Researchers identify a small number of 'cheating' cancer cells under hypoxia

Researchers at UConn Health, Yale, and Johns Hopkins have identified that some cancer cells can "cheat" through escaping constraints imposed by lack of oxygen, allowing the cancer cells to continue to grow.

This discovery was recently published in Cell Systems by Kshitiz, assistant professor in the Department of Biomedical Engineering, in collaboration with researchers Chi V. Dang at Johns Hopkins and Andre Levchenko at Yale.

Nearly a decade ago, the researchers observed a strange phenomenon while looking at cancer cells under hypoxia-;or a lack of oxygen.

As tumors grow and become large, they run out of oxygen and new blood vessels are created. This results in scarcity of oxygen, called hypoxia. Under hypoxia, cells are supposed to slow down their growth, but of course, cancers continue to grow larger. This presents a conundrum, yet unsolved."

Kshitiz, Assistant Professor, Department of Biomedical Engineering

The researchers determined that a small number of cells were "cheating"-;or rewiring their signaling to allow them to divide and grow. Solving the mystery of how the cells were cheating-;and how this phenomenon applied to cancer diagnoses-; soon became a focus of the researchers' work.

Under hypoxia, cells stabilize a protein called HIF-1, which is a master regulator of oxygen response in the cells. When oxygen goes down, HIF-1 signaling becomes high and takes the cells to a non-functioning state. HIF-1 directs the cell division machinery to stop working, jump starts anaerobic respiration using a large quantity of glucose, and makes cells secrete proteins to bring blood vessels towards themselves.

In the study, the researchers noted that a small percentage of cells did not stabilize HIF-1, but instead oscillated the protein-;moving it up and down. As HIF-1 oscillated, and went from up to down to up again, cells could escape the HIF-1-;imposed pause. In this way, these oscillating cells cheated and continue to divide, despite very low oxygen levels.

"To find cheaters within a population of cancer cells, which are themselves cheating the normal cells, is interesting at so many levels," Kshitiz says.

"We have observed oscillations in many systems, but oscillations in HIF-1 activity was not recorded before, and it is truly remarkable," Levchenko adds. "We are particularly interested in how oscillations like these can be recognized as a signal triggering specific genes."

Additionally, researchers found that the cancer cells communicate with each other, allowing cells to sense other cells' density. When HIF-1 is high because of hypoxia, cells produce energy without oxygen. A byproduct is lactate, the same molecule that gives us cramps during exercise if the muscles are not well oxygenated. Cancers accumulate a lot of lactate in their environment. Kshitiz worked with researcher Junaid Afzal at University of California San Francisco to work out the detailed mechanism that caused lactate to destabilize HIF-1.

"Excess lactate forces cells to undergo respiration, even when oxygen is scarce, and that caused degradation of HIF-1 in lysosomes, the recycling centers in a cell," says Afzal.

However, questions remain-;are these observations under a microscope meaningful in real cases of cancer? Current technologies do not offer an effective way to test these predictions in animal subjects-;let alone human subjects.

Kshitiz, along with Yasir Suhail, a postdoctoral student in Kshitiz's lab at UConn Health, used this newfound information and looked at the genetic makeup of different cancers that occur in humans.

"What we found was truly astounding," says Kshitiz. "Most genes behaved as expected, but there was a group of genes which behaved opposite to what is expected in hypoxia. It did not make much sense; why should genes which turn on in hypoxia, turn off when hypoxia is oscillating? Clearly something is at play."

To understand further, Suhail looked at these genes in all human cancers and found a universal phenomenon. The genes that were turned off by oscillations were turned off in most cancers-;showing that the oscillation in HIF-1 levels could possibly decrease tumor suppressor genes and contribute to cancer growth in most cancers.

Kshitiz says, "The most interesting aspect is the universality of the phenomenon in all cancers. It seems this effect is pan-cancer, and not just in any cancer."

The research-;unraveling this unique phenomenon-; answers several conundrums about cancer, while opening new lines of scientific inquiry.

"It is a large collaboration across many institutions, a testament to how deep scientific questions require the integration of many types of expertise to come together," says Kshitiz.