An international research team reports that defects in mitochondria, play a key role in the transition from normal cells to cancerous ones. When the scientists disrupted a key component of mitochondria, otherwise normal cells took on characteristics of cancerous tumor cells.
Their study (“Disruption of cytochrome c oxidase function induces the Warburg effect and metabolic reprogramming”) is published Oncogene and was led by members of the lab of Narayan G. Avadhani, Ph.D., the Harriet Ellison Woodward Professor of Biochemistry in the department of biomedical sciences in the school of veterinary medicine at the University of Pennsylvania. Satish Srinivasan, Ph.D., a research investigator in Dr. Avadhani's lab, was the lead author.
In 1924, German biologist Otto Heinrich Warburg observed that cancerous cells consumed glucose at a higher rate than normal cells and had defects in their grana, the organelles that are now known as mitochondria. He postulated that the mitochondrial defects led to problems in the process by which the cell produces energy, called oxidative phosphorylation, and that these defects contributed to the cells becoming cancerous.
“The first part of the Warburg hypothesis has held up solidly in that most proliferating tumors show high dependence on glucose as an energy source and they release large amounts of lactic acid,” said Dr. Avadhani. “But the second part, about the defective mitochondrial function causing cells to be tumorigenic, has been highly contentious.”
To see whether the second part of Warburg's postulation was correct, the researchers took cell lines from the skeleton, kidney, breast, and esophagus and used RNA molecules to silence the expression of select components of the mitochondrias' cytochrome oxidase C, or CcO, a critical enzyme involved in oxidative phosphorylation. CcO uses oxygen to make water and set up a transmembrane potential that is used to synthesize ATP, the molecule used for energy by the body's cells.
The biologists observed that disrupting only a single protein subunit of cytochrome oxidase C led to major changes in the mitochondria and in the cells themselves. “These cells showed all the characteristics of cancer cells,” noted Dr. Avadhani.
They displayed changes in their metabolism, becoming more reliant on glucose and reducing their synthesis of ATP. Instead of conducting oxidative phosphorylation, they largely switched over to conducting glycolysis, a less efficient means of making ATP that is common in cancer cells.
The cells lost contact inhibition and gained an increased ability to invade distant tissues, both hallmarks of cancer cells. When they were grown in a 3D medium, which closely mimics the natural environment in which tumors grow in the body, the cells with disrupted mitochondria formed large, long-lived colonies, akin to tumors.
The researchers also silenced cytochrome oxidase C subunits in an already-tumorigenic breast and esophageal cancer cell lines. “We found that the cells became even more invasive, heightening their malignant potency,” added Dr. Srinivasan.
Finally the team looked at actual tumors from human patients and found that the most oxygen-starved regions, which are common in tumors, contained defective versions of cytochrome oxidase.
“That result alone couldn't tell us whether that was the cause or effect of tumors, but our cell system clearly says that mitochondrial dysfunction is a driving force in tumorigenesis,” explained Dr. Avadhani.
The researchers observed that disrupting CcO triggered the mitochondria to activate a stress signal to the nucleus, akin to an SOS alerting the cell that something was wrong. Dr. Avadhani and his colleagues had previously seen a similar pathway activated in cells with depleted mitochondrial DNA, which is also linked to cancer.
Building on these findings, Dr. Avadhani and members of his lab will examine whether inhibiting components of this mitochondrial stress signaling pathway might be a strategy for preventing cancer progression.
“We are targeting the signaling pathway, developing a lot of small molecules and antibodies,” said Dr. Avadhani. “Hopefully if you block the signaling the cells will not go into the so called oncogenic mode and instead would simply die.”
In addition, they noted that looking for defects in cytochrome oxidase C could be a biomarker for cancer screening.