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Breast cancer spread triggered by a cleaver-wielding protein on cancer cell's surface

Scientists from the University of Helsinki and from UCSF have exposed a cell pathway that breast tumor cells use to destruct local tissue neighborhood. Cancer cells may use this pathway to free themselves from mammary epithelial tissue architecture, to spread to surrounding tissues. The cell pathway, the researchers found, is a biochemical chain of events leading to activation of a protein-cleaving enzyme on the surface of the tumor cells.

Cancer rarely kills unless it evolves the ability to spread beyond the tissue in which it developed, to grow into surrounding healthy tissues. An important roadblock for tumor spread is membranous scaffolding, basement membrane, which lines epithelial cell layers in tissues. Normal epithelial cells and even early-stage tumor cells remain tightly tethered to basement membrane, which segregates healthy and likewise cancerous epithelial cells from surrounding tissues. Breakdown of this barrier allows tumor cells to escape from the tethers of the epithelium, launching a tumor invasion to healthy tissues.

Finnish scientists from the University of Helsinki, together with UCSF researchers, have identified a molecular pathway in breast tumor cells leading to activation of a protein-cleaving enzyme hepsin on the surface of breast tumor cells. Tumor cells use hepsin to chop basement membrane proteins - to break free from ties and matrix binding them to local neighborhood in their native epithelial tissue, the investigation suggests. The study will be published in the 16th January edition of the Proceedings of the National Academy of Sciences (PNAS).

"If we could delay or prevent a tumor from switching from one that grows in place to one that invades, then that would be a major milestone in cancer treatment," according to study co-author Zena Werb, PhD, a professor of anatomy at UCSF. Werb has for decades studied the ways in which the behavior of tumor cells is influenced by their surroundings, with a focus on breast tumors.

Working with genetically engineered mouse mammary glands and mammary epithelial fragments isolated from these glands (organoids), University of Helsinki scientist and Finnish Academy Research Fellow Juha Klefstrm, PhD, along with a University of Helsinki graduate student Johanna Partanen designed and led experiments that resulted in the discovery of a biochemical chain of events that is likely to be initiated by many breast tumor cells when they become invasive.

The research collaborators initially studied a tumor suppressor gene (a gene that prevents the growth of tumors) called Liver Kinase B1 (Lkb1). They found that shutdown of this gene disturbs development of parts of the mammary gland, including milk-secreting tissue structures. Especially, basement membrane, which normally surrounds tissue structures of the mammary gland was damaged and degraded.

A culprit for basement membrane damages was pinpointed: Lkb1 shutdown disconnected hepsin from normal regulation and the protein mistakenly started to blanket the surface of mammary epithelial cells, causing degradation of the basement membrane. Researchers found that inactivation of hepsin allowed the basement membrane to recover. These events may take place in many tumors, as the research found that Lkb1 is missing and hepsin is abnormally expressed in 1 out of 4 human breast cancer samples.

"These findings led us to ask an obvious question: does Hepsin mediated degradation of basement membrane make epithelial cells more cancer-prone?" says Johanna Partanen. She knocked out Lkb1 in the mouse mammary gland. Evidently, hepsin expression was abnormal and basement membrane shattered but even after a year, she did not observe any tumors forming in the glands .

"I was disappointed with the results. However, then I realized that even though broken basement membrane may give more freedom for cells to proliferate, the cells may just lay there, resting, and not start to over-proliferate unless they are pushed to cell division cycle."

Partanen re-engineered mice so that she combined cell cycle "driver" oncogene Myc with inactivated Lkb1 gene in the mammary glands and she soon noticed very fast growing mammary tumors in the mice. "We think that it is the combination of Myc empowered cell cycle and cell's ability to destroy basement membrane, which contributed to the vicious tumor formation" says Partanen.

Will these findings help us to fight cancer? - Maybe, says Juha Klefstrm: "Hepsin is of a type of protein known as a protease and proteases have been successfully targeted in drug development. We found that deactivation of hepsin in the mammary gland organoids prevents formation of a cancerous phenotype. This finding excites us as it leads us to think that inhibition of Hepsin by drug-like molecules could restrain breast cancer progression. However, we do not know yet if we can cure already formed tumors by blocking hepsin activity. We need to first improve our experimental systems to properly address this question."

According to Zena Werb, "In humans, breast cancers that have diminished amounts of Lkb1 show strong hepsin expression. Since hepsin sits on the cell membrane, it should be accessible to drugs. We believe that hepsin forms a novel target for treatment of a subset of breast cancer patients."

Contact: Dr. Juha Klefstrm
University of Helsinki

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Breast cancer spread triggered by a cleaver-wielding protein on cancer cell's surface
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