Itt írjon a(z) hypoxia_oxygen-ról/ről

THE ROLE OF HYPOXIA/OXYGEN IN THE TREATMENT OF TUMORS

Introduction

Hypoxia is the deficiency of oxygen in the body. As a tumour develops, it will rapidly outgrow its blood supply (Brown and Bicknell, 2001). Abnormalities of the newly formed blood vessels (neovascularization) can cause malfunction creating a hypoxic environment, which is related to malignant progression and resistance to treatment such as chemotherapy and radiation. Therefore, tumour hypoxia is important in the prognosis of the disease in survival rate and also the expected disease-free time. It is important to know how the cancer cells change under these conditions, so we understand the impact they have on different therapeutic methods.

HIF-1

Birner et al (2000) describes HIF-1 (hypoxia inducible factor 1) as a transcription factor made of an alpha and a beta subunit. The alpha subunit is oxygen regulated, which means that the alpha subunit will determine HIF-1 activity. This transcription factor is essential for tumour growth and progression, and can thus be used in prognosis of the disease, because it is known that a strong HIF-1 alpha expression will lower the survival rate and decrease the disease-free time (Birner et al, 2000).

Oxidative stress

Oxidative stress is caused by reactive oxygen species (ROS), which are oxygen radicals that damage DNA, causing mutations and then later malignant progression (Brown and Bicknell, 2001). ROS are made in the mitochondria during aerobe metabolism, but also in neutrophils and macrophages used in the immune response (Brown and Bicknell, 2001). Tumour cells are known to overproduce ROS (Brown and Bicknell, 2001). The oxidative stress cause resistance to apoptosis, which cause treatment failure (Brown and Bicknell, 2001). Therapies like chemotherapy and radiotherapy are dependent on apoptosis as a response to oxidative stress and ROS induced DNA damage caused by the treatment (Brown and Bicknell, 2001).

VEGF and angiogenesis

Jain et al (2007) describes that the growth of both primary and metastatic tumours relies on their own ability to take over the host vessels, forming new vessels (from preexisting vessels) by angiogenesis and/or by recruiting bone-marrow-derived cells (vasculogenesis). Vascular endothelial growth factor (VEGF) has a major role in neovascularization, and can stimulate these tumors to form new blood vessels (Rakesh et al, 2007). These formed vessels ends up being anatomically and physiologically abnormal and contributes to a hostile microenvironment. This microenvironment present increased interstitial fluid pressure and decreased oxygen tension, which leads to increased lethality and even more malignant phenotype (Rakesh K et al, 2007).

Types of treatment

Hyperbaric oxygen treatment

Hyperbaric oxygen treatment (HBO) is used for treating hypoxia and ischemia (lower oxygen level than normal). HBO is used to increase the amount of dissolved oxygen in the blood plasma, and the oxygen delivery to the tissues. The administration of HBO is 100% oxygen at a higher than normal atmospheric pressure.

HBO in relation to cell death, shows that hyperoxia over a longer period of time elevate the reactive oxygen species (ROS), which overwhelm the antioxidant defence and can lead to damage to the cells and possible organ dysfunction. To ensure the survival of the cells during oxygen treatment, cell type, concentration of oxygen used and the duration of the exposure should be carefully monitored.

Angiogenesis in tumours is an important factor for growth and metastasis. HBO has shown to have an antiangiogenic effect in mammary tumour models and in glioma. At the same time, several other studies showed no particular change in angiogenesis with oxygen treatment.

A recent study showed that HBO leads to a mesenchymal to epithelial transition, reducing the aggressiveness of the tumours phenotype in mammary tumours, therefore also reducing the metastasis and invasiveness of the cells. (Moen and Stuhr, 2012)

Research studies shows that HBO has an effect on some types of cancer, reducing growth rate of tumours. This was shown in breast cancer, while bladder and cervical cancer did not show any difference in growth rate after HBO treatment. HBO did not show the ability to cure cancer as the only method of therapy, but it can be a powerful supplement to conventional therapy in some types of cancer. (Moen and Stuhr, 2012)

Radiation Therapy

Radiation therapy uses ionizing radiation to kill cancer cells in two ways; direct and indirect. Directly it can kill cells by damaging the DNA, and indirectly it damages the water molecules in the cells, which leads to production of free radicals. Free radicals are small, independent molecules that contains an extra electron which makes the free radicals very reactive to other molecules. (Desouky et al, 2015)

