Hi Hanie... These are some thoughts regarding the question you asked.

Carcinoma cells acquire a mesenchymal shape through EMT (Epithelial-to -Mesenchymal Transition) that is when a cell starts to lose its epithelial characteristics and adopt a newly mesenchymal expression profile. Molecular-wise, cells will upregulate EMT markers such as Vimentin, N-Cadherin, Twist...etc and downregulate other genes, especially those pertaining to MET such as E-Cadherin.This will drive cancer cells to slowly detach from the basement membrane, as cells are no longer in close proximity to one another (also loss of gap junction, contact inhibition, and many other cell-to-cell junction), and give the cells the 'freedom' to invade locally. Indeed, cells can locally invade by digesting the ECM through a group of enzymes termed 'proteolytic enzymes' such as MMP2 and MMP9. That alone is insufficient for cancer cells to locally invade then leave their primary site, there is indeed a need of acquired motility associated with a rearrangement of the cytoskeleton: cancer cells will upregulate proteins involved in motility, such as Cdc42 or RhoA which drive 1. the remodeling of actin cytoskeleton, 2. a contractile force for cells to move, 3. invasive actin structures associated with an invasion status such as invadopodia. Because of the uncontrolled cell growth, there's definitely a lack of nutrient and oxygen which drive a state of hypoxia, if you take a tumor growth or foci, and you cut it, you would realize that cells in the core have undergone necrosis because of hypoxia. This phenomenon will induce cancer cells to increase the transcription of HIF1-alpha which drives the secretion of growth factors that enable 'de novo' blood vessels formation, a good example of these factors would be VEGF. Blood vessels will start to orient themselves towards a chemoattractant (VEGF) that is secreted locally, nearby the cancer cells. It should be noted here that blood vessels formed due to angiogenesis are generally leaky, and poorly structured as pericytes are loosely attached to them.  Moreover, the tumor microenvironment plays a crucial role in the process of invasion. In breast cancer cells for example, a very important member of the tumor niche is Cancer Associated Fibroblasts (CAFs) which promote invasion through secretion of local growth factors and cytokines that drive cancer cells to metastasize. Immune cells also play an important role in the supply of growth factors and cytokines as these are essential for the growth and survival of cancer cells. Now, when blood vessels reach cancer cells, a heterotypic interaction between endothelial cells and the latter cells take place: Cancer cells are indeed capable of transcribing connexin genes (mainly Cx43) to interact with the endothelial cells, and because gap junctions are one of the sources of molecular exchange between cells, cancer cells can cross the the endothelial layer (that is one proposed example of how cancer cells can cross and get into the bloodstream). A recent paper in Cancer Cell came out in April 2014 where it was shown that breast cancer cells secrete mir-105 that is capable of destroying the vascular endothelial barrier to promote metastasis. This suggests that there isn't a generalized theory on how cells get into the bloodstream, but many studies have particularly focused on that. The big question to ponder now: How can cells that have never been exposed to the blood environment i.e. blood pressure, pH change, immune cells,.. adapt so quickly (survive) and can invade into a secondary site? Many cells will die throughout the process, and few will make it and form foci. 

Hope this helps,

Patrick  

There's a variety of factors that will trigger the migration of some mesenchymal clones (in organism). Their ability to respond to pro-migration signals (cytokines, chemokines, soluble factors and growth factors, components of ECM...) is strongly related to their metastatic potential (clonal selection). These signal influences not just motility of these cells, but also phenotype, expression profile and so on. So in general there must be some interplay 1) to change phenotype and 2) to trigger motility ;)

In vitro experiments with migration usually uses established cell lines (which of course won't migrate towards serum free medium, cause there's no benefit from that) and chemoattractants like TGFb, EGF or FBS supplemented medium. But you can also use a variety of chemoattractants.

I suppose nobody can give a complete answer at present. One factor we know is important is oxygenation or hypoxia in the primary tumour. Probably acute hypoxia is a particularly strong driving force. The mechanism at a more physical-chemical level is not known as far as I can see.

Thank you so much Prof. Pettersen! I had totally forgotten about hypoxia!

