Understanding Metastatic Cancer: Hallmarks And Targeting Sustaining Proliferative Signaling

Hallmarks of Metastatic Cancer

Question:

Discuss about the Identifying Hallmark of Cancer Biology with detailed.

Metastatic cancer is a type of cancer in which cancer cells transfer to different organs in contrast to the site where it was first formed. The other part of the body in which it forms the new tumor is called as metastatic tumors. Metastatic of cancer takes place in a complex system of heterogeneous cell population. One of the hallmarks of metastatic cancer is the invasion of cell from local areas to distance tissues. There are many other mechanism underlying development of metastatic cancer such as sustaining proliferative signaling, evading growth suppressors, overcoming immune destruction, tumor promoting inflammation, invasion, angiogenesis, mutation, resisting cell death and deregulation energetic (1). This essay gives more insight into each hallmark of cancer and its contribution to cancer development. Furthermore, the essay also discusses in more detail about the hallmarks of sustaining proliferative signaling and resisting cell death and the role of specific chemotherapy drugs targeting these hallmarks.

The 10 hallmarks underlying metastatic cancer development and their role in metastatic dissemination are as follows:

1. Sustaining proliferative signaling: The most fundamental mechanism for cancer cell development is their ability to proliferate constantly. Compared to normal cells, cancer cells deregulate the growth promoting signals resulting in unlimited growth as they are no more dependent on proliferation signals. Tumor cells acquire the property to sustain proliferative signaling by producing their own growth factors causing autocrine stimulation. The production of paracrine signal also enhances growth of normal cells. In addition, their reliance on growth factors are reduced by the constitutive activation of downstream signaling pathways (2). The downstream signaling pathway is activated by somatic mutations in the catalytic subunit of phosphoinositide 3-kinase (PI3-kinase) (3). This indicates the mechanism by which cancerous cell grow and develop.
2. Evading growth suppressors- Evading growth suppressors is that hallmarks of cancer cells that complements the process of sustaining proliferative signaling. The ability to evade growth suppression is a necessary process to sustain growth signal as this mechanism acts to prevent all those pathways that negatively influence cell proliferation. TP53 and RB are some tumor suppressive protein coding gene involves in inhibiting cell growth and mutation or deletion of these genes results in the developments of cancerous tumors in patient (3). Another mechanism by which cancer cells inhibits the function of tumor suppressor genes includes interaction of the ncRNA fragments with tumor suppressor proteins. This results in release of high levels of PSF-binding RNAs from tumor cell line (4). Hence, role of ncRNAs in evading growth suppressors gives clear insight into mechanism behind cancer development.
3. Avoiding immune destruction: Immune evasion or avoiding immune destruction is also one of the hallmarks of cancer development. This process also acts as a major barrier in designing anti-cancer therapeutic strategies. The evasion of immune attack occurs by creating an immune suppressive environment by means of tumor variants resistant to immune effectors. Cytotoxic T cells and CD4⁺ and T helper cells produce interferon and cytotoxin to inhibit the development of cancer cells, however the process of chronic inflammation counteracts immune response and promotes cancer development. The tumor cells also exploit regulatory T cells (Tregs), defective antigen presentation and immune suppressive mediators and apaptosis mechanism to evade immune response. In the case of cancer metastasis also, the mechanism of detaching from primary tumor and travelling through the surroundings tissues occurs by avoiding immune destruction (5).
4. Enabling replicative immortality: Enabling replicative immortality is the third trait of cancer which indicates the potential of cancer cells towards unlimited replication. Normal cells cannot pass through large number of cell division cycles; however tumor cells possess the potential to unlimited replication. Tumor cells possess unlimited replication potential by way of circumventing the loss of telomeres that determines the number of cell division cycles. Tumor cells are able to control the loss of telomeres by the expression of telomerase enzyme and the activation of telomere tandem lengthening pathway. Long ncRNAs also plays a role in replicative immortality as it acts as the regulator of genome stability and replication (3).
5. Tumor promoting inflammation: In the process of cancer development, it has been found that the negative effect of the immune system results in cancer development. The presence of white blood cells in tumor cells gives indication of the relation between inflammation and cancer development. The complex interplay between immunity and inflammation causes the development of cancer cells. Tumor promoting inflammation (TAM) and anti-tumor immunity regulates the pathway for formation of tumor. Tumor associated macrophases also provides the environment for tumor growth, invasion and metastasis (6). From this evidence, it can be said that TAM cells plays a major role in tumor promoting inflammation and cancer development. This knowledge can be effectively utilized to design cancer treatment options.
6. Activating invasion and metastasis: Another mechanism that is regarded as a hallmark of cancer includes their ability to invade and form distant metastases. The step towards invasion and metastatis initiates when morphological changes occur in cancer cells. The invasion-metastasis reaction occurs when cancer cells are able to escape immune surveillance and move from primary regions to target tissues to form micromestastases. The expression of E-cadherin, a cell-to-cell adhesion molecule is also one of the important factor of the invasion-metastasis cascade. In contrast to down regulation of E-cadherin in human carcinomas, N-Cadherin is upregulated in invasive tumors (7).
7. Inducing angiogenesis: Inducing angiogenesis is also a trait found in cancer cells. The main advantage of this trait is that the process of angiogenesis prevents the natural diffusion limit of oxygen and nutrients. The process of angiogenesis is necessary for wound healing and tumor progression turns on the angiogenic switch thus helping to sustain tumor growth. The mechanism behind angiogenesis includes binding of the angiogenic regulators to receptors on the endothelial cells (8). Certain evidence has also given indication about the role of ncRNAs in facilitating the angiogenic process (3).

