The idea of cancer hallmarks was first proposed by Hanahan and Weinberg in a seminal paper in 2000 and an expanded and updated version in 2011 (D. Hanahan and Weinberg (2000),Douglas Hanahan and Weinberg (2011)). Specifically, they proposed that the many complex mutations and heritable changes could in fact be organised into a limited number of recognisable traits, or “hallmarks.”The concept of ‘hallmarks’ describes features shared by all cancer cells which are thought to be integral to cancer progression.
These traits also differentiate between normal cells and transformed cancer cells, i.e., they are essential for the development of cancer but not equally important to normal cells. For example, the ability to proliferate in an uncontrolled fashion independent of physiological signals is a trait shared by all cancers.
The hallmarks traits do not develop spontaneously, rather they can be found in all cancers due to the process of somatic evolution.A cell may randomly acquire a epi-/mutations. If this heritable change translates into a beneficial change of phenotype cells can gain fitness advantage which due to natural selection will lead to a dominance of the cellular clone (daughter cells) carrying these mutations.Thus from all the random possible mutations and epi-mutations, ultimately those that bring a selective advantage for cancers, a sort of ‘superpower’ (e.g., more rapid proliferation), will be selected by the process of somatic evolution.As transformed cells gradually transform into the malignant cancers that eventually threaten to kill the patients they acquire over time each of the proposed hallmark that therefore eventually will be found in all clinical cancers.
Initially, the authors proposed six hallmarks (Figure 11.1):
Figure 11.1: Cancer hallmarks as originally proposed by Hanahan and Weinberg
11.1 Hallmarks overview
11.1.0.0.1 Original Hallmarks
- Sustained proliferative signalling. Cancer cells stimulate their own growth independent from normal signalling to achieve sustained proliferative signalling.
- Evading growth suppressors.Cancer cells avoid or overcome the signals that normally counteract and balance proliferation evading growth suppressors.
- Resisting cell death. Cancer cells manage to balance pro-apoptotic (programmed cell death) and anti-apoptotic signals or in other ways find means to resist undergoing apoptosis.
- Inducing angiogenesis. Cancer cells acquire the ability to stimulate blood vessel growth signalling to induce angiogenesis.
- Activation of invasion and metastasis. Cancer cells develop the ability to invade surrounding tissue and spread and colonise distant sites by activating invasion and metastasis programs.
- Enabling replicative immortality. Cancer cells overcome physiological limits on the total number of cell divisions (Hayflick limit) to achieve limitless replicative potential.
Emerging HallmarksAbout ten years after the publication of the paper first proposing the idea of cancer hallmarks Hanahan and Weinberg published an update (Figure 11.2) in which integrated insights on the patho-physiology of cancer that had accumulated in the intervening years (Douglas Hanahan and Weinberg (2011)).
Specifically, additional hallmarks and tumour enabling tentative hallmarks were introduced:
- Avoiding immune destructionCancer cells actively evade detection and destruction by immune cells
- De-regulation of cellular energeticsCancer cells fundamentally change their cellular energy metabolism to support continuous cell growth and proliferation
Enabling Characteristics
- Genome instability and mutationCancer cells develop genomic instability generating random mutations including chromosomal rearrangements
- Tumour promoting inflammationCancer cells benefit from the inflammatory state induced by immune cells in pre-malignant and malignant lesions promoting tumor progression
It is important to bear in mind that this ‘acquisition’ of specific traits is not an active process that the cancer cells control or influence but in fact the result of somatic evolution, i.e.the gradual selection of cell populations which have a beneficial pheotype after having undergone random epi-/mutations. While it is not possible to predict in which sequence these hallmark traits will be acquired malignant tumours eventually acquire all of them.
In addition, Hannah and Weinberg highlighted the role of the ‘tumour organ,’ i.e.the fact that cancer cells do not exist in isolation; they exists as part of a tissue that contains other cells and structures (fibroblasts, immune cells, connective tissue, extracellular matrix, and blood vessels) that interact with the cancer cells. It is this ‘tumour organ’ which forms the environment in which cancer cells compete with healthy cells and other populations of cancer cells.
Figure 11.2: The updated cancer hallmarks
11.2 Hallmark: Inducing angiogenesis
Typically all cells in the body are within around a hundred micrometres of a blood capillary to guarantee supply of nutrients etc. In order to grow over a size of a few millimetres, tumours therefore require additional blood vessels; without those they can not expand and continue to grow. Tumours may thus exist asymptomatically in the body in a ‘dormant’ form (i.e.unable to grow much in size) for extended periods of time. While the cancer cells at this site may not be able to proliferate sufficiently to create a tumour they continue to adapt based on the principle of somatic evolution. Clones develop, potentially acquire further mutations and compete in this local biotope. While without blood vessel growth they may appear dormant because of a lack of growth in volume these cells may still becoming more malignant.
