IRIC - Institute for Research in Immunology and Cancer

Published on July 1, 2013

IRIC researchers explain why a class of antitumor molecules has only limited effects.

3D model of a RAF protein dimer in which one of the two proteins (in red) is bound by an anticancer compound (green) inhibiting its enzymatic activity A research team from IRIC, led by Dr. Marc Therrien and his colleagues at the Samuel Lunenfeld Research Institute at Mount Sinai Hospital in Toronto, recently elucidated the molecular mechanism limiting the action of inhibitors of an enzyme called RAF involved in many cancers. This important study just made the cover of the prestigious scientific journal Nature Chemical Biology.

The enzyme RAF controls proliferation and cell survival. Normally, RAF is activated only when two RAF proteins to join together to form a pair (or dimer) following reception of signals from outside the cell. However, in many tumors RAF dimers form spontaneously, independently of external signals, resulting in the uncontrolled proliferation and survival of cancer cells.
Because of its important role in tumor formation, pharmaceutical companies have invested considerable effort in the development of molecules that inhibit the enzymatic activity of RAF. Although some clinical success has been reported, it appears that as a whole, treatment with these inhibitors tends to stimulate the enzymatic activity of RAF in cancer cells rather than reduce it, a paradox that has puzzled an entire community of researchers and clinicians. Recent studies suggest that these inhibitors tend to stimulate the formation of dimers and that when the RAF dimers contain a protein unbound by the inhibitor, this one gets activated by its compound-bound inactive partner. This stimulates proliferation and survival of cancer cells, counteracting the desired effects of the drug.
Why do RAF inhibitors have this effect? To further investigate this phenomenon, the team of Dr. Therrien adapted a technology originally developed by Dr. Michel Bouvier of IRIC and engineered biosensors to directly measure the formation of RAF dimers in living cells. These tools allowed them to confirm that a number of RAF inhibitors induce dimer formation in a dose dependant manner. Together with the team of Dr. Frank Sicheri of Toronto, the team of Dr. Therrien also sought to determine the impact of these molecules on the three-dimensional structure of RAF. They found that RAF inhibitors block the protein in a “closed” conformation that looks just like an active form and that this conformation causes dimerization with another unbound RAF protein forcing it to adopt an active conformation.
Understanding this mechanism is a first step in the development of new anticancer drugs. Medicinal chemists may now go back to the drawing board to design new inhibitors capable of overcoming this complication.

Cited Study:
Lavoie H, Thevakumaran N, Gavory G, Li J, Padeganesh A, Guiral S, Duchaine J, Mao DYL, Bouvier M, Sicheri F, Therrien M. Inhibitors that stabilize a closed RAF kinase domain conformation induce dimerization Nature Chemical Biology