A Brief Understanding of the Cancer Hallmarks and the Tumor Microenvironment: Part 2
This writeup aims to give an overview of the hallmarks needed for cancer cells to form a new organ and explains the tumor microenvironment (Part 4 of the illustration)
(iii) How do cancer cells form a big heterogenous mass of cells and invade other tissues?
In the early stages of neoplasia formation, immune cells like cytotoxic CD8 T cells and NK cells eliminate the more immunogenic cancer cells and the less immunogenic ones continue to proliferate. This is called immunoediting. As a tumor becomes bigger in size, it needs supporting cells like endothelial cells and pericytes to form blood vessels for oxygen and nutrients, fibroblasts for scaffolding, and help with other aspects of tumorigenesis as well as pro-tumorigenic immune cells. The heterogeneous tumor mass forms a dynamic environment with cross-talk between various cells. They also undergo phenotypic plasticity and transdifferentiation to contribute to a diverse pool of cells with different lineages. At the same time, it develops spatiotemporal gradients of hypoxia, meaning limited nutrients and oxygen conditions at some regions, with the same energy and metabolic demands. The cells thus utilize an efficient way of energy metabolism called oxidative glycolysis, also called the Warburg effect. At later stages, too they find smarter ways or immune evasion. The cancer cells also become ready to explore new niches and undergo invasion and metastasis, becoming even more exposed and adapted, adding to tumor heterogeneity. Once they find appropriate soil, they start from scratch and prepare to adjust to another microenvironment. Metastasis accounts for 90% of deaths. Once a tumor is diagnosed which it is only at macroscopic sizes and treatment like chemo- or radio- or immunotherapy is given, the cancer cells mostly die (necrosis releasing DNA and adding to heterogeneity and resistance) or senesce (again causing tumorigenic effects by SASP). Even microbiota within the tumor and in the gut can symbiotically act to promote tumorigenesis. The TME is a very dynamic environment that is ever-changing and co-evolving.
Some of the key mechanisms associated with the hallmarks needed to sustain a big tumor mass are given below:
Ø Evading immune destruction
§ Tumor cells need to surpass the immune system which is responsible for eliminating incipient cancer cells.
o Some examples of the tumor-fighting immune cells are CD8+ CTLs, CD4+ Th1 helper cells, NK cells
§ Highly immunogenic cancer cells are eliminated in competent hosts in a process called immunoediting.
§ Cancers recruit immunosuppressing cells like Tregs and myeloid-derived suppressor cells (MDSCs) as they suppress cytotoxic lymphocytes.
Ø Angiogenesis
§ Tumor cells initially lack the angiogenic ability and gain it in the early to mid-stages.
§ Tumors switch to the angiogenesis inductors instead of the inhibitors with the help of proteases.
o E.g., proteases release βFGF stored in ECM or inhibitors like angiostatin accessible
§ In sprouting capillaries proteases are connected with integrins which help them invade the tissue.
§ Many tumors have increased VEGF expression and/or FGFs and suppressed inhibitors such as thrombospondin-1 or IFNβ.
o Ras activation can lead to increased VEGF and p53 loss can lead to increased thrombospondin-1
§ VEGF can be upregulated by hypoxia and oncogenic signaling.
§ Different tumor cells have distinct angiogenic mechanisms/strategies, thus a single anti-angiogenic therapy might not suffice and there needs to be a diverse catalog of angiogenic therapeutics, each addressing a specific mechanism.
§ Pericytes or supporting cells that line the blood vessels are loosely associated with neo vasculature of most tumors.
§ Variety of BM-derived cells like macrophages, neutrophils, mast cells, and myeloid progenitors are present at tumor margins to aid angiogenesis and have protective effects against anti-angiogenic drugs.
Ø Changes in energy metabolism
§ There are changes in energy metabolism in cancer cells to adjust for increased cell division and growth requirements.
§ In aerobic conditions, glycolysis is followed by oxidative phosphorylation where glucose is metabolized to pyruvate in cytosol and then to CO2 in mitochondria, but in anaerobic conditions, there is no oxidative phosphorylation. Cancer cells exhibit a Warburg effect or oxidative glycolysis, wherein even in aerobic conditions they don’t undergo oxidative phosphorylation.
§ Cancer cells have higher rate of glucose metabolism, and thus the amount of ATP in normal cells vs cancer is comparable at any given point of time.
o Evolutionally, a faster rate of ATP production rather than yield has a selective advantage when competing under limited energy resources.
§ They upregulate glucose transporters, notably, GLUT1, which increases glucose import into the cytoplasm.
§ Ras and hypoxia can independently increase glycolysis.
§ Supports the biosynthetic requirements of uncontrolled proliferation i.e., de novo synthesis of nucleotides, lipids, and proteins required for the formation of organelles and molecules for assembling new cells.
§ The physical volume available per cell may limit mitochondria number and thus any energy and biomass that exceed the limited mitochondrial capacity need to be produced from aerobic glycolysis.
§ Since cancer cell populations vary with oxygen gradients within the tumor, there are some that use glucose as fuel and secrete lactate as waste which can be imported and used by better-oxygenated cancer populations as an energy source.