Cells that are well supplied with oxygen makes the radiation more effective, and the range of DNA damage and killing cancer cells is high. In hypoxic areas the radiation is half as effective, which enables some cancer cells to survive radiation therapy. This is one of the reason for patients with cancer to receive radiation therapy in several cycles. (Rockwell et al, 2009) Almost all cancer patients receives radiation therapy at some point in their treatment, and the ability for hypoxic cancer cells to resist the radiation is a serious problem, and these hypoxic cancer cells that do survive- can lead to recurrence of the tumor and, in worst case, development of and even more aggressive tumor phenotype. (Rockwell et al, 2009)

Hypoxia may generate tumor resistance to radiation therapy directly and indirectly. Directly it induce tumor resistance through the elimination of molecular oxygen. Indirectly it stimulates or inhibits post transcriptional effects and gene expression, and by that change the cell-cycle position or numbers of cells existing in the G0-phase of cell division. (Harrison et al, 2004)

Figure 1. Shows how radio sensitivity declines when tumor pO2 is <25-30mmHg

Chemotherapy

Chemotherapy is the use of drugs to treat a disease. This kind of therapy will be able to work in the whole body, so it will be a good way to reach not only the primary growth site, but also reach metastasized tumour cells. (Moen and Stuhr, 2012)

Hypoxia in tumours are connected to resistance to therapy, and growth with metastasis of the tumour. Angiogenesis forms pathways for the tumour cells to propagate to other parts of the body, but they will also have to survive in the circulation and attach to a secondary growth site in order to multiplicate. Chemotherapy is working systemically, and will reduce this option for the tumour cells. (Blackwell and Harrison, 2004)

Hypoxia occurs in most solid tumours, and can influence the response to treatment. Under hypoxic conditions, the tumour cells have a number of responses to ensure the survival of the cells, primarily the Hypoxia-inducible factors (HIFs). In normal conditions where the oxygen levels are good, the HIF subunits are degraded by Prolyl hydroxylase domain (PHD) enzyme. As soon as the conditions are hypoxic, the PHD enzymes are inactivated and degradation is reduced. Then the more stabilized HIF-1α subunit molecules translocate to the nucleus and bind to DNA, as a response gene transcription is induced to adapt the cells to the hypoxic environment. (nevnt før)

During adaptation we can see abnormal vascularization, reduced blood flow and therefore reduced 02 diffusion in the tumour (Blackwell and Harrison, 2004). (nevnt før)

Tumour hypoxia, or hypoxic regions, leads to several molecular and genetic changes, that can make the treatment difficult (Blackwell and Harrison, 2004). (nevnt før)

Altered cellular metabolism can reduce the chemicals cytotoxicity, some substances need a sufficient supply of oxygen to be maximally cytotoxic. Genetic instability can lead to development of drug resistant cells, making the therapy difficult. Lack of oxygen will also make the availability of the tumour less efficient, and thus the administration can be affected. Use of hyperbaric oxygen treatment could increase the tumour perfusion and the cellular sensitivity to the drugs (Moen and Stuhr, 2012). Since the tumour is harder (through the circulation?) to reach when in hypoxic conditions, there have been research on bioreductive drugs targeting hypoxic cells.(used together with chemo? define here not in end to understand?) Hypoxia activated prodrugs (HAPs) are reduced and activated in hypoxic conditions, and will therefore target the hypoxic regions in the body. Normally, healthy tissues do not have hypoxic regions, so the tumour is targeted only. A useful HAP will have the ability to reach hypoxic cells, have features that makes it easy to metabolise, and have a long lasting effect on tumour cells. This strategy combined with radiation or conventional chemotherapy could be a good strategy to treat prostate tumours. (Mckenna, Errington and Pors, 2018)

Surgery

During surgery the tumour and surrounding tissue is removed. Its immediate efficiency will not be altered by the effects of hypoxia and ROS, but as cancer is known to spread quicker and more frequently under these conditions, surgery as treatment will still have a higher risk of failure compared to non-hypoxic and ROS affected cancer types (Höckel et al, 1996).