However, this makes me wonder what happens to the benign tumor cells during hypoxia (assuming they wouldn't migrate). Would angiogenesis be good enough to provide cells with enough oxygen. Here again, if angiogenesis provides tumor cells (benign & the metastatic cells) with enough oxygen, then I am afraid hypoxia might not play a very highlighted role in migration.

It's all hypothesis and nothing is for sure so far.

hello 

i do agree with Pettersen, because cancer cells proliferate abnormally consuming all the needed oxygen and nutrients they might require. This insufficiency of oxygen and nutrients might trigger a form of migration to look for better area to start their population. Cancer metastasis is most at time said to be similar to evolution traits like "founder's effect" and "bottleneck effect". The highly proliferative cells need some source of energy propeling them to migrate to find new fertile soil. one can not exclude the possibility of chemoattractants as it has been reported that melanoma cancer cells create their own gradient density that distinguishes them from their outside envt propeling them to undergo invasion.

i have attached two papers which might be of help

cheers bro!!!!!!!!!!!!!!!

http://www.nature.com/nrc/journal/v9/n4/pdf/nrc2622.pdf

http://www.plosbiology.org/article/info%3Adoi%2F10.1371%2Fjournal.pbio.1001966

Since cancer researchers have had their main focus on the process of metastasis the effect of hypoxia on the ability of non-malignant cells to migrate and eventually invade have not had much focus. 

Hi Hanie... These are some thoughts regarding the question you asked.

Carcinoma cells acquire a mesenchymal shape through EMT (Epithelial-to -Mesenchymal Transition) that is when a cell starts to lose its epithelial characteristics and adopt a newly mesenchymal expression profile. Molecular-wise, cells will upregulate EMT markers such as Vimentin, N-Cadherin, Twist...etc and downregulate other genes, especially those pertaining to MET such as E-Cadherin.This will drive cancer cells to slowly detach from the basement membrane, as cells are no longer in close proximity to one another (also loss of gap junction, contact inhibition, and many other cell-to-cell junction), and give the cells the 'freedom' to invade locally. Indeed, cells can locally invade by digesting the ECM through a group of enzymes termed 'proteolytic enzymes' such as MMP2 and MMP9. That alone is insufficient for cancer cells to locally invade then leave their primary site, there is indeed a need of acquired motility associated with a rearrangement of the cytoskeleton: cancer cells will upregulate proteins involved in motility, such as Cdc42 or RhoA which drive 1. the remodeling of actin cytoskeleton, 2. a contractile force for cells to move, 3. invasive actin structures associated with an invasion status such as invadopodia. Because of the uncontrolled cell growth, there's definitely a lack of nutrient and oxygen which drive a state of hypoxia, if you take a tumor growth or foci, and you cut it, you would realize that cells in the core have undergone necrosis because of hypoxia. This phenomenon will induce cancer cells to increase the transcription of HIF1-alpha which drives the secretion of growth factors that enable 'de novo' blood vessels formation, a good example of these factors would be VEGF. Blood vessels will start to orient themselves towards a chemoattractant (VEGF) that is secreted locally, nearby the cancer cells. It should be noted here that blood vessels formed due to angiogenesis are generally leaky, and poorly structured as pericytes are loosely attached to them.  Moreover, the tumor microenvironment plays a crucial role in the process of invasion. In breast cancer cells for example, a very important member of the tumor niche is Cancer Associated Fibroblasts (CAFs) which promote invasion through secretion of local growth factors and cytokines that drive cancer cells to metastasize. Immune cells also play an important role in the supply of growth factors and cytokines as these are essential for the growth and survival of cancer cells. Now, when blood vessels reach cancer cells, a heterotypic interaction between endothelial cells and the latter cells take place: Cancer cells are indeed capable of transcribing connexin genes (mainly Cx43) to interact with the endothelial cells, and because gap junctions are one of the sources of molecular exchange between cells, cancer cells can cross the the endothelial layer (that is one proposed example of how cancer cells can cross and get into the bloodstream). A recent paper in Cancer Cell came out in April 2014 where it was shown that breast cancer cells secrete mir-105 that is capable of destroying the vascular endothelial barrier to promote metastasis. This suggests that there isn't a generalized theory on how cells get into the bloodstream, but many studies have particularly focused on that. The big question to ponder now: How can cells that have never been exposed to the blood environment i.e. blood pressure, pH change, immune cells,.. adapt so quickly (survive) and can invade into a secondary site? Many cells will die throughout the process, and few will make it and form foci. 