Ø Tissue invasion and metastasis
§ Invasion-metastasis cascade has discrete steps:
o local invasion
o intravasation of cancer cells into blood vessels
o transit of cancer cells through lymphatic systems
o release, or extravasation into the parenchyma of distant tissues
o formation of small nodules of cancer cells
o growth into large tumors
§ The cells first need to detach from the primary tumor with loss of tethering molecules like E-cadherin.
o By mutations in gene or proteolysis of cadherin in most epithelial tumors
§ In some cancers, N-CAM switches from highly to poorly adhesive or reduces its expression
§ A migratory cancer cell is highly adaptive and goes through a lot of microenvironmental shifts, evolving and changing during the journey,
o E.g., changes in expression of integrin α or β subunits, thus, profiling the diversity and complexity of integrin signaling and subunits is another challenge
§ Extracellular proteases which are produced by cancer, stromal and inflammatory cells are upregulated, and inhibitors are downregulated.
§ Epithelial to mesenchymal transition (EMT) is a process in which there is loss of cadherin junctions and conversion from polygonal/epithelial to spindle/fibroblast morphology, expression of matrix-degrading enzymes, and apoptotic resistance.
§ Interaction of cancer cells with different cells in the TME can help develop these invasive traits, and cells at the margins are more EMT prone than cells at the core.
o E.g., inflammatory cells present at boundaries can produce matrix-degrading enzymes promoting invasion.
§ Most EMT might be reversible/plastic except for the colonization step as the cells acquire new traits resembling a hybrid of epithelial and mesenchymal traits and new surface receptors for a new ECM and through the passage in blood vessels.
§ Colonization is a complex problem and needs huge trial and error, even though it might start early as cells lack hallmark capabilities for a new environment and nutrient starvation may cause autophagy and reversible dormancy and growth begin after long durations when nutrients are apt.
§ Cancer cells find an appropriate soil or another place for themselves where nutrients and space are not limiting, which requires all other 5 hallmark capabilities.
o E.g., There may be anti-growth signals in a new environment or ECM and tumor-suppressing effects of ECM but certain microenvironments are actually fertile for growth (seed and soil theory).
§ The tumor cells may circulate back to the primary site but with evolved characteristics making them more heterogeneous/diverse in terms of gene expression programs and phenotypes.
§ There are 2 modes of invasion-
o collective, involving movement of a mass of cancer cells into adjacent tissues, and rarely metastatic.
o amoeboid, allowing individual cells to slither through the tissue interstices rather than clearing paths as in collective and EMT.
What is the tumor microenvironment or TME?
The tumor microenvironment (TME) is a hypoxic and acidic milieu constituted of cellular and noncellular components. The cellular component is composed of various stromal cells, including endothelial cells (ECs), cancer-associated fibroblasts (CAFs), myeloid-derived suppressor cells (MDSCs), tumor-infiltrating lymphocytes (TILs), and tumor-associated macrophages (TAMs). The noncellular component includes non-soluble or semi soluble substances, such as the extracellular matrix (ECM), and soluble substances, such as interstitial fluids, various cytokines and chemokines, growth factors, and metabolites. Cancer and stromal cells engage in reciprocal interactions and support tumor growth, progression, metastasis, and therapy resistance. They also co-evolve during the process and are highly dynamic.
Some of the main cell types in the TME are:
Ø Endothelial cells and pericytes
§ Role in tumorigenesis is poorly understood because high interstitial pressure in TME collapses blood vessels.
§ Apart from angiogenesis (VEGF, angiopoietin and FGF signals), other signal transudcing ligands in ECs are Notch, Neurophilin, etc.
§ Pericytes are finger-like projections which wrap around blood vessels, supporting tumor endothelium.
§ Pericytes collaborate with ECs to synthesize vascular basement membrane, which helps vessel walls to withstand the hydrostatic pressure of blood flow.
Ø CAFs
§ Desmoplastic stroma characterizes many carcinomas and CAFs are a dominant part of it.
§ Have more pro- and anti-tumorigenic roles in the TME.
§ Play a role in several aspects of tumorigenesis through heterotypic signaling.
§ Co-evolve with cancer cells and take heterogenous roles.
(CAFs are elaborated on in other articles).
Ø Immune cells
§ Cancer represents persistent inflammation and attracts tumor-promoting immune cells.
§ Include macrophages, mast cells, neutrophils, T and B cells and release EGF, VEGF, FGF, MMP-9, proteases to support many aspects of tumorigenesis like angiogenesis, invasion , proliferation, etc.
§ One type of myeloid cells which express macrophage marker CD11b and neutrophil marker Gr1, suppress CD8+ cytotoxic T cells and NK cells and are identified as MDSCs.
References:
- Hanahan D, Weinberg RA, “The hallmarks of cancer”, Cell (2000) 100(1):57–70.
- Hanahan D, Weinberg RA, “Hallmarks of cancer: the next generation”, Cell (2011) 144(5):646–74.
- Hanahan D, “Hallmarks of Cancer: New Dimensions”, Cancer Discov (2022) 12(1):31–46.
