Hallmarks of Cancer

Although cancer is a group of highly complex diseases, cancer cells exhibit a set of distinct attributes across tumor types that enable them to grow and metastasize to distant organs.¹

Collectively, these hallmark features provide a broad mechanistic framework within which the multistep transformation of a premalignant cell to its lethal metastatic counterpart is understood.¹

Tissue invasion and metastasis are integral components in how tumor cells escape from the primary site and disseminate into distant organs. The process of tissue invasion and metastasis is not well understood, but, in general, it involves changes in the way cells attach to other cells and to the extracellular matrix.²

This process has several steps, including²

• Local tissue invasion
• Intravasation
• Transition through the blood and lymphatic system
• Colonization of foreign tissue

Molecular cross-talk between tumor cells and neoplastic stroma suggests that metastases do not arise from a cell-autonomous model but require input from surrounding tissue.²

• One example of this is the involvement of tumor-associated macrophages (TAMs) that supply cancer cells with epidermal growth factor (EGF) and stimulate macrophages with CSF-1, facilitating intravasation.
  

Molecular cancer research into the complexity of metastatic growth also shows that different malignancies exhibit different characteristics^2,3:

• Distinct modes of invasion are seen in metastatic and nonmetastatic diseases. The reasons for this remain elusive, but they may be due to different cell-biological programs
• Genetic pathways, such as that of tumor necrosis factor α (TNF-α) in bone dissemination, may facilitate tumor metastasis to preferred organ destinations

Figure 1. Activating invasion and metastasis⁴

Tumor cell migration is promoted in part through a paracrine loop involving CSF-1, EGF, and their corresponding receptors, which are differentially expressed on carcinoma cells and macrophages residing in the tumor microenvironment.⁴

Normal cells have a finite replicative ability. An intrinsic cellular mechanism allows normal cells to divide a finite number of times and blocks cell division beyond a certain limit.²

Cancer cells overcome this by overexpressing telomerase, an enzyme that maintains telomere length, which protects the ends of chromosomes and allows the cell to continue proliferating. This process is also aided in part by the loss of tumor-suppressor genes, such as TP53.²

In recent years, molecular cancer research has uncovered additional functions of telomerase that are independent of telomere maintenance and may aid in tumor growth²:

• Enhancement of cell proliferation and/or resistance to apoptosis
• DNA damage repair
• RNA-dependent RNA polymerase function
• Association with chromatin

Figure 2. Enabling replicative immortality^2,5

A shortening of telomere length activates replicative senescence in normal cells; however, tumor cells overcome the finite replicative ability by overexpressing telomerase, an enzyme that maintains telomere length.^2,5

Cell proliferation in normal cells is a tightly controlled process wherein the pro- and antiproliferation signals coordinate their activities at the cell-cycle level. Particularly, the G₁ phase of the cell cycle is a vital checkpoint wherein the antigrowth signals exert their influence to block cell proliferation.⁶

Antigrowth signals in normal cells can block proliferation in multiple ways^6,7:

• Induction of the G₀ phase
• Induction of a postmitotic state, usually involving terminal differentiation of the cell

However, most cancer cells circumvent normal growth suppressors in order to continue proliferating.²

The 2 tumor suppressors most commonly dysregulated in cancer cells are retinoblastoma protein (Rb) and tumor protein p53 (TP53). In normal tissue, these proteins are part of a large network that controls the cell cycle.²

• Rb actively inhibits cell passage through the restriction point in the G₁ cell-cycle phase
  • Cancer cells with mutated Rb remove this gatekeeper and allow for ongoing cell proliferation
• The TP53 protein functions as a central regulator of apoptosis because it arrests the cell cycle in cells with DNA damage
  • Loss of the TP53 tumor suppressor gene allows for cell-cycle progression despite DNA damage and cellular stresses

Figure 3. Evading growth suppressors⁸

Rb and TP53 are 2 common tumor suppressors that are inactivated in tumor cells, leading to uncontrolled growth and proliferation.⁸

Immune surveillance is an essential cellular process that proactively prevents tumor formation in the human body. Preclinical studies have suggested that an active immune system continuously recognizes and eliminates the vast majority of cancer cells before they establish themselves and form a tumor mass.²

