Saturday, September 24, 2011

Understanding Cancer Metastasis: Unmasking A Killer

We continue with understanding how metastasis occurs in its transformations in the body.

Primary cancer, CTCs and distant Metastasis
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Second Hurdle: The cell, having found a perch, now has to migrate longer distances to reach a source of easier nutrition and better residence. For that it has to find a vehicle of transportation. Various means are available for just such purposes. There is the lymphatic chain - slow, thin-walled easily invaded system. Cancers originating in the Colon, Breast, Melanomas and Lung are adept at such invasive coercions. Their travel through the lymphatics establishes new sites for growth and residence -in places wherever those lymphatic chains end – the lymph nodes (gateway of the lymphatic system).
They reach the widespread circulation in the vascular system, traveling miles in minutes.

These lymph nodes, process the defense and measure of the cancer cell by using the elaborate mechanisms of immune surveillance and serve as the primary protective gate-keepers. They try to destroy and prevent the cancer cell from laying a foundation and a foot-hold.  It is by sheer misinformation and some disinformation that the cancer cell “fools” the host immune cells. Sometime it just uses sheer physical force of a large army to subdue and claim victory, as in leukemia. The more aggressive cancer cells find means to kill the joys of defense by insinuating themselves into blood vessels directly, mostly the venous system, since the veins are less thick-walled then the arteries. They reach the widespread circulation in the vascular system, traveling miles in minutes.

Main Lymphatic System
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The Main lymphatic system originating in the abdomen also ends up in the vascular system eventually, it is called the Cisterna Chylii and dumps its lymph via the thoracic duct into the large subclavian venous system on the left side of the neck. This is why there is a revitalized interest in the CTCs (Circulating Tumor Cells). The presence of the CTCs is object of renewed interest to try to detect their presence and decipher their vulnerabilities. 40% of all comers with breast cancer have spread of cancer cells in the bone marrow and an equal number of lung cancer in early stage I state have cancer cells in the pleural sac that lines the lung. No wonder there is recurrence in early stage disease at a later date. The “cat had already climbed out of the bag.”
Every line of defense proposed by the host is subject to mitigation and denial of sustainability by the cancer cell. This co-opting mechanism is the hard and tedious work of Darwin’s old adage, “The survival of the fittest.”

One can detect their (CTCs) (cancer cells) genomic make up and fire targeted therapies at them in an attempt to cure. Although it may seem that the cancer cell has it all in its favor. Not so! Another hurdle for the free-floating CTCs in the blood-stream is that having lost cell to cell adhesion, in their free-floating mode, they rapidly undergo anoikis (apoptosis or cell death) When observed morphologically, it is unknown if these CTCs are alive, playing possum or just dead and effete waiting for the dirt heap. The cancer cell “Mission Impossible” smart, adaptive and self-protective, capability has devised elaborate special anti-apoptotic mechanisms by expressing Bcl-2, Bcl-XL or FAK genes, to keep itself alive from death till it finds a host tissue organ. The self-destruct message is not invoked that easily. No poof! Only more smoke and mirrors. Some of these cells may live in the body undetected for months to years and resurface again. It is well known that years later the same cancer, considered cured, can raise its ugly head and new molecular data seems to shed some light on this phenomenon. The argument remains whether it is the cell that clicks its machinery into action or the abrogation of responsibility by a taxed immune system that allows such spread, no one is certain just yet.

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Third Hurdle: From the blood vessels migrating back into an organ is another deterrent for the cancer cell. There are two issues with this hurdle: One, for the cancer cell to re-invade the endothelial and muscular layers of the blood vessel wall to reach the outside reference of the vascular system is difficult and, two, to find a location where the cell is comfortable in its existence. (Someone who loves the beach will want a beachfront property and those desiring mountaintop abodes will find their destinies). So there is a “soil” issue that remains to be resolved. Oh yes these cancer cells are picky! 
We will uncover that truth eventually and relegate it at some intermediate level of a “chronic disease’ and eventually to the vast heap of “past maladies.” Whenever the end-result, the journey is far from over and ripe for the taking.

The cancer cell has protrusions on its surface that look for target sites on the organs. For instance colon spreads to the lymph nodes and the liver, breast cancer spreads to the lymph nodes and the bones, prostate cancer spreads mainly to the bones. All these predilections are based on the “homing devices” the cancer cells carry on their surfaces. Not to say that the cancers do not deviate and attack other organs, they do, but the majority, behave in the fashion elicited for a reason. A transcription factor 3-gene being an adaptive-response gene may serve to integrate stromal signals from the tumor micro environment and in so doing bring the invader to its selected host site. Or in other words the signals from the host organ are received and processed by the cancer cell and appropriate obfuscating response is given to the host to confuse and thus allow easy access. (It is akin to throwing a steak to the dog). In Osteosarcoma (A sarcoma (cancer) of the bone) where 20% of patients have metastases at the time of initial diagnosis and 40% in later stage, 80% of the metastases are to the lung! Thus a special proclivity to spread to the lung tissue exists in osteosarcoma cancer cell.

The metastatic behaviour of OS (Osteosarcoma) is very distinct as over 80% of all metastases arise in the lungs and other organs usually remain unaffected. This suggests that the circulating tumour cell specifically ‘homes’ to distinct molecules that are expressed on the endothelium of the organ of preference.