8. Genome instability and mutation: Genome instability is also one of the properties of cancer cell. Genome instability is the increased likelihood of genomic alterations during cell division. Genome stability is necessary for cellular integrity, however the opposite process of genome instability leads to the progression and development of tumor. The presence of genetic unstability factor in cancer cells results in a shorter cell cycle and evasion of immunological control mechanism. This provides cancer cells the advantage of proliferation and transforming into a malignant cell. The process of genetic instability is associated with structural changes like variations in base pair mutation and function of microsatellite. There various contrasting evidence regarding the mechanism underlying genetic instability. One hypothesis is that occurs by the loss of gene function (9). More research in this area may help to identify chemotherapeutic drugs that target this hallmark.
9. Resisting cell death: Another established hallmark of cancer development is the ability of cancer cells to resist cell death. Three mechanisms contribute to cell death. Firstly, the mechanism of apoptosis leads to controlled cell death. This occurs because the cancer cells lose the ability to show mutation and activates the expression of anti-apoptotic regulators like Bcl-2. TP53 induces apoptosis. Autophagy is the second mechanism attributing to cell death. The property of cells to break down their organelles provides many beneficial effects to cancer cells (10). Necrosis is the third mechanism contributing to cell death. Necrosis is a phenomenon in which necrotic cells releases their content into the local tissue microenvironment. By this process, it releases many pro-inflammatory signals into the surrounding tissue. The necrotic cells act as the source that facilitates the process of angiogenesis, proliferation and invasion in cancer cells. PCGEM1 is regarded as the ncRNA that plays a role in anti-apoptotic functions (3) (Refer-Figure 1).

10. Deregulating cellular energetic: Deregulating cellular energetics is also one of the attributes of cancer cells. Normal cells produce energy the process of glycolysis, whereas the malignant cells increase their source of energy by the upregulation of glycolysis. This phenomenon of increases utilization of glucose is known as the Warburg effect. Hence. The continuous activation of gylcolysis in cancer cells leads to activation of oncogenes and progression of cancer. The tumor suppressor genes and the mutated oncogenes plays a major role in deregulating cellular energetic. Normal cells have several signaling networks that activates metabolic pathways for cell reproduction. However, many byproducts produced during aerobic metabolism such as reactive oxygen species result in DNA mutation and cell damage. This change in cell metabolism is the factor resulting in tumorigenesis. The mutations in the tumor suppressor genes and oncogenes change many signally pathways thus triggering the process of tumor development (11).

Hence, from the above evidence, it can be said that the Warburg effect is the mechanism that results in metabolism of tumor. Cancer cells finds glycolysis as a source of energy. Hence, it can be said that there is direct relations between tumor malignancy and glycolytic ATP production. By understanding this phenomenon behind cancer cell development and the dependence of cancer cells on glucose utilization, many therapeutic interventions can be designed.

