Finding effective therapeutic targets for cancer has never been an easy undertaking. Cancer cells can quickly become resistant to treatments through adaptation, making them notoriously tricky to defeat and highly lethal. Yet now, a team of investigators, led by scientists at Cold Spring Harbor Laboratory (CSHL) Cancer Center, have uncovered some of the underlying mechanisms that drive cancer drug resistance. While investigating the basis of “adaptive resistance” common to pancreatic cancer, the researchers discovered one of the backups to which these cells switch when confronted with cancer-killing drugs. Findings from the new study were published recently in PNAS through an article titled, “Oncogenic KRAS engages an RSK1/NF1 pathway to inhibit wild-type RAS signaling in pancreatic cancer.”
“For decades, KRAS interactors have been sought after as potential therapeutic targets in KRAS mutant cancers, especially pancreatic ductal adenocarcinoma (PDAC),” the authors explained. “Our proximity labeling screen with KRAS in PDAC cells highlights RSK1 as a notable mutant-specific interactor.
KRAS is a gene that drives cell division. Most pancreatic cancers have a mutation in the KRAS protein, causing uncontrolled growth. But drugs that shut off mutant KRAS do not stop the proliferation. The cancer cells find a way to bypass the blockage and keep on dividing.
“You take away your main engine, and you’re kind of on some backup engines. But it’s getting by on those,” noted lead study investigator Derek Cheng, a former medical scientist training program student at CSHL. “The ship isn’t sinking yet. It’s still moving at a slower pace. Ultimately what we want to do is sink the ship.”
Senior study investigator David Tuveson, PhD, CSHL Cancer Center director and his team wanted to figure out the “backup engines” in these cancer cells. They used a technique called biotin proximity labeling to identify what other proteins interacted with mutant KRAS.
“We used proximity labeling to identify protein interactors of active KRAS in PDAC cells. We expressed fusions of wild-type (WT) (BirA-KRAS4B), mutant (BirA-KRAS4BG12D), and non-transforming cytosolic double mutant (BirA-KRAS4BG12D/C185S) KRAS with the BirA biotin ligase in murine PDAC cells,” the authors wrote. “Mass spectrometry analysis revealed that RSK1 selectively interacts with membrane-bound KRASG12D, and we demonstrated that this interaction required NF1 and SPRED2.”
“I basically attach a spray can to my favorite protein, or rather least favorite protein, in this case,” Cheng added. “And so it attaches biotin, basically spraying biotin ‘paint’ to nearby proteins, and we’re able to analyze it to figure out what proteins were labeled.”
The scientists found “biotin paint” on a protein named RSK1, which is part of a complex that keeps a nearby group of proteins, called RAS proteins, dormant. The scientists were surprised to discover that when they inactivated mutant KRAS, the nearby RSK1 complex stopped working as well. This allowed the RAS proteins to activate and take over the work of the missing mutant KRAS.
“We find that membrane RSK1 mediates negative feedback on WT RAS signaling and impedes the proliferation of pancreatic cancer cells upon the ablation of mutant KRAS,” the authors concluded. “Our findings link NF1 to the membrane-localized functions of RSK1 and highlight a role for WT RAS signaling in promoting adaptive resistance to mutant KRAS-specific inhibitors in PDAC.”
Stopping pancreatic cancer cells may require drugs that can simultaneously target multiple molecules. Tuveson hopes to uncover more of the players contributing to adaptation in cancer cells to improve future treatments.