Slums in big cities and tumor invasion

In this assay we compare two phenomena: tumorigenesis and the development of slums in big cities, and propose that not only the rules that control their existence are similar but also that the strategies in order to eradicate them are equivalent and that the lessons learned from one problem can be used in the other.

Slums are a grave problem in big cities in underdeveloped and in development countries. In 2007 in São Paulo, the biggest city in Brazil, there were approximately 2,000 slums with a total population of more than 400,000 families living in sub-human conditions. Besides the social problem of this population deprived of minimum sanitary conditions, slums are also safe haven for organized crime and drug dealers and gradually grow by engulfing neighborhoods of the city whose real-state is downgraded by the proximity with them.

Tumors are believed to be created by the relentless replication of genetically unstable cells that, through mutations and selection from microenvironment, acquire a set of phenotypes that allow them to invade healthy tissue, promote angiogenesis and colonize new regions of the host and create new tumors [1, 2], eventually reaching a state of tumor burden that is fatal to the host.

Both phenomena, slums and tumors, often develop in the periphery of the host (carcinomas develop from epithelial tissue separated by host by basement membrane while slums have their origin in the outskirts of towns where real state is less expensive) where resources are limited and uncontrolled growth lead to gradients of resources and harsh conditions.

Both systems invade by “trashing” their surroundings: tumors invade healthy tissue by both degradation of extracellular matrix and by causing death of healthy cells; it is known that tumors constitutively metabolize glucose anaerobically producing lactic acid [3, 4] even in presence of oxygen. It was proposed that this glycolytic phenotype would be a mechanism through which tumors would intoxicate their surroundings in order to kill healthy tissue and make room for new tumor cells [4]. A similar mechanism is found in the periphery of growing slums: a wave of devaluation of real state moves outwards of the slum propagated by criminality which imposes a “bad reputation” to the neighborhood, scaring the dwellers away and leaving room for new residents from the slum periphery or from outside of the system.

Solid tumors are often avascular during the early steps of tumorigenesis and are only able to promote angiogenesis as they achieve a critical mass. The fragile infrastructure of slums is no different from solid tumors: as one progresses into the settlement, the roads become narrower until cars cannot traffic, what considerably reduces efficiency of law enforcement. This inability of law enforcement and a minimum infrastructure for the survival of the slums is similar to what happens in solid tumors. In one side poor perfusion prevents a faster growth of tumor but on the other hand it protects the tumor by preventing the action of the immune system, chemotherapy and radiotherapy by limiting diffusion of drug, inducing quiescence in hypoxic tumor cells and by generating a heterogeneous microenvironment that confers robustness to attack [5].

We have discussed some aspects in how carcinomas and slums develop in a similar manner, notably by uncontrolled population growth in an area in the edge of the host/city with poor infrastructure but also with small or no interference from immune system/law enforcement, as is the case with carcinomas which are separate from immune system by a basement membrane.

Both systems appear to be robust to brute force attacks (toxins and antibiotics in cancer, and law enforcement and eviction in slums) not only because these approaches cause higher side effects in the “host” than in the target but also because the forces that promoted the initial development of these systems remain unchanged (genetic instability and microenvironment-imposed selection for cancer, and social inequality in slums) and thus will promote regrowth of the original system or other similar ones in other areas.

We propose that the most promising strategies for eradicating and preventing carcinomas and slums are those that target the forces that promote their emergence. For carcinomas these strategies would focus on intratumoral pH normalization, use of glucose competitors, use minimum amounts of therapy necessary to arrest tumor growth and delay patient relapse, and finally assess tumor response to therapy in a closed-loop approach. For slums, whose emergence is due to a considerable mass of poor people, the most promising approach would be to invest resources into bringing this share of the society into more equal conditions, which can be achieved by full-time public education with meals and recreational activities in order to keep the children away from one environment permeated by violence, drugs and poverty. Work laws that ensure minimum wages and social programs to provide credit to families to finance homes are also more immediate actions. Finally, the problem of slums in big cities will never be solved if the flow of migrants from poorer underdeveloped regions of the country remains. It is important thus that such an action for reduction of social disparities happens country-wide.

As a final note, we would like to stress that even though slums carry within criminality and major social and public health problems, they only exist and grow because of the initial advantage of cheap labor they offer to the richer population of the cities. An interesting point is that in carcinomas the cells that develop as tumors are exactly those that are isolated in the periphery of the host and considered as “expendable”.