However, cancer immuno-editing, an emerging hallmark, includes 3 key phases—elimination, equilibrium, and escape.⁹

• The immune system successfully recognizes and eliminates cancer cells, a process often described as the elimination phase
• Tumor cells not eliminated by the immune system proceed to the equilibrium phase, in which the immune system controls cancer cell growth but does not completely eliminate the transformed cells
• Tumor cells not susceptible to immune destruction progress into the escape phase. In this phase, the “escaped” tumor clones—not effectively detected and destroyed by the immune system—continue to divide and grow

Clinical examples also support this finding, demonstrating that colon and ovarian cancer patients with an increased immune response have a better prognosis than do those patients with a reduced immune response.²

Figure 4. Evading immune destruction^10,11

Cancer immuno-editing, an emerging hallmark, comprises 3 key phases—elimination, equilibrium, and escape. Cancer cells that successfully navigate these phases acquire the ability to evade immune destruction.⁹

Multiple alterations in the genomes of cancer cells serve as the foundation for many oncogenic processes. Cancer cells take advantage of increased rates of mutations in order to accumulate several mutations needed to foster tumorigenesis. They do this through²:

• Increased sensitivity to mutagenic agents
• Breakdown in 1 or more of the cell’s DNA repair mechanisms mediated by genes such as TP53 or breast cancer type 1 susceptibility protein (BRCA1)
• A combination of these factors

Accumulation of these mutations is accelerated by altering DNA-maintenance machinery, or “caretaker” genes. These genes are responsible for²:

• Detecting DNA damage and activating repair machinery
• Directly repairing damaged DNA
• Inactivating or intercepting mutagenic molecules

By inactivating or suppressing caretaker genes, tumor cells can increase the rate of mutations and, subsequently, tumorigenesis.²

Analyses of cancer cell genomes also reveal function-altering mutations and demonstrate that genomic instability increases during tumor progression.²

Figure 5. Genome instability and mutation¹²

Cancer cells take advantage of mutations in DNA repair pathways to promote genomic instability. Depicted above is one such mechanism, resulting from the defective BRCA signaling pathway.¹²

In tumor cells, the process of angiogenesis, or the formation of new blood vessels, is critical for sustained tumor growth and metastasis. Tumor angiogenesis is a multistep process and involves signaling input from several pro-angiogenic growth factors.^13,14 The moment at which a tumor begins to overexpress pro-angiogenic factors, such as vascular endothelial growth factor (VEGF), is generally referred to as the “angiogenic switch.”¹³:

Unabated angiogenesis enables tumor expansion and local invasion through¹³

• Delivery of oxygen and nutrients
• Production of growth factors that benefit tumor cells

Molecular cancer research also suggests that metastases can ultimately exit through the new tumor vasculature into systemic circulation.¹³

Two additional components play a role in tumor neovasculature²:

• Pericytes are supporting cells that have long been associated with normal tissue vasculature; however, recent studies reveal that pericyte coverage is also important for tumor angiogenesis
• Molecular cancer research also indicates that bone marrow–derived cells, such as macrophages and neutrophils, are recruited to lesions and may help initiate the angiogenic switch

Figure 6. Inducing angiogenesis¹³

Tumor angiogenesis is a function of multiple signals from a number of cell types residing in the tumor microenvironment.¹³

Reprogramming energy metabolism has been identified as an emerging hallmark in cancer cells.²

To sustain uncontrolled proliferation, cancer cells make adjustments to their energy production by²:

• Reprogramming their glucose metabolism
• Upregulating glucose transporters such as glucose transporter 1 (GLUT1)
• Depending on alternate metabolic pathways

Although limiting energy production to the glycolysis phase decreases the amount of adenosine triphosphate (ATP) produced, it also allows cancer cells to divert glycolic intermediates to various pathways, including those required to assemble new cells.²

Molecular cancer research also reveals a number of activating mutations in enzymes found in glioblastoma that confer an advantage to altering tumor cell energetics.¹⁵

Additionally, reprogramming energy metabolism is widely applied in clinical settings today through the use of [¹⁸F]fluorodeoxyglucose positron emission tomography (FDG-PET) technology that helps capture an image of tumors with increased glucose uptake.¹⁶