One of the major hallmarks of cancer is sustaining proliferative signalling (13). One of the major characteristic of cancer cells is their ability to proliferate at a constant rate even in the absence stimuli coming from the external growth factors (13). Normal cells strictly manipulate the production of the growth initiating and inhibiting factors in order to ensure a tight control of the tissues and cell number, integrity of the tissue and architecture. However, tumour cell physiology is completely different from the normal cell lines. They showcase deregulated signalling cascades that promote them to be more or less free from the effect of the signals of proliferation which results in the unlimited cycles of proliferation. In order to attain this immortal capability, the tumours cells attain the capability of sustain proliferative signalling. This power of sustaining proliferative signalling is acquired by the tumour cells in different ways for example they generate their own growth factors and their complementary receptor molecules thereby resulting in the process of autocrine stimulation (14). Other pathways which are undertaken by the tumour cells in order to attain sustaining proliferative signalling include paracrine signalling which help in the production of numerous growth factors that support the development of cancer cells. The growth factor receptors are also constantly expressed in the tumour cells and this help in sustained proliferation via continuous binding of the growth factors with its receptors on the tumour. In extreme cases, the cancer cells become totally independent from the effect of the exogenous growth factors because of constitutive activation of the downstream signalling pathways or other disruption of negative-feedback mechanism (14).

Sustaining Proliferative Signaling in Cancer Cells

In order target the cancerous cell in the ground of sustained proliferative signalling, PI3K/Akt pathway is being targeted by the chemotherapeutic drugs. This is because, PI3L/Akt signalling pathways plays an important role not only towards the growth of tumour but also act as a potential response of tumours towards anti-cancer treatment (15). Acquired resistance towards treatment with chemotherapy, radiation and targeted therapy. Cisplatin, a chemotherapeutic drug, induces the activation of Akt pathway and thereby promoting cell death. Active Akt has been found to be present in numerous cisplatin-selected chemo-resistant lung, ovarian and glioma cancer lies in comparison with the sensitive parental counterparts. Combined treatment done under the application of Ly294002, a PI3k inhibitor not only helps in the attenuation of cisplatin induced Akt activity, but also increases cisplatin induced cytotoxicity and this indicates that the process of Akt activation is caused under the effect of upregulation of PI3K and this makes cancer cell lines more resistant towards cisplatin. Numerous mechanisms have been have been proposed in order to mediate the cisplatin-induced PI3K/Akt activation (15) (Refer: Figure 2).

The cisplatin-resistant NSCLC a549 cells on stepwise exposure of increased concentration of cisplatin has been found to increase Akt1 activity via increasing the protein level and increased gene expression in comparison to that of the parental cells. On the other hand, the levels of pAkt signals in the lung cancer tumour are inversely proportional towards the cisplatin sensitivity towards the primary cultured cancer cell lines of the lung from identical tumour tissues. The Cisplatin-induced Akt activation is dependent on the EGFR activity which lies upstream of PI3K. Cisplatin-induced phosphorylation of EGFR is associated with EGFR internalization along with ATM and ATR-dependent activation of p38 and this occurs as a result of cisplatin-induced DNA damage. This activated p38 causes the internalization of epidermal growth factor receptor (EGFR). Internalized EGFR causes downstream phosphorylation of the tyrosine residues and thereby active further downstream tumour suppressor protein p85. This p85 is another important component of PI3K and thus help in the prevention of sustained signalling and thereby promoting cell death (16).

Another chemotherapeutic drug that is found to prevent sustained signalling in cancerous cell line is Etoposide. Etoposide is a podophyllotoxin which is kown to casr peliotropic actions within the cells including inhibition of the action of topoisomerase II, production of reactive oxygen species (ROS) and subsequent induction of DNA damage. This DNa damage actives PIK3-Akt kinase signalling pathway which in turn found to cast an effective impact towards the treatment of gastric cancer (17)(18).