Figure 11.3: The updated cancer hallmarks.
Once these dormant tumour sites acquire the ability to induce the growth of new blood vessels they are able to ‘flick’ the angiogenic switch allowing tumours to expand rapidly (exponentially) and are now able to metastasise via the blood vessels.
The physiological growth of new blood vessels depends on activators (e.g.VEGF) and inhibitors (e.g.endostatin) which are balanced. In response to conditions in the local tissue environment cells respond and the balance of pro- and anti-antigenic factors changes to trigger blood vessel growth.
Tumours achieve this by subverting the physiological angiogenesis pathways with signalling to normal (untransformed) vascular endothelial cells which triggers the process of new blood vessel formation. Without theses new blood vessels tumours remain harmlessly small because of the lack of nutrients etc. thus making angiogenesis a promising drug target.
11.2.0.1 Anti-angiogenic therapy
Angiogenesis therapy is attractive as its is universally applicable (all tumours) and the targeted endothelial cells remain untransformed (low risk of cell resistance).
- Examples of angiogenesis targeting drugs include
- Avastin (bevacizumab) ⟶ extracellular (binding VEGF)
- Sunitinib (Suten) ⟶intracellular endothelial cells (tyrosin kinase inhibitor)
There are limitations to this otherwise very attractive strategy, e.g., diagnosis typically happens after the development of vasculature has already started so therapy may not be as effective. In addition, the angiogenesis is regulated in complex multi-node pathways and it is therefore unlikely that a single ‘stop’ node will fully block angiogenesis. Furthermore, the network provides multiple options to bypass blocks and thus could allow development of resistance. Successful therapy might mean that tumour growth is stopped but not necessarily all tumour cells killed (i.e.tumour - cytostatic vs.cytotoxic) thus making continuous therapy necessary.
11.3 Hallmark: Activation of invasion and metastasis
One of the clinically most significant changes in the progression of cancer is that from a local to a systemic disease and the change in therapeutic approach from surgical removal of the tumour to treatment of potential spread with chemotherapy. It is the metastatic forms of cancer where cancer cells have spread to other sites of the body to grow into secondary (or tertiary) tumours which is most likley to kill the patient.
Metastasis is a process that can occur well before the diagnosis of the disease: when diagnosed with cancer more than 50% of patients will already have detectable metastatic disease and a significant proportion will have metastases which may not be clinically detectable.It is a multi-stage process (metastatic cascade) in which cancer cells have to undergo somatic evolution process to acquire distinct changes in their pheontype that enable metastasis (Figure 11.4).Before metastasis can occur cancer cells have to acquire the ability for invasion i.e.they need to be able to disrupt the normal tissue boundaries. This involves changing the way the cancer cells interact with neighbouring cells (reduced cell adhesion), requires the development of means to disrupt the surrounding extracellular matrix and basem*nt membrane (degradating enzymes), and induction of the ability to change location (motility and migration).
Figure 11.4: The process of invasion and metastasis.
Once cancer cells have acquired the ability to disrupt and migrate into neighbouring tissue metastasis requires for them to reach blood or lymphatic vessels. Given the limitations to tumour growth in the absence of novel blood vessels (highlighted above) it seems likely that the ability to induce angiogenesis may need to be acquired at the same time.
Once cancer cells have invaded the surround tissue and reached the blood or lymphatic vessels they need to disrupt endothelial cells to gain access to the vessel (intravasation) to be washed away to other body sites where a mirror process of blood vessel walls adherence and following extravasation takes place. Finally, the cache cells need to establish themselves in the new tissue.
Stephen Paget (1889) as assistant surgeon to the West London Hospital and the Metropolitan Hospital wanted to answer the question whether the organ distribution of metastases observed in patients was random or there was evidence of preferential dissemination or taking hold in specific organs (Paget (1889)).
He therefore studied the case histories of 753 fatal breast cancer and it became clear that metastases tended to occur in specific organs; one potential reason for this is related to the access and volume of blood flow of an organ. However, Paget observed that liver metastases were much more likely to occur than would be expected from the blood flow alone, e.g.compared to spleen. He thus postulated that a specific interaction exists that favours metastatic growth in the target organ, i.e.the liver. He proposed that the process of cancer metastasis is analogous to the planting of seeds, i.e.when the plant seeds are scattered in the wind only a few seeds will germinate depending on landing on the right soil or environment.
In 1980 Hart and Fidler studied this further in experimental cancer models: using experimental models they transplanted organ tissue to different locations in the body. They then tested where experimental metastases would grow and were able to show that it was indeed the nature of the tissue rather than location relative in the blood stream that determined where metastases would occur (Hart and Fidler (1980)).
11.4 Hallmark: Resisting cell death
11.4.1 Apoptosis - physiological role
- Destruction of cells is potentially dangerous
- Wrong cells, wrong trigger?