Figure 7. Reprogramming energy metabolism¹⁷

Cancer cells convert available glucose to lactate irrespective of the availability of oxygen (the Warburg effect), thereby diverting glucose metabolites to useful anabolic processes that accelerate cell proliferation.¹⁷

Normal cells may initiate apoptosis in response to DNA damage, among other cellular stresses. In contrast, cancer cells are generally less sensitive to similar stresses and tend to avoid apoptosis.¹⁸

Apoptosis occurs through 2 pathways: the intrinsic, or mitochondrial, pathway that is initiated by intracellular stresses; and the extrinsic, death-receptor pathway that is initiated by engagement of cell-surface receptors with specific ligands.¹⁸

The intrinsic pathway may be important in cancer, as many of the cellular stresses encountered by cancer cells are activators of the intrinsic pathway. These include DNA damage and growth factor deprivation, as well as treatment with chemo- and immunotherapeutics. The intrinsic pathway is tightly regulated by a group of related proteins called the BCL-2 family.

Consistent with its role in the regulation of apoptosis, many cancers are able to resist the apoptotic pathway through dysregulation of BCL-2 family members. Cancer cells are thought to achieve this through 2 main mechanisms: a down-regulation of pro-apoptotic proteins, or an increase in BCL-2 expression.¹⁹

Still, cancer cells may even avoid apoptosis further upstream by dampening the stress signals that promote the initiation of the BCL-2 pathway, or conversely, decreasing downstream effector molecules. For example, mutated TP53, which normally can couple cellular stress to increased expression of pro-apoptotic proteins, resulting in cells less sensitive to DNA damage²⁰; down-regulation of caspase-3 has been linked to apoptotic resistance in some tumor types.²¹

Other cell-death or cell-death–like pathways exist, such as autophagy and necrosis, and may be dysregulated as a means of cancer cell survival, although their specific roles are currently not well understood.¹⁸

Figure 8. Evading cell death¹⁹

Growth signaling in normal cells is a highly regulated process wherein proliferative signals are activated whenever necessary and deactivated when no longer necessary; this tight regulation ensures cell homeostasis. However, in cancer cells, this regulation is compromised.²

One of the fundamental traits of cancer cells is their ability to proliferate without a controlled signaling input. They achieve this in a number of ways²:

• Increasing growth factor production
• Stimulating normal cells in the microenvironment to provide cancer cells with growth factors
• Increasing the number of receptors on the cell surface
• Structurally altering receptors to facilitate cancer cell signaling
• Activating proteins in the downstream signaling pathway

Recent studies also highlight the ability of cancer cells to disrupt negative feedback loops that constitute a safety mechanism to dampen a signaling pathway whenever a mitogenic signal is hyperactivated. One key example of this is the Ras oncoprotein.²

• Oncogenic activity of Ras is not the result of overactive Ras signaling but rather the disruption of normal negative feedback mechanisms operated by the oncogenic GTPase
• Other examples of this process include loss-of-function mutations in phosphatase and tensin homolog (PTEN), which amplify phosphatidylinositol 3-kinase (PI3K) signaling

Figure 9. Sustaining proliferative signaling²

Tumor cells disrupt negative feedback loops in the oncogenic Ras signaling pathway, leading to sustained proliferative signaling in tumor cells.²

The tumor microenvironment is often infiltrated by innate and adaptive immune system cells that enable tumors to mimic inflammatory conditions seen in normal tissues. Current molecular cancer research indicates that this tumor-associated inflammation might aid in tumor growth.²

Emerging research also indicates that tumor-associated inflammation may aid in tumor growth by supplying the tumor microenvironment with²:

• Growth factors
• Survival factors
• Pro-angiogenic factors
• Extracellular matrix (ECM)–modifying enzymes that promote angiogenesis, invasion, and metastasis
• Inductive signals that activate epithelial-mesenchymal transition (EMT) and other hallmark-facilitating mechanisms

Additionally, inflammation is often seen in early stages of neoplastic disease. Early inflammation can release chemicals into the tumor microenvironment and may lead to genetic mutations that enable and accelerate the formation of a tumor.²

Figure 10. Tumor-promoting inflammation²

Tumor-associated inflammation may promote tumor growth by supplying the microenvironment with growth factors, survival factors, and factors that promote angiogenesis.²