underlying myoepithelial cells(arrow heads). Ducts are surrounded by dense ﬁbrous interlobular adipose tissue (at). (b) Ductalcarcinoma in situ disorganization of ductal epithelium (∗) stromal inﬁltration of inﬂammatory cells(arrow)in dense collagenous stroma are evident. (c) Mammary carcinoma in which ductal epithelialcells(∗) form aberrant glandular structures and grow in cords without gland formation, surroundedby dense collagen bundles produced by activated ﬁbroblasts.
These cells line the walls of capillaries and larger blood vessels, and lymphatic ducts Their growth is encouraged by angiogenic factors (VEGF)• Endothelial cells release PDGF to attract pericyteswhich support the endothelium with VEGF and ANG-1 structural stability & ability to resist the forcesof the blood pressure• In tumors, VEGF stimulation without normalbalance of other factors, such as ANG-1 and -2, capillaries seem unable to attract pericytes andsmooth muscle cells to make well-constructed, physiologically responsive structures Normal Tumor
Normal Dysplasia CarcinomaThe leakiness of tumorvasculature (red-dextran) leakiness allows deposition offibrin bundles throughout thetissue
The gaps between cells in tumor capillaries allow leakage of fluid into the tumortissue. This results in the increase in fibrin deposits within the tumor describedearlier, and a higher interstitial pressure within the tumor, making delivery ofanticancer agents more difficult.