- Collateral damage (Enzymes, signalling molecules, hostile?)
- Programmed/controlled cell deathAims
- Safe triggering & efficient execution
- Demolish the cell with out damageApoptosis – greek - leaves falling
- Other forms of cell death exist
11.4.2 Apoptosis vs other forms of cell death
Apoptosis is often described as a form of cell “suicide”; it differs from other forms of cell death, in particular necrosis (which is often referred to as ‘murder’) , in a number of ways. Apoptosis is a process that is well regulated and allows the controlled shutdown of cells based on triggering of gene expression programs that lead to a sequential and safe shutdown of cells. Apoptosis plays an important role in many physiological process (e.g.development) but is also involved in a range of pathological situations.
The controlled shutdown allows the body to dispose of potential detritus and toxic breakdown products with minimal tissue disruption and avoids induction of a inflammatory response.
At the histological level apoptosis is characterised by a number of distinct changes to the cell morphology. Specifically, cell undergo shrinkage, peptide & DNA cleavage, membrane blebbing, fragmentation, and finally phagocytosis.
Necrosis, by contrast, typically describes a unregulated rapid destruction of cells in which breakdown products are release into the surroundings leading to inflammation. The morphology of necrotic cells is characterised by swelling, lysis, breakdown and random DNA cleavage.
| Apoptosis | Necrosis |
|---|---|
| “Suicide” | “Murder,” trauma, overwhelming |
| Controlled by genes, sequential, safe shutdown | Tissue dysfunction |
| Physiological and pathological | No controlled sequence |
| No inflammation, minimal tissue disruption | Pathological |
| Typical morphology | Inflammation |
| Shrinkage, peptide & DNA cleavage, | Toxins, lack of blood supply |
| Membrane blebbing, fragmentation, phagocytosis |
11.4.3 Regulation of apoptosis
Apoptosis can be triggered along different pathways depending on the nature of the trigger:
an extrinsic/external pathway is activated after binding of “messengers,” i.e.specific ligands such as FAS, TNFa.
an intrinsic pathway which is activated from range of internal signals when ‘things go wrong,’ stress, DNA damage, cell cycle problems, hyperactive oncoproteins…
- A number of upstream sensors exist that act as nodes e.g.TP53 signals to Noxa, Puma nd BH3
The apoptotic response depends on a balance of pro and anti-apoptotic signals - therefor the absence of survival factors could also lead to apoptosis.
11.4.4 Therapeutic relevance
Most anti-cancer drugs work by triggering apoptosis.
These pathways are relevant to the development of novel therapies if these signals can be controlled in cancer cells.
Mutations can reduce efficacy of therapies that exploit these pathways and can therefore contribute to (drug resistance)[#resistance].
Figure 11.5: Intrinsic and extrinsic apoptosis pathways.
11.4.5 Molecular mechanism
The extrinsic pathway is mediated by the FAS death receptor. FasLigand activates the Fas receptor by binding to the extracellular death receptor domain. Fas receptor also contains a cytoplasmic motif known as the death domain (DD), which is also found in the adaptor proteins FADD, TRADD and RIP.
The DD of Fas binds to the DD of FADD, whereas FADD interacts with procaspase-8/-10, through another motif designated death effector domain (DED). The formation of this complex of FAS/FADD/procaspase-8/-10 (DISC complex) is required for the activation of caspases.
Caspase-8, in turn, cleaves both effector caspases and Bid, a proapoptotic member of the Bcl-2 family of proteins.
The processed Bid (tBid) activates Bax and Bak, members of the Bcl-2 family that oligomerize to promote MOMP (mitochondrial outer membrane permeabilization (MOMP).
tBid inhibits the function of the antiapoptotic members of the Bcl-2 family (Bcl-2 and Bcl-xL), which normally prevent the oligomerization of Bax and Bak, inhibiting MOMP and apoptosis. MOMP allows the release of various proapoptotic mitochondrial proteins such as cytochrome c, Smac/Diablo and HtrA2/Omi that further activate the apoptotic cascade.
Cytochrome c induces the heptamerization of the cytosolic protein Apaf-1, which binds procaspase-9 to form the active apoptosome complex for cleavage of effector caspases, whereas SMAC and HtrA2 act as inhibitors of IAPs.
11.5 Hallmarks therapeutic relevance
Somatic evolution leads to cancer cell populations acquiring the hallmark traits. This provides a growth and survival advantage but also highlights traits and pathways in which cancer cells differ from normal cells and on which cancers depend in order to progress. Consequently, disruption of the pathways that drive these traits offers exiting opportunities for effective and selective cancer therapies and the identification of drug targets (Figure 11.6).
Figure 11.6: Cancer hallmarks map onto molecular targets with a central role in cancer progression and are a key focus of anti-cancer drug discovery.
