Metastasis is responsible for 90 percent of cancer deaths. Rapidly growing tumor cells are tightly packed. They quickly exhaust their available supplies of oxygen and nutrients. By breaking away from the original tumor, migrating cancer cells have a chance to escape starvation and wind up in a less crowded environment with more nutrients.
Researchers have shown that inhibiting autophagy, a self-devouring process used by cells to degrade large intra-cellular cargo, effectively blocks tumor cell migration and breast cancer metastasis in tumor models. In a study, published in the journal Cell Reports, they demonstrate that the process is essential for tumor metastasis and describe the mechanisms that connect autophagy to cell migration.
Researchers noticed that when they placed metastatic breast cancer cells on a dish and monitored them with time-lapse microscopy, the control cells were "active, constantly moving around the dish," author said. But cancer cells that the team had altered, by knocking down autophagy-related genes Atg5 and Atg7, "didn't move at all. They appeared to be stuck."
When they injected these gene-altered cancer cells into the mammary fat pad of female mice, the cells multiplied, forming large primary breast tumors, but these cancer cells were unable to metastasize to the usual distant sites, the lungs, liver or bone. A closer look showed that these cells were morphologically very different. Their focal adhesions, large structures at the edge of the cell that are crucial for cell movement, were more numerous and abnormally large.
As the cell travels forward, focal adhesions form at the front of the cell and establish dynamic connections to the extracellular matrix. As the cell passes over them, these adhesions drift back to the trailing edge of the cell. Then autophagy intervenes, disassembling the focal adhesion, breaking down its contents and allowing the back edge of the cell to disengage from the extracellular matrix and be pulled forward by traction from the front end.
Researchers show that if autophagy is inhibited, these metastatic tumor cells cannot move. Adhesions that don't get turned over grow larger and larger. They anchor the cell in place.
"They literally just get stuck," author said. "Through the microscope, you can see the cell wobbling, trying to move, to put out new protrusions, to migrate. But it can't, because it is stuck, unable to dissolve the adhesions at the back end of the cell. Basically, autophagy-deficient tumor cells cannot migrate and as a result cannot travel to another location. This is why we think that inhibiting autophagy could block tumor metastasis."
A closer look at the process revealed the biochemical connection between autophagy and focal adhesions. A protein called paxillin, found in focal adhesions, is used to link the internal components of cellular protrusions to cell migration. When the cell needs to disassemble a focal adhesion, the autophagy process uses LC3, a key protein in the autophagy system, to engulf paxillin and transport it to a lysosome, where it is degraded.
"The interaction between LC3 and paxillin is regulated by SRC," author said. SRC, which promotes cell migration and metastasis, was the first defined oncogene. Authors showed that SRC's ability to promote metastasis depends on autophagy.
"If you inhibit autophagy, SRC can no longer drive migration of metastatic tumor cells," author said. "This is a major finding."
There are approved drugs, some now being evaluated in clinical trials that can disrupt autophagy. One of them, hydroxychloroquine, is FDA approved to prevent and treat malaria and is currently being tested in clinical trials as a way to slow tumor growth.