Mutations in the cell adhesion molecule known as integrin alpha 7 (integrin ?7) lead to unchecked tumor cell proliferation and a significantly higher incidence in cancer spread, or metastasis, in several cancer cell lines, report researchers at the University of Pittsburgh School of Medicine in a study being published today in the Journal of the National Cancer Institute.
These findings suggest that integrin ?7 represents an important new target for cancer therapy and prevention.
Integrin ?7 belongs to a major class of cell membrane proteins that play a role in the attachment of a cell to the extracellular matrix (ECM), which is the material that holds cells within a particular type of tissue together. Integrins also help cells attach to one another and are involved in transmitting chemical signals between cells and the ECM.
In this study, the researchers, led by Jianhua Luo, M.D., Ph.D., associate professor in the division of molecular and cellular pathology, University of Pittsburgh School of Medicine, examined whether this gene is mutated in specimens of various human cancers as well as whether the level of integrin ?7 expression is associated with clinical relapse of human cancers. They also investigated whether integrin ?7 has tumor suppressor activity.
To determine whether mutations in integrin ?7 contribute to cancer, Dr. Luo and his collaborators sequenced the integrin ?7 genes from 66 human cancer specimens and cell lines representing a number of different kinds of cancer, including cancer of the prostate, liver, brain (glioblastoma) and muscle (leiomyosarcoma).
They found mutations in the integrin ?7 gene, particularly those that resulted in an abnormally shortened protein product, or truncation, in 16 of 28 prostate cancers. They also found truncation-inducing mutations in five of 24 liver cancer samples, five of six glioblastomas, and one of four leiomyosarcomas.
Integrin ?7 mutations also were associated with a significant increase in the recurrence of cancer among patients. Nine of 13 prostate