Hope this helps,

Patrick  

Wow Patrick! That was great! Some other questions popped in my mind regarding your answer:

1. What causes EMT? I mean what kind of signal induces this transition (same goes with MET).

2. what is/are the primarily factor(s) responsible for the varied gene regulations which will end up in the malignant cells to become loose and free from their counterparts?

3. You mentioned Cancer Associated Fibroblasts (CAFs) which secrete local growth factors and cytokines. Are these secreted within the tissue or within the blood stream? Does this means that cells response to a gradient of GFs/cytokines?

Thank you for your informative answer.

Hi again, 

Many factors participate in promoting EMT, for instance TGF-beta signaling pathway has been shown to be greatly implicated in the transition. However, the process is indeed complicated (as any other process) and needs further investigation. But for sure, mutations and epigenetics are at the heart of the disease, which answers (in a very vague way) your second question. There is definitely a differential gene expression that takes place, driven by mutations and epigenetics. As one cell divides, the risk of committing DNA errors increases: let alone the fact that generally cancer cells are aneuploidy, and the chromosomal territory is no longer respected. It's pretty chaotic in the nucleus (and cytoplasm of course), so you can imagine what erroneous DNA might lead to. 

Good mutations i.e those that will increase survival of cancer cells in an oxygen and nutrient deprived environment will be selected for. Recent studies have been interested in the role of the microenvironment in promoting survival and metastasis of cancer cells but also, in its putative tumor-suppressive role. CAFs will secrete diverse cytokines and molecules that will act on cancer cells in a paracrine manner, and on CAFs themselves in an autocrine fashion. Mostly these factors are secreted within the heterotypic milieu. 

Your questions are thought-provoking, there isn't an ultimate answer. 

Hope this helped, 

Patrick

Tumor tissue is composed not only cancer cells but also various types of cells in the stroma; Cancer-associated fibroblasts (CAF), tumor-associated macrophages (TAM) etc. Cancer cells show invasion and metastasis in collaboration with CAF and TAM. Accumulating evidence suggests that tumor cells educate stroma cells as they enhance the malignant potential of tumor cells. Given that cancer cells frequently have mutations in the cell-cycle regulatory proteins, tumor cells are mistaken as homogeneous. However, there exists a cellular heterogeneity in tumor tissues, which is why therapeutic response to the molecular targeting drugs is different depending on the genetic or epigenetic background of each tumor cell. Thus, the high degree of heterogeneity renders tumor tissue more easily to acquire the resistance to the molecular-targeting therapies focusing on “oncogene/oncogenic signal-addiction”. Only the resistant clones survive, so that the level of heterogeneity is transiently diminished by the anti-cancer treatment, which is called “bottleneck effect”. Generally speaking, this “bottleneck effect” is lost and the heterogeneity is again recognized in the recurrent

tumor tissue.

An emerging concept of the cancer stem cells (CSCs) simply

explains how the hierarchy of tumor cells is formed. CSCs have the selfrenewal

ability and multi-lineage differentiation potential, both of which are necessary to architect the secondary heterogeneous tumor lesion from a single cell (Figure 1). This diversity is responsible for the formation of minimal residual disease (MRD), which leads to the latent relapse and the distant metastasis. MRD is frequently enriched in therapy-resistant clones with the “stemness”.

Epithelial-mesenchymal transition (EMT) has widely been recognized as a crucial step in the invasion and metastasis as well as normal tissue development and wound healing. EMT has long been believed to increase the number of CSCs at the invasive front and metastatic foci.

Ref

The Novel Anti-tumor Therapy Targeting the “Functional” Cancer Stem Cell Markers

Yoshida GJ and Saya H,

Clin Exp Pharmacol 2014, 4:2

There are two things happening in this case 

1) Cells get detached (the cell adhesion proteins are cleaved) from the chunk

2) The random migration of the cells begins. They tend to cross the blood vessels because there are protein from the endothelial cells that associate with these individual cells and causes the transduction of several intercellular signaling cascades (such as Rho mediated) that assists the migration process 

Rest of the process is pretty random. It can end up anywhere depending on the local environmental conditions. 