Fourth Hurdle: Once inside the organ, the cancer cell has to fight off the natural defense of cellular density (cramped quarters) within the organ of its desire. Again the EMT and MET come into play at the host site.
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 A critical molecular event underpinning the dissolution of cell-cell contacts during EMT is the loss of E-cadherin, a key component of adherens junctions… The loss of E-cadherin releases β-catenin into the cytosol and elicits activation of the canonical Wnt signaling pathway [1,3]. Moreover, the impairment of E-cadherin function, together with the downregulation of components of tight junctions and desmosomes (for example, claudins, occludins, desmogleins and desmocollins) and polarity genes, contributes to the dissolution of inter-cellular contacts and the loss of apico-basal polarity… The ensuing reorganization of the actin cytoskeleton and the intermediate filament network and the acquisition of front-back polarity dramatically alter the cellular architecture, while the secretion of extracellular matrix components and matrix metalloproteinases remodels the extracellular matrix. Collectively, these changes disrupt the contiguity of the tissue epithelium and render the cells intrinsically able to migrate - independent of one another - and to invade the underlying stromal compartment by breaching the basement membrane.

It does so by “talking” to its neighbors and coercing them into allowing residence and ultimately though vascular fiat of co-opting the vascular endothelial factor release it finds itself a stream of palatable nutrition (Employing the same previously well-managed method). Cancer cells also block the immune network by sending alternate and confusing signals to the immune cells, (T and B lymphocytes, Dendritic Cells and the macrophages). While the first robber is decoding codes of protection, the second robber is sighting, gathering up the valuables and laying bare the foundations of an existence.

dendritic cell click to enlarge
Tumor cells, either circulating or at the site of metastases, can modulate the immune system of the host in order to achieve a survival advantage. Down-regulation of cell surface receptor HLA class 1 is one of such mechanisms. This impairs the recognition of tumor cells by the host cytotoxic T-lymphocytes. Tumor cells can also induce the production of immunosuppressive molecules such as IL-10… Fas also plays a role in immune evasion. Fas expression leads to recognition by, and activation of cytotoxic natural killer (NK) cells and promotes elimination from the circulation by the host immune system.

Additionally they send out alarms of inflammatory signals throughout the blood system. (IL18, NFkB, TNFa etc.) These cytokines have the ability to modulate the genetic makeup of the immune cells by bringing them in full force to bear on the enemy (in this case the cancer cell) and then by disinformation and direct assault causing attrition forcing the defensive host army to laying down the arms and capitulating. Thus the cancer cell kills the immune-surveillance for the single and simple act of self-perpetuation.
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Hypoxic (low oxygen) environments are not conducive to most cells. Metastatic cancer cells reap the rewards in a low oxygen environment where they find themselves frequently, both in the initial phases when the framework of blood vessels have not yet been established and late stages of growth, where there are too many cancer cells in the center of the tumor where the co-opted blood vessels cannot reach by distilling out HIFs. They use hypoxia as a mediator to release (Hypoxia Inducible Factor) s HIFs to force other signaling proteins to modify the DNA to build more bridges over breached dams for their own survival.

Low oxygen tension in the primary tumor is associated with metastasis in soft tissue sarcoma, cervix carcinoma and carcinoma of the head and neck. Multiple mechanisms may be involved in hypoxia-induced metastasis. Thus, hypoxia followed by reoxygenation may induce point mutations and DNA strand breakage leading to deletions, amplifications and genomic instability…Moreover, hypoxia may induce a temporary increase in the expression of gene products involved in the metastatic cascade, either through gene amplifications or through normal physiological processes by activating oxygen sensors, hypoxia signal transduction pathways and DNA transcription factors.

Every line of defense proposed by the host is subject to mitigation and denial of sustainability by the cancer cell. This co-opting mechanism is the hard and tedious work of Darwin’s old adage, “The survival of the fittest.” The cancer cell having acquired devious means, wants to survive. The cancer cell is unfortunately better equipped since it has understood the molecular machinery of survival better then the host has determined new adaptive lines of defense. There is more tit than tat. The fight between the host and its cargo of renegades, is a battle of intense planning, scrutiny, subterfuge and all out assault that rages in deft silence.
Understanding of this elaborate and intricate fabric woven together by a rag-tag group of wayward cells is the difference between life and death.

Therapeutic Options for metastases exist currently in the form of surgery (metastatectomy), radiation therapy, chemotherapy and biologic therapy. The former is surgical removal of the metastatic sites. There is proof at least in Osteosarcoma, and in Colon cancer where known only-metastatic cancer sites when removed (osteosarcoma metastases to lung and colon cancer metastases to the liver (in colon cancer, as long as no intervening spread or in-transit spread into lymph nodes has occurred) yield better survival.  It is also well known that patients with concurrent metastases have a worse survival then sequential ones. The precept being that the cancer cell has already achieved multiple DNA transgressions in the former and acquires them later in the latter. The steps needed for metastases to occur in osteosarcoma specifically are detailed in the table below: (listed in references):

Steps of metastasis
Molecular involvement
Migration and invasion
(a) Anoikis resistance
BcL family
(b) Apoptosis resistance
Evasion of immune system
Arrest and extravasation
Angiogenesis and proliferation

Understanding of this elaborate and intricate fabric woven together by a rag-tag group of wayward cells is the difference between life and death. The matters remain intricate and more and more complexity continues to be added to this heavy tome of discussion. The answer may lie at fundamental levels. There may be a final common pathway, or as we now know, there may be many layers to the finite bottom of this “rabbit hole.” Each step and each experiment brings us closer and sometimes inadvertently leads us further from the truth through the minutia of “discussions.” We will uncover that truth eventually and relegate it at some intermediate level of a “chronic disease’ and eventually to the vast heap of “past maladies.” Whenever the end-result, the journey is far from over and ripe for the taking.

Parvez Dara, MD FACP
Twitter: JediPD

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