1. Goldie JH: Drug resistance in cancer: a perspective. Cancer Metastasis Rev 2001, 20:63-68.
2. Hanahan D, Weinberg RA: The hallmarks of cancer. Cell 2000, 100:57-70.
3. Gatenby RA, Gillies RJ: A microenvironmental model of carcinogenesis. Nat Rev Cancer 2008, 8:56-61.
4. Gatenby RA, Gawlinski ET, Gmitro AF, Kaylor B, Gillies RJ: Acid-mediated tumor invasion: a multidisciplinary study. Cancer Res 2006, 66:5216-5223.
5. Kitano H: Cancer robustness: tumour tactics. Nature 2003, 426:125.

Mets are Vets?

The other day I was talking to one of my advisers and he mentioned that a reviewer had just critiques the fact that the Acid Mediated Tumor Invasion does not consider the fact that the aggressive cells selected by tumor microenvironment (hypoxic and acidic) should lose this phenotype once they progress through healthy tumors, since the selective pressure on these cells decreases as they invade further from original tumor.
A metaphor used by that time was that tumor cells are like Navy Seals who, once far from their training environment, relaxing in a paradisaical beach would eventually get soft like other regular people.
The point here, I believe, is that the metaphor is wrong for tumor cells are not trained to be tough, they are selected, they are traumatized like 17-year old boys who are sent to war. Those who are able to adapt, to toughen up, survive and come back home, but many of them cannot adapt back to normal life.
The perfect example is John Rambo. When Rambo, and many other Vietnam war vets, returned to US, he no longer could live in society as we do. He would see threats everywhere, he would trust no one and would not accept the authority of civilians. In the movie Rambo escapes from prison and hides in the woods nearby the city. When the police and National Guard try and capture him he turns back to the Green Beret M.O. and kills them all.
At the end of the movie he destroy much of the city including the Police Station.
I believe that the cells that survived the stress created by tumor development and reached a more healthy environment to live will, in their majority, settle down. The cells that start new metastases however are like John Rambo: they cannot re-adapt to normal environment so they tend to re-create the original environment that traumatized them.
If this is true, there may be ways of acquiring an aggressive phenotype through reversible ways (most of tumor cells) but some of these ways may yield to irreversible conversion and eventually lead to cells that can generate metastases.
A microarray analysis as well as protein expression of cells recovered from metastases compared to the original tumor cells might yield to clues on how this commitment happens.

Substrate-induced starvation in cells?

I was just reading an article sent to me by my PhD adviser about evolution of metabolic networks.
It is impressive how metabolism is structured in a complex network with apparent redundancies, inefficiencies and bottlenecks.
But this post is about something extremely interesting I read in one of the references: apparently some of these metabolic pathways are "boosted up" by an energy-dependent step. These are called "Turbo-designed pathways" and glycolysis is one of them.
In glycolysis, before glucose catabolism can yield 4 molecules of ATP, first it consumes 2 molecules.
When yeast is grown in culture with high glucose concentration, negative feedback mechanisms prevent the cell from wasting too much energy in accumulating substrates intracellularly and at the cost of a lot of energy. Yeast just uptakes enough glucose, phosphorilates it (2 moles of ATP consumed) and then metabolizes it to get enough energy.
However, when yeast is grown in media with high malate (analogous to glucose) concentration, these cells will deplete all their energy uptaking this substrate and before they are able to make out the ATP surplus, they are already dead.
It is like someone spending all their money in seeds and not saving anything for sowing the crops.

The funny thing is that tumor cells are known for increased glucose metabolism, not only increased glucose consumption but anaerobic glucose metabolism, which is much less efficient. So wouldn't these cells just starve to death? I guess they would, if the increase in the metabolic flux where in the ATP consuming steps, what would lead to accumulation of metabolites and energy deprivation. But if the increase is in the ATP generating steps (after glyceraldehyde-3-phosphate dehydrogenase) then it would yield to energy surplus.

This conclusion is very interesting because it suggests that any increase in expression of glycolytic enzymes pre-GAPD should come only after the ATP-surplus enzymes have already been mutated (or its expression increased) to increase its reactions fluxes.