The aim of immortality for the cancer cells can be easily achieved if the cancerous cell acquires the hallmark of resting cell death. There are three important pathways that promotes cell death and careful regulation of three of these pathways help in the achievement of the immortality of the cancerous cell. The first mechanisms that promote towards controlled cell death include apoptosis. The process of apoptosis is initiated via numerous external and internal stimuli and numerous studies have highlighted that cancerous cell which are malignant in nature can attenuate the process of apoptosis and thereby becoming resistant towards cell death. In normal cell, the induction of DNA damage is one of the main markers towards the initiation of apoptosis. DNA damage, promotes the expression of the pro-apoptotic proteins Noxa and Puma (upregulated modulator p53, apoptotic gene) causing cell death. However, more than 50% of all human cancers have lost the signalling pathways mediated by p53 gene or p53 gene remain mutated and this lead to termination of the process of apoptosis even during the cell damage. Alternatively the tumours represents show an increased level of expression of numerous survival factors or other anti-apoptotic regulators like Bcl-2 and Bcl-xL. This can be regarded as one of the second important mechanism that repress controlled cell death or autophage. The cellular mechanism of autophagy operates at low levels within the cell line. However, it can be alternatively activated via numerous kind of cellular stress factors like nutrient deficiency. Autophagy is considered as a cell-recycling program that enables the cell to break down into their organelles and then to employ the degradation of the products towards the process of fuel biosynthesis pathways or for re-usual for subsequent energy production within the body. However, autophagy can act both as strength and weakness of cancerous cell. This weakness comes in the form of blocking the pathway for carcinogenesis. The last mode of cell death that is negatively impaired in cancer cell line is necrosis. Necrosis is defined as a process of un-controlled cell-death that occurs mostly with the damaged or injured cell lines. Similar to that of autophagy, necrosis can also serve to be beneficial as well as harmful for the cancerous cell lines. In the beneficial grounds, the negative regulation of necrosis in cancer cell lead to the uncontrolled cell proliferation and the other hand, necrotic cell lines have also found to attract the proinflammatory cell mediators causing death of the cancer cells (14).

In order to induce apoptosis in cancerous cell, the chemotherapeutic drugs like the cisplatimum or etoposide triggers apoptosis via the activation of TP53 pathways (2). Alterative research has showed that over-expression of the anti-apoptotic protein from the Bcl-2 family like Bcl-2, Bcl-xl is found to contribute chemotherapeutic resistance in cancers cell. One of the strategy that is used to destroy this anti-apoptotic protein include application of interfering oligonucleotide that downregulate the expression of the Bcl2 family of proteins. Controlled expression of Bax protein or application of BH-3 peptide has been found to abrogate protection againt the antiapoptotic protein in cancerous cell. One agent that is present gaining importance in the clinical trials is oblimersen. It is a nuclease-resistant antisense oligonucleaotide that targets Bcl2 mRNA. However, Oblimersen is still in Phase II and II clinical trials and is used to treat a wide range of adult and childhood tumours (19) (20). Oblimersen is still not approved for the treatment of melanoma this is because the results published from the phase III trials failed to show any extended survivals of the patients. On the other hand, oblimersen has been shown to produce favourable outcome when it is combined and injected along with docetaxel in the patients who are suffering from hormone-refractory prostate cancer (19)(20).

Another drug that is used to induce aopotosis is 5-Fluorouracil (5-FU). It is mainly used for the treatment of colorectal and breast cancer. 5-FU mainly targets p53 mediated cell apoptosis. However, one of the major disadvantage of 5-FU is, it becomes non-functional among the p53 independent cells (21). 5-FU is an uracil analogue. It has fluorine atom located at the C5 position of the pyrimidine ring. Once 5-FU is transmitted inside the cell, it gets converted into active metabolites like, fluorodeoxyuridine triphosphate (FdUTP), fluorodeoxyuridine monophosphate (FdUMP) and fluorouridine triphosphate (FUTP). These metabolites promotes global RNA metabolism via incorporating FUMP ribonucleotide into RNA as well as DNA. This incorporation either ocuurs directly or occurs via thymidylate synthase (TS) inhibition leading to a wide range of abnormal biological effects which trigger controlled cell death or apoptosis (21). 

Thus from the above discussion, it can be concluded that the concept of “hallmarks of cancer” is a powerful guide in the domain of translational research which is aimed at improving and developing early detection process of cancer, effective screening and thereby improving the quality of life of the cancer patients. However, the use of the chemotherapeutic agents can impose several threats to the normal cell line due to non-specific target. Recent researchers are targeting ncRNA (Non-coding RNA) as an effective means of treating cancer. ncRNA has several advantage over the chemotherapeutic drugs and this are regarded as novel diagnostic and prognostic markers.

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