Proposed Role Of Stem Cells

Published Date: 02 Nov 2017

Introduction.

It has been suspected for many years now that stem cells play a major role within the development of colon cancer. It’s also known that the process takes many years, often over 30 years [1], the changes happen in steps, which in fact are all reversible up until an actual carcinoma forms, at which point it is very likely that progression will continue, specifically when mutations are continuing to happen without being checked by the body’s own protective cell cycle system.

Cancer stem cells (CSC’s) would seem to be the Holy Grail when it comes to either the progression of tumourigenesis or fighting against and elimination of these colonic tumours. Olszewski et al 2011, explains that due to their low proliferation rates and the over-expression of drug resistance proteins on the cell surface they are a very hard target to hit with traditional chemotherapies [2]. Add to this the stem cell niche that all stem cells inhabit, and which CSC’s seem able to form, makes it almost impossible for tradition therapies to hit these targets.

The stem cell niche is important in many ways as well as in normal circumstances with adult stem cells (ASC’s) performing a renewal system for the colon in which cells are turned over every 5 to 7 days, they are able to undergo symmetric division in which they can form further ASC’s or progenitor cells also sometimes called transit/amplifying cells, these can then further divide via asymmetric division to form differentiated cells which then mature as they move towards the apex of the crypt (Fig.1).

Figure 1. Showing areas within the crypt and which pathways are active at which stage.

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Picture from Gene expression patterns of human colon tops and basal crypts and BMP antagonists as intestinal stem cell niche factors.

In normal colon tissue it has been observed that there are generally 1-16 ASC’s at the base, this was supported by observation after radiotherapy where only a single ASC left alive within a crypt was able to regenerate the area damaged [11].

Mutation pathway.

Mutation starts early in colon cases taking many years to eventually turn into a malignant tumour. Changes usually start with a polypoid phase, with the loss or mutation of the Adenomatous polyposis coli (APC) gene, this is the trigger mechanism for further changes to occur. This can be followed shortly after by a mutation to the CDC4 gene, this is part of a ubiquitin ligase complex which targets Cyclin-E, C-Myc and C-Jun for destruction. Removal or mutation of this allows for progressive drive towards cell cycling.

A further mutation occurs in the KRAS (40% in colon cancers) and BRAF genes, RAS genes are an important target for mutation, it is common to find a Gain of Function (GoF) mutation within this area in cancers. The RAS genes encode for proteins that interact with p21 on the inner cell membrane, this transduces signals from the membrane to the nucleus of the cell and so RAS genes along with p21 have a vital role in normal cell growth. RAF genes help to control cell growth by regulating MAP kinase activity.

Next along the timeline is PTEN from chromosome 10, it is a potent tumour suppressor, it antagonises the phosphoinositol-3-kinase/AKT signalling pathway which in turn suppresses cell survival and proliferation. Knocking this gene out allows for longer lived cells allowing a further build up in mutations, perhaps it is this that drive the cell from polyposis into a carcinoma state.

At this stage the cancer has formed and so further mutations are picked up at a greater speed the major one being that of TP53.

TP53 tends towards a missense mutation rather than deletion or truncating mutation this leads to a protein that can produce a Gain of Function (GoF)[7,8,9], that is instead of just becoming a useless protein or being created and broken down by the cells enzymes it gains further uses in the oncogenic process [5][6]. GoF enabled proteins seem to drive tumourigenesis towards Migration, invasion and metastasis [10,5]. p53 mutations appear early on in the carcinogenic stages and reports indicate that mutant p53 exert a differentiation-blocking activity.

The Transforming Growth Factor (TGF)-Beta signalling mutation is one of the last stages to take place, it regulates transcription factors by controlling SMAD complexes within the cell, thus increasing growth, proliferation, and reduced apoptosis.

Stem cells and their proposed role.

Cancer stem cells are almost impossible for traditional chemotherapies to work against, as discussed they are extremely well protected by the stem cell niche which as shown can soon be repopulated by a single surviving CSC.

The ability for these cells to regulate the LGR-5 from positive to negative upon contact with a damaging agent allows these cells to survive chemotherapy and not long after switch back to a positive status and start to re-grow the surrounding tumour. LGR-5 is a Wnt target gene and has been indicated in colon cancer cell lines, they also form adenomas upon deletion of the APC gene. Toraro et al. puts forward a case for these cells being highly resistant due to the expression of various proteins such as antiapoptotic protein surviving, this increases between G2 and M phase of the cell cycle and as such cell survival comes down to which part of the cell cycle is it in at the time.

Wnt is an important protein concentrated at the base of the crypts, it attaches to Frizzled receptors triggering a cascade in the development of tumours and its expression is highest at the point in the crypt where tumourigenesis occurs most frequently. The Wnt pathway regulates cell proliferation via CmyC (transcription factor) and Cyclin D1 (Cell cycle regulator protein). It is the upregulation of Wnt that enhances cell proliferation, with a controlled Wnt pathway, Wnt is switched off and ubiquitin like modifiers attach to β-catenin it is then degraded by the APC, Axin, GSK-3β-complex before it can enter the nucleus and so cell proliferation is halted. In the case of Wnt not switching off then β-catenin is attached to Rac1 (Rho-GTPase family, membrane trafficking G-Protein) and then enters the nucleus for transcription along with NLK (Serine/threonine-protein kinase). This is a first step to transformation of SCs into CSCs.

Stroma has been seen to proliferate in tumourigenic processes and indeed in human polyposis this is the case, it would indicate that mesenchymal stem cells have the ability to speed up the growth of colon tumours and the spread of metastasis.

It seems that it is this area between the stem cell niche and the proliferative compartment that is the real problem when trying to target colon cancer tumours. The potential tumour cells that are between these stages can be converted either forwards further up, or backwards down the crypt so may move into a quiescent phase where they are not moderated by Wnt and so are radiation and chemo resistant, where the Wnt modulated areas are cycling and indeed sensitive to both these treatments. As said earlier the quiescent cells can then become Wnt modulated again and begin cycling. This is a normal process within the colon and the very process that repairs damage to the lining.

Conclusion.

As explained inactivation of p53 not only holds the cell away from apoptosis but can drive the cell towards further malignant change.

Are these stems cells mutated and genetically altered Stem cells or are they AP cells that have been dedifferenciated back to a stem like state.