A team led by researchers from Stony Brook University on Long Island have isolated and targeted what appears to be an Achilles heel for pancreatic cancer in a protein called keratin 17 (K17), which plays a pivotal role in controlling tumor growth and aggression.
The discovery of this vulnerability not only identifies K17 as an effective negative biomarker for disease severity, but also points to a potential novel approach to treat some forms of pancreatic ductal adenocarcinoma (PDAC), the most common and lethal form of the disease.
"These studies provide the basis for the development of novel biomarker-based therapeutic approaches using small molecular inhibitors to target K17 active sites or the pathways altered by this protein in PDAC," said Cindy V. Leiton, PhD, a Research Assistant Professor from the Renaissance School of Medicine's Pathology Department at Stony Brook University.
As outlined by Leiton, who presented the study's findings at an AACR Virtual Special Conference on Pancreatic Cancer in September, the team has identified small molecule inhibitors and agents that disrupt nuclear trafficking of K17 in cell lines for PDAC.
"Our study is the first to explore therapeutic targeting of K17 in PDAC, and if successful, could provide a novel and more effective approach to treat these patients," added Kenneth Shroyer, MD, PhD, Chair of the Stony Brook Pathology Department and co-principal investigator of this project.
Patients with pancreatic cancer have the lowest 5-year survival rate of any major cancer at just 9 percent. And according to data from the Pancreatic Cancer Action Network, pancreatic cancer is projected to become the second-leading cause of cancer-related deaths in the United States by 2020.
"Despite advances in clinical management, PDAC, the most common form of pancreatic cancer, remains one of the most lethal of all cancers," said Shroyer. "This poor prognosis for PDAC patients is due a large extent to the fact that these tumors are intrinsically resistant a wide range of chemotherapeutic agents and immunotherapy."
This latest advance from the Stony Brook-led team builds on several years of study into the role played by K17-one of 54 different types of keratins in the body-in a cross section of tumors of the breast, cervix, stomach, gall bladder, skin, and head and neck.
Among other things, researchers at Stony Brook and elsewhere previously determined that the quantity of K17 present in these tumors can indicate how aggressive it is and help determine a patient's prognosis. Studies further suggested that K17 doesn't cause cancer itself, but promotes an inflammatory and immune response that can allow the disease to develop more aggressively.
Ongoing Pancreatic Research
About 5 years ago, Shroyer and colleague Luisa Escobar-Hoyos, PhD, MSc, revealed another way K17 works to promote the growth of cancer cells while inhibiting standard-of-care chemotherapy.
In a study published in the September 2015 issue of the journal Cancer Research, the team reported that K17 contains an amino acid sequence-likened to a chemical password-that permits the protein to enter and leave the inner sanctum of the cell's nucleus (2015; doi: 10.1158/0008-5472.CAN-15-0293). Once inside the nucleus, K17 binds to p27-a tumor suppressor protein that controls cell division-and then shuttles this cargo outside the nucleus where it is degraded. The progressive loss of this tumor suppressor is known to trigger the onset of malignancy.
"This work provided evidence that K17 drives tumor aggression and mediates chemoresistance," said Escobar-Hoyos, who is now Assistant Professor of Therapeutic Radiology at Yale University, but continues to co-direct this project with Shroyer.
In the August 2, 2019, issue of the journal Scientific Reports, the Shroyer/Escobar-Hoyos team revealed how the level of K17, determined through mRNA analysis and immunohistochemical staining of cells, could separate patients with a poor prognosis from others with better overall survival (2019; doi: https://doi.org/10.1038/s41598-019-47519-4). Those whose tumors expressed lower K17 benefited from standard-of-care chemotherapy, while those with high levels of K17 expression did not.
"Keratin 17 independently identifies a subset of patients with worst outcome within advanced stage and negative margin groups," the researchers wrote in the Scientific Reports study. "Lower K17 tumor expression may therefore provide a new indicator of the potential PDAC patient long-term survivors."
The results spurred new questions, Leiton said, including how does K17 target p27 and is this a druggable site? A second question: is this nuclear export blockade a druggable mechanism in K17-expressing PDACs?
To answer the first question, the researchers examined proteins known to bind to p27 and performed amino acid sequencing to identify similar binding domains in K17. A domain called MRAIL was found to directly dock to p27, with two such domains found in K17. In PDAC cell lines expressing either wild-type K17 or another form containing point mutations in MRAIL-like domains (MLD) essential for p27 binding, the team found that those with mutant MLD domains accumulated p27 in the nucleus. Their studies indicated that once this vulnerability was targeted, K17 lost its ability to shuttle p27 out of the nucleus to promote degradation.
In laboratory animals, the team also found that loss-of-function in the MRAIL domains resulted in an almost four-fold increase in extended survival, further suggesting that crippling these domains could improve disease outcome.
"We are now exploring the rational design of potent and effective inhibitors that block K17-MLD function as possible PDAC-targeted therapies," said Leiton.
To answer the second question, the researchers turned to studies with selinexor, an oral selective inhibitor of nuclear export (SINE) that binds to and inhibits a nuclear export protein called exportin-1 (XPO1), leading to the accumulation of tumor suppressor proteins in the cell nucleus. K17 impacts the nuclear export of about 50 percent of the proteome and depends on XPO1.
Selinexor, currently in Phase II clinical trials in PDAC, is now being tested to determine if K17-expressing PDACs are more sensitive to treatment with this drug based on its nuclear export functions.
"We found that PDAC cell lines expressing K17 demonstrated enhanced cytotoxic and cytostatic effects upon selinexor treatment, leading to a two-fold increase in cell death compared to control cells, suggesting that nuclear export by K17 in PDAC cells could be therapeutically targeted," the team wrote in an abstract prepared for the AACR meeting.
"These results suggest that K17 is highly dependent on nuclear export to promote tumor progression and that patients whose tumors express high K17 may be the best candidates for selinexor treatment," Leiton noted.
Warren Froelich is a contributing writer