There are studies that shows there could be a chemoattract mediated migration. These could be the soluble proteins from the ECM that interacts with protein such as integrins cell adhesion molecules or growth factor receptors, and int urn turns on the migration process which be directional.

Hi Hanie & all the other ones,

Thank you for your inspiring discussion!

I'd like to add that a few examples in which migration and invasion have key roles in physiological processes:

• Melanocytes (Khamal already mentioned melanoma):

These cells are derived from the neural crest, from where the spread all over the body and find their way into every single square mm of the skin and into every single hair follicle. To me, their personal history seem to explain the very special biology of melanomas.

• Development of the anterior pituitary (adenohypophysis):

Source of this organ is a portion of epithelium from the upper palate, moving 'upstairs' towards the brain. In order to fuse with the posterior pituitary, cell masses from so-called Rathke's pouch need to cross a couple of barriers.

• Mammary gland development (which would not work without EMT):

This requires quite motile cells, and may be switched on at any age - simply by changes in steroid hormone status. We see this in patients receiving anti-androgen therapy for advanced prostate cancer. Their mammary glands (initially having 100 cm² within less than a year (unless patients receive a low-dose radiotherapy of their mamillary region).

• Formation of paranasal sinuses:

At birth, our sinuses were extremly small, but throughout life, they continously expand by some kind of mass invasion into the bones of our skulls.

So we must keep in mind that cancer cells do not 'come up' with invasion as something new, but they literally have it in their genes. Carcinomas in situ (Cis) may grow into large masses without being invasive, and I guess that additional mutations are required to activate those 'old pathways' and give rise to an invasive subpopulation which may 'take over', then. A link to an interesting paper on the role of EMT in breast development and cancer is attached below.

Hope this helps, somehow,

andreas

http://www.ncbi.nlm.nih.gov/pubmed/23086238

 You answer gave me a new insight on the whole thing. Thank you Andreas!

I undrestood  that the answer to why/how/where migration happens is directly related to the developmental procedure of each cell. Cells with unique embryonic source can have multiple determined destinations. Maybe that is why malignant cells can't colonize in every tissue and they actually choose their next destination?

Hanie,

Patrick, Andreas, and Vishnu have given excellent and informative responses.  Cancer cells, which are genetically very unstable, acquire more and more mutational events over time, allowing them to transition from benign to malignant, non-metastatic to metastatic, noninvasive to invasive, etc.  Thus, certain mutations (perhaps few) actually allow a subpopulation of clonal cells to acquire a more aggressive transformed and metastatic phenotype.  Upregulation of hepatocyte growth factor receptor and its activity is sufficient not only to drive mitosis, but also EMT, motility and metastasis.  Genetically stable normal cells from the same tissue , while possessing the same genetic information and intrinsic ability to carry out these cellular processes, suppress them.  As Patrick mentioned above, secondary microenvironmental cues, brought on by the sheer growth of the tumor (such as nutrient and oxygen deficiency), can also play a major role in promoting these cellular programs, as well as angiogenesis.  In fact, in certain cancers, these cues appear to play a significant role inducing transdifferentiation of the tumor cells themselves from an epithelial to an endothelial phenotype, permitting them to incorporate into and function in the tumor vasculature.  Again, normal cells of the tissue are mostly spared since these microenvironmental cues occur mostly in the interior part of the tumor.  As already mentioned, CAFs can play a major role as well by producing growth factors and cytokines, which can act directly on the tumor cells and induce motility.  Hence, acquisition of a specific set of mutations gives rise to a tumor cell clone which then expands to a subpopulation having increased survival capacity and that possesses exquisite "cellular plasticity" capable of EMT, invasion, circulation,  evasion of immune cells, MET, and establishment of a metastatic colony.

Hope this was helpful.

Collective Invasion (paper attchd)

Here attached are some interesting articles.

Best regards

Robert