The role of Hypoxia in Cancer types

Pancreatic cancer

About 85% of the pancreatic tumors develops in the exocrine tissues of the pancreas. These tumors arise in the epithelial cells lining the pancreatic ducts and accounts for the vast majority of cases. This type of pancreatic cancer is know as Pancreatic Adenocarcinoma. These tumors usually form in the head and neck of the pancreas, but sometimes also in the tail. Around 5% of all cases are pancreatic endocrine tumors. (Ilic, 2016)

Figure 2 shows the complex relationship between pancreatic cancer cells and the pancreatic stromal/stellate cells generating hypoxia in pancreatic cancer tissue, and initiating a cascade of events. These events will lead to the stromal/stellate cells forming a dense extracellular matrix (ECM) that reduces capillary function, and the pancreatic cancer cells to secrete antiangiogenic factors that will limit new vessel formation. The combination of these two events leads to lack of blood flow- which results in formation of hypoxic areas within the tumor tissue of the pancreas (Erkan et al, 2015). Hypoxia-inducible-factor-1 (HIF-1) initiates first as a response to hypoxia, which further heighten the stromal/stellate cell to create a vicious cycle which leads to severely hypoxic areas throughout the tumor tissue (Erkan et al, 2015). HIF-1 also mediate adaptive responses of pancreatic cancer cells, which allows the cells to survive by improved aerobe glycolysis, inhibition of apoptosis and constant proliferation whilst increasing the aggressiveness of the disease through maintenance of cancer stem cells (CSC), invasion/metastasis and treatment resistance. (Erkan et al, 2015)

Pancreatic cancer has a very poor prognosis because it is very invasive and has a rapid progression (Stocken et al, 2008). Pancreatic cancer is one of the deadliest types of malignant carcinoma and is typically diagnosed at a late stage, with a 3% 5-year survival rate in the US (Li et al, 2018). For the best chance at long term survival surgery is the best option, but chemotherapy and radiation therapy is also used. (Li et al, 2018)

Prostate cancer

Prostate cancer is the most common malignancy in men, and it is the second most lethal type of cancer. Statistics from the American Cancer Society shows that 220,800 new cases and 27 540 deaths are estimated in the United States in 2015. (Bibby, Choudhury and Roberts, 2017)

Hypoxia in prostate cancer tumours occurs early during the tumour development, and are related to the more aggressive phenotypes of prostate cancer. The Hypoxia and hypoxia-related biomarkers are associated with the progression of the disease, and also failure of treatment. (Bibby, Choudhury and Roberts, 2017)

Under hypoxic conditions, several signalling pathways are activated in the tumour cells. Some of the activated factors include HIF, PI3K/Akt/mTOR, NOX (NADPH oxidase), Wnt/ β-catenin and Hedgehog. These pathways are communicating and ensuring the tumour cells survival in the harsh hypoxic conditions, inducing formation of blood vessels -angiogenesis, rapid reproduction of the cells, stemness, spread and further growth. The cancerous cells will also do a basic transformation – ependymal to mesenchymal transition. This transition is leading to cancer cells with a more invasive or metastatic phenotype.

Exomes will also play an important role in growth of tumours, and in generating a suitable microenvironment to survive the hypoxic conditions. Exomes are nanosized vesicles that can transfer information in form of protein, lipids, sugars and nucleic acids from a hypoxic cell to normoxic cells, and therefore promotes growth and progression of the cancer.

Therapy options are surgery, radiation therapy, hormone therapy, chemotherapy or a combination. Under hypoxic conditions there is harder to reach the tumour cells in an adequate way, since the 02 pathways are limited. A way around this could be chemotherapy that targets hypoxic cells. (Mckenna, Errington and Pors, 2018)

Surgery is a good option, but the metastasis have to be under control to remove enough of the tumour cells. In radiotherapy, the primary factors leads to radio resistance, and the radio sensitivity declines with lower 02 levels (Blackwell and Harrison, 2004)

Cervical cancer

20-35% of all cervical cancer patients are expected to die from their disease (Höckel et al, 1996). Hypoxic tumour types are known to increase this percentage, caused by the more aggressive phenotypes developed due to hypoxic induced development of the tumour cells, which will affect the spread and life expectancy of the disease (Höckel et al, 1996). The strengthened resistance of hypoxic tumour cells will lead to a decrease in responsiveness to radiotherapy and chemotherapy, which are typically used in cervical cancer treatment (Höckel et al, 1996).

