According to one very gross estimate, there are approximately 37.2 trillion cells in the human body. They all have different jobs and all need to work together in order for the organism to function properly. At the same time, they need to not get in each other’s way and look out for each other in an attempt to fend off disease. Imagine, if you will, the world’s most complex traffic junction. In order to make this type of junction work, you would need several complex traffic lights, all working together to promote the seamless movement of traffic. This is essentially what goes on in human cells on the molecular level. Of course, instead of red, yellow and green light bulbs there are chimerical signals which instruct cells on when to move, when to stop, when to reproduce and when to commit suicide (for more on organised cell death, see my previous post of apoptosis).
Cancer is essentially a disease of de-regulation where cells begin behaving out of control and stop respecting the molecular traffic lights that dictate every detail of their everyday live. For instance, cancer cells need to learn when to ignore the “stop” signals that tell them to stop replicating and, even worse, that tell them to commit suicide when it’s appropriate. Even more disturbingly, they need to learn how to move and replicate even when there is not “green signal” telling them to reproduce or move. There two hallmarks – learning to become independent for growth signals and from stop-growth signals, can both be clustered together as cancer cells gaining the ability to ignore traffic lights.
Much like a real motorist that ignores traffic lights, the eventual fate of an organism where cells do not respect road signals is doomed. Cancer cells replicating uncontrollably form cancerous masses, which in turn become unsustainable for the rest of the body. Cancer cells moving uncontrollably form secondary tumors, or metastases, which are in fact what ends up killing about 90% of cancer patients. The molecular traffic lights that regulate our cellular lives also usually act as fail-safes for cancerous cells. For example, cells moving uncontrollably that move away from cancerous masses and ending up in a different location from the one where they originated would end up dying due to the lack of growth factors. Each cell in the body is designed to recognize the chemical components of its “home” environment, which act as a signal for the cell to remain alive and even to proliferate. When away from its own environment, the cell lacks the “green light” required for continued survival and proliferation and therefore ends up committing cellular suicide.
However, cancer cells moving away from the main tumor mass develop the ability to survive and even grow in the absence of their “home” signals. This most often happens by cells starting to recognize other signals as “home” factors. In other words, these cells begin to read any signal as a “green light” and therefore are able to thrive in inappropriate locations. This is how breast cancer cells end up forming secondary tumors in the brain or in the bone and how pancreatic cancer cells can form new tumors in the lungs.
A big part of cancer research these days is to figure out how cancer cells go around different aspects of the molecular traffic lights that regulate them. Genes that are involved in these mechanisms are excellent targets for new drugs, which have the potential to rewire the circuitry of the cell and force it to follow the rules.
This post belongs to the series Understanding Cancer. Please go check out the other posts in the series here.