For references:

The danger of metabolic pathways with turbo design. Teusink et al., 1998
Glycolysis, turbo design and the endocrine pancreatic β cell. Iynedjian, 1998

Humankind and cancer

One Day I was talking to my postdoc mentor about how a therapy for total cancer eradication should look like. We were not arguing any specific strategy bust just were trying to imagine how a cure for cancer would look like and how we would describe it if we found a Djinn in a desert.
The dialog proceeded a bit like this:

Mentor-An average tumor should have something around 5-10 billion cells. This tumor, however, is often spreaded around the body in regions from which it cannot be extracted without causing patient death.
I-But can't we help the body fighting it? Cancer immunotherapy proposes to identify over-expressed proteins that can be targeted by immune system.
M-Do you know the success rate of these therapies?
I-5-10%?
M-You'd be surprised by what a placebo can do...
I-So how could we envisage a therapy? Any sort of cure? Maybe keeping cancer as a chronic disease, just slow down its growth?
M-Coming back to the tumor size, imagine humankind where a tumor on earth, there are around 6 billion people, spreaded all over the world. They eat differently, have different resistant to diseases, live in different weathers, humankind is pretty much as heterogeneous as tumors...and as pernicious to the planet as tumors are to human body.
I-So the problems are equivalent: how to eradicate human kind and save the planet is like eradicating a tumor?

Now how can we overcome such a challenge? The more we proliferate on Earth and the more we pollute, destroy natural ecosystems, the closer we get to our own extinction, but will life on the planet survive our extinction or shall we bring all life with us like a real cancer?

Why do tumor cells glycolyse?

Since Pasteur and Warburg it is known that tissue tends to increase glucose uptake in low oxygen environments and that tumors continue metabolizing glucose anaerobically even in presence of oxygen.
Warburg originally hypothesized that this effect not only was a hallmark of cancer but also that is was its main cause. Warburg believed that all tumor cells had malfunctioning mitochondria and thus were forced to rely on anaerobic energy metabolism.
Gatenby and Gillies recently proposed the acid mediated tumor invasion hypothesis, through which, not only glycolysis was necessary for tumor proliferation and invasion, but also that this phenotype is selected by the micronenvironment were epithelial tumors are formed.
This hypothesis proposes that the use of abundant glucose in serum anaerobically would provide enough energy for tumors to develop and also produce acicity that promotes degradation of extracellular matrix and induces apoptose in normal surrounding tissue.
One interesting point, however is that tumor do not entirely abandon aerobic metabolism. Oxygen is necessary for keeping the basic functioning of TCA cycle that not only produces energy but also is linked to production of fatty acids and and many other metabolic pathways in cells. It has been shown that tumor regions in anoxic conditions become too acid for even tumor cells to survive.
I propose that the reduction of O2 consumption in tumors is an adaptation at population level. The cells that are in tumor rim, closer to oxygen-rich tissue, consume less oxygen which diffuses through them towards the cells in the inner core of tumor, preventing excessive acidosis and allowing tumor growth.
This kind of behavior is not normally proposed in tumors, that are normally considered as a heterogenous disorganized mass, but maybe these systems of cooperation between different subpopulations in a heterogeneous environment are exactly what differentiate malignant tumors from the many neoplasias that are periodically destroyed in our bodies without our knowing.

Democracy and Cancer

For long time it has been known that molecular signaling in cells is done through scale-free networks, both at gene and protein levels.
Such networks possess an architecture that confers resistance to failures but fragility to attacks.
The Internet uses such architecture: every computer is connected to an "internet access provider" which is connected to other servers that are interconnected all over the world. The design of internet addresses is an example of this hierarchical architecture, where each node is directly connected to approximately the same number of nodes (or at least the same order of magnitude).
A failure in a random computer in the network is not likely to affect the network performance since the chances of this computer being in a node in the lowest levels of the network (a user computer) is a lot lower than being one of the servers. Also, the catastrophic consequences of a failure in one main node in the network, associated with the low number of computers in this level, allow spending more money in protecting these high-level nodes with backup systems.
This architecture also allows control of network flux (it is sufficient to control a few high level nodes in order to control most of network flow) but also raise the risk of attacks.
Many examples exist of hacker attacks directed to websites, corporative networks and even entire countries.
How does this links to cancer?
It is widely accepted that cancer is originated from genetic instability. Many other factors, such as population heterogeneity (also born from genetic instability) and environmental conditions (selective forces, inflammation, exposure of body to carcinogens, etc.) are also important and may harness the genetic variability and direct towards malignancy.
Genetic instability can be considered as failures in the regulatory networks of the cell. These failures are most probable to happen in nodes in the lower levels of the network, for these are the most numerous. The mutations can occur in three ways: the first would be either a silent mutation or a mutation that would cause little effect on cell state, the second being a mutation that would alter cell state in a way that it would activate cell programmed death mechanism.
The third and most dangerous way if a mutation that alters cell state but bypasses apoptosis. Such mutations, if accumulated, would allow cell to escape body's control system and start a tumor.