Höckel et al (1996) describes how hypoxia is known to enhance the genetic diversity of the cancer cells. Evolution will therefore develop aggressive phenotypes typical for hypoxic tumour cells, such as the increase of growth of the primary tumour, increased spread to the connective tissue of the uterus and increased frequency of lymph-vascular spread.

HIF-1 alpha used in early prognosis of cervical cancer HIF-1 alpha stabilizes the tumour suppressor protein p53, which causes cell death (Birner et al, 2000). In cervical cancer, p53 is usually inactivated by the viral oncoprotein E6 leading to further stimulation of the HIF-1 alpha expression, which will affect the tissues response to treatment as the cancer suppressing proteins does not show significant activity (Birner et al, 2000). ‘

Breast cancer

Brown and Bicknell (2001) suggests that oxygen radicals give an important indication of the prognosis, and therefore explains how antioxidants are important in therapy, as they can antagonize the effect of ROS. If antioxidants are proven effective in the treatment of breast carcinoma, it would be very interesting since they are a low toxicity substance (Brown and Bicknell, 2001).

The insufficient blood supply of breast carcinoma cells will cause low glucose levels (Brown and Bicknell, 2001). Pyruvate supplies will decrease when glucose levels are low, so the cell will no longer to be able to decompose the oxygen radicals, which enhance the accumulation and damage further (Brown and Bicknell, 2001).

Brown and Bicknell (2001) also describes possibilities of how oxygen stress can aid tumour spread, as it causes easier detachment of the cell from the basal membrane, and increased permeability in the microenvironment of the tumour, and can thus spread through the blood or lymph easier. This vascular spread is also promoted further through two ways in breast carcinoma (Brown and Bicknell, 2001): HIF-1 alpha levels can be increased by oxygen radicals and hypoxia, which will promote VEGF production. This newly formed vascularization may cause spread to lymph nodes. Increases the risk of spread by blood caused by increased blood supply through vasodilation activation.

HBO in the treatment of breast cancer is a promising field of research as the treatment is known to: Decrease the malignant cell spread (Granowitz et al.,2005) Create a less aggressive tumour type (Moen et al., 2009) Decrease cell proliferation and increase cell death frequency (Raa et al., 2007)

Brain cancer

Brain tumors can be categorized by their primary site location, which can be supratentorial or infratentorial tumors. The tumors are named after the cell type involved, eg: Astrocytoma, which is a tumor formed by mutated astrocytes. Glioblastoma is a type of astrocytoma that is the most common and aggressive malignant primary type of adult brain tumor, and it is found in the cerebral hemisphere. (Jain et al, 2007).

Pervasive hypoxia is one thing that characterizes glioblastomas. It is uniformly fatal, despite radiotherapy, chemotherapy and surgery (Jain et al, 2007). Researchers have found out that anti-VEGF therapies can be effective in the treatment of glioblastoma because it can, for a little while, bring the tumor vessels back to normal. This means that the usage of anti-VEGF in glioblastoma can optimize the combination of chemotherapeutics and radiation as treatment. (Jain et al, 2007)

Hyperbaric oxygen (HBO) treatment has shown to reinforce normoxia in hypoxic regions in brain tumors (Graham et al 2018). Graham et al (1996) explained in their article that there were one study that showed glioma treated with HBO and radiotherapy improved survival, but at the same time another study showed that it can form complications such as necrosis and seizures, so this is a exiting field but it still need more research.

Conclusion

Those tumour cells who survive hypoxic stress will have an increased multiplication rate, causing increased oxygen deficiency in the body. The reproduction of these cells will further enhance the cells ability to cope with the low-oxygen environment, which will make the tumour cells a more dominant type, becoming more difficult to treat with their improved ability of survival and more aggressive spreading. The transcription factor HIF-1 will also make the treatment more difficult, as a strong HIF-1 alpha expression will lower the survival rate and decrease the disease-free time (Birner et al, 2000).

Some ways to counteract these responses in the body can be oxygen treatment, or targeted chemotherapy to reach the tumour cells in an efficient way. It is shown that different types of cancer, need different kinds of therapy. The same therapy method does not give the same result in every case.

Therapies like chemotherapy and radiotherapy are dependent on apoptosis, and as the oxidative stress cause resistance to apoptosis, hypoxia can lead to treatment failure.

Understanding the role of oxygen and hypoxia in the treatment of different cancer types is crucial to create an optimal treatment plan, as they are both factors known to alter the way different treatments would work under normal conditions.

REFERENCES

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