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By doing these experiments, they were able to go beyond simply asking which genes are upregulated and which are downregulated in the tumor cell. Instead, they are determining which genes are regulated as a direct result of FAK expression.

Furthermore, Schlaepfer and his colleagues established in vivo models in which they can effectively take away the ability of FAK to invade tissues. They used an inhibitor of FAK activity to selectively disrupt the invasion component alone. The inhibitor is actually just a fragment of the FAK gene itself that competes with endogenous FAK for binding to integrins.

“We’re throwing a wrench into the FAK signaling system to answer the question, if we stop its function, what happens?” says Schlaepfer.

Interestingly, they found that stopping FAK takes away, from tumor cells, the ability to metastasize but does not affect their motility. This enabled them to dissociate the role of FAK in motility versus its role in invasion. It also led to an interesting direction for the research.

FAK in Motility and Invasion

FAK has a role to play in motility and invasion because it is present in the projections that cells form when they are invading new tissue. In the parlance of cell biologists, these feet are referred to as “invadopodia” or “pseudopodia” Podia, in Latin, means feet.

Pseudopodia are foot-like extensions that cells use for probing an area and crawling. And within these pseudopodia, FAK is highly expressed. Staining cells growing in culture for phosphotyrosine, a sure sign of FAK activity, will show hotspots at the ends of actin filaments, where the FAK signaling is taking place.

“During invasion, these same feet squeeze between cells,” says Mitra. “We’ve seen FAK specifically enriched [in invading cell extensions].”

Another important cancer enzyme that is often overexpressed in cancer cells and is localized to pseudopodia are enzymes known as matrix metalloproteinases (MMPs).

MMPs are secreted enzymes that play a number of important biological roles in both the early development of organ structures and in tissue remodeling. Their physiological function is to remodel the extracellular matrix, and because of the potential damage that this could do to tissues, MMPs are one of the most highly regulated enzymes in the body.

“If they weren’t regulated,” says Schlaepfer, “our bodies would dissolve, basically.”

Unfortunately, this sophisticated regulation does not prevent cancer cells from subverting MMPs for their own purposes—cancer cells secrete these enzymes in order to break free of the extracellular matrix and tissue stroma, allowing them to move. It also allows them to dissolve barriers that come in their way to the bloodstream or to distant tissues during metastasis.

Significantly, when FAK is upregulated in a tumor cell, that cell will correspondingly upregulate MMP expression and activity as well. This leads to the tantalizing possibility that FAK is one of the signaling proteins that cancer cells use to activate MMPs and achieve metastasis. Schlaepfer and colleagues are testing the connections between FAK, MMPs, and metastasis.

“If we can figure out how FAK is functioning, and if we can get a good inhibitor then we might be able to stop cells from metastasizing,” says Schlaepfer. “These drugs might contain a tumor, preventing it from spreading if it is found early enough.”

In addition to the regulation of MMPs, Schlaepfer is also looking at the effect of FAK inhibition on certain other genes within the cells. Looking at these “peripheral” markers that are up- or down-regulated by FAK expression, might be the easiest way to gauge the effectiveness of a future FAK inhibitor in vivo and could be a useful application for testing whether any given FAK inhibitor works in a clinical setting.



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This FAK-null tumor cell has been reconstituted with FAK and stained to show where the protein localizes. The staining shows FAK concentrated in areas of focal contact (top arrowheads) and of pseudopodia formation (lower arrow).