Another point I'd like to point out is that the scale-free architecture from cell networks are optimized for evolutionary purposes: when a node is connected to many different others, and each of these others is connected to many others, it is possible for this system to gradually evolve to selection through increase of weight of influence from one of the 2nd degree nodes but also show more dramatic evolution due to modification of weight of a 1st degree neighbor or of the node itself.

Lastly, this architecture is very similar to a democracy, where most of the people has one low value vote, while fewer have higher level votes (congressmen) and very few have rights to alter the state of entire groups (governors, president). The idea of democracy is not that the majority is smarter than individuals but rather that a great number of people are less likely to take decisions that will harm the entire group. Due to practicity, democracy had to be stratified to allow decisions to be taken more rapidly, and thus appeared the scale free network of decision making known today is most democratic nations.

Most of the efforts today in cancer treatment are focused on finding targets that can be treated by highly specific drugs. It is believed that if these targets, which are often mutated in cancer, are silenced, cancer cells would die or at least lose its advantage over normal cells.
This idea would be correct if the only mutated nodes were the ones selected by the therapy. Unfortunately cancer is not based on attack but rather to failure and thus the probability is that, if a high level node has been mutated, this event was preceded by many more other mutations in intermediate and lower levels nodes all over cell signaling network.

Instead of a corrupt governor or president, cancer is more likely to be the result of a coup d'êtat that gradually formed from the infiltration of enemies all over the hierarchy of cell network. If one leader is removed from the network, it is likely that many other still exist in a lower level and will eventually replace the lost comrade.

In such a situation, the strategies to be used would be similar to the ones that have had success in fighting corruption in poor countries: the development of education and the increase of level of life of citizens tends to reduce the chances that corrupt leaders will be elected. The same is true for cells: if the micro environment of tumors is changed to endanger the survival of cells, there will be an opportunity for these cells to suffer mutations and diverge towards cancer. Conversely, modification in micro environment may reverse this tendency by preventing new cells from turning into cancer cells and also by reducing the efficiency of the tumor phenotype which is adapted to adverse conditions.


Regulatory network of genes with known role in cancer.
From "A map of human cancer signaling."
Qinghua Cui, Yun Ma, Maria Jaramillo, Hamza Bari, Arif Awan, Song Yang, Simo Zhang, Lixue Liu, Meng Lu, Maureen O'Connor-McCourt, Enrico O Purisima & Edwin Wang doi:10.1038/msb4100200

A 3D Computer Model to Map Modifications in Cellular Metabolism into Different Tumor Phenotypes

In this work we propose an approach for studying tumor
development as an iterative process that searches for the
minimum set of modifications in cell metabolism that would lead
healthy tissue to develop an invasive tumor.

There are many models in the literature describing putative
steps necessary for a homogenous population of healthy
epithelial cells to generate an invasive tumor, however these
studies do not account for the cellular metabolic modifications
that drive and support cancer development and progression
(hyperplasia, high aerobic glycolysis and acid resistance).

We propose an integrated process for discovering the minimal
sets of modifications in cellular processes that lead tumors to
become invasive (growth of tumoral tissue, invasion of normal
epithelial cells and basement membrane).

The development of tumors such as Ductal Carcinoma In Situ
(DCIS) is a complex process where a genetically heterogeneous
population of tumoral cells is submitted to selective pressure
in a dynamic environment. Here, we represent DCIS as a 3D
computer model based on a tubular structure of 40 x 40 cells
dimension (diameter x length), composed of endothelial cells,
basement membrane and a layer of normal epithelial cells
(Figure 1).

One region of this layer of healthy cells is chosen to be
mutation prone, meaning that these cells have a higher than
normal mutation rate due to environmental changes such as
chronic inflammation or a mutation in DNA repair mechanisms.
This will lead to appearance of different phenotypes - a
process that will ultimately lead to evolution of a malignant
population.

Each individual cell was built with algorithms to perform
duplication or apoptosis, and three metabolic pathways:
Glycolysis, TCA cycle, and pentose phosphate cycle. These cells
were therefore programmed to duplicate, die, metabolize glucose
and oxygen and generate ATP and excrete lactate. In order to
account for dynamics in different time scales, the model is
simulated in three levels: the first one being the
reaction-diffusion of chemical species, the second is the
metabolism of each cell and the last level is the cell
duplication.

Within this model, one simulation is performed for each set of
phenotypic modifications in tumoral cells and the set of
modified cells that have successfully achieved invasiveness are
selected. For each of these sub-populations the search moves
one step further and details the reactions composing that
pathway in search for the minimum set of enzymes that might be
responsible the modifications in the phenotype previously
observed. This drill-down process proceeds until reaching the
genes responsible for the synthesis of the proteins that
catalyze or regulate these metabolic reactions. Finally, the
candidate genes found are compared to oncogenes known in
literature and assessed experimentally for validation of the
model.

This approach may improve understanding the intricate factors
behind the development of tumors as well as test for treatments
such as modification of acidity, oxygen and other substrates in
the tumor microenvironment.

On the left, a snapshot of the simulation of a tumor developing in an epithelial duct (3D view on top right) and a 2D view of a transversal slice of the model showing the PH, oxygen and glucose concentration gradients. In the 3D view, red dots are blood vessels, gray represent the basement membrane, pink are healthy epithelial cells and the others are tumoral cells with different phenotypes. On the right, histology of DCIS shows the expansion of tumor cells within the duct (from Gatenby and Gilles, 2004).

External Simmetry versus Internal Assimetry

Most mammals display a remarkable external simmetry (same number os limbs on both sides of body, same numbers of fingers on each hand/foot, eyes placed in symmetric positions, etc.) in spite of a considerable asimmetry of organs within the body.

First we can point some organs that are placed unevenly in the body such as the heart and organs whose size are different such as the two lungs. Other organs such as pancres, liver and appendix are placed in one side of the body and have a corresponding counterpart of completely different function on the respective position on the opposite position in the body.

One way of seeing the point is to state that the position of these organs does not matter so much as long as they work properly. This could be true if we found human beings with livers on either sides of the body and the same for pancreas as well as intestines winded clockwise and counterclockwise but the reality is that these organs's placements is quite conserved.

Why do we have two kidneys if one unique bigger kidney would do the same job? Many theorists state that Nature does not work with redundancy, meaning that Natural Selection would not creatures with two kidneys so they could survive a kidney failure: instead of that, natural selection would remove those that carry alleles that will lead to liver conditions. This said one could expect the second kidney to be either a mandatory step during development of a mammal or simply a 'bug' during the development program and this bug will be soon corrected.

A bug, however, shouldn´t have spreaded all over the mammalian kingdom meaning that the second kidney development must really have a mandatory presence.

Why do we grow up?

We are born from a single cell that duplicates itself many times until we reach a volume and mass so many times bigger than our original self that one could very well wonder if we keep any link to that tiny part of us.

One could argue that development is needed in order to achieve a level of specialization of tissues where our organs will work properly and the inumerous tasks to be performed by our organism will be fulfilled. However, even after our organs are fully developed and in their right place, we countinue to grow up.

Let´s arbitrarily say that a 10-year-old child is completely functional - its teeth are all there, lungs, heart and intestines are all working and so is its brain and mind- then why does it keep growing, getting denser (more cells per cubic centimeter) and reach a point where many of its main functions will begin to decay (ageing)?

One hypothesis could be that a human being must grow until an optimal point for its survival and then push the envelop a little further so its offspring will be favored. A bigger mother can certainly bear more children and during more time (longer pregnancy, more developed children) but on the other hand this oversize will certainly claim its share and the life expectancy of the mother will be shortened.

Anoter reason for being born smaller than our maximum size is because an adult cannot fit inside another one, it´s mandatory that a life be born more simple than it will be at is apex. This rule can only be bent in cases such as unicelular organisms but even in this case the 'mother' cell must double its volume so when it splits into two 'daughter cells' the two newborns will be approximately the same size as an adult cell.

Development is as necessary for pluricelular organisms as the simplification of mechanisms is for life itself. Life is only possible if we are capable of breaking its functions into smaller more simple ones that will be themselves broken into more simple ones and so on.

One interesting question would be if death by ageing (and ageing-related diseases) is somehow linked to this dynamics and also if there is a correlation between the number of final cells of the organisms, its development time before birth, and the life expectancy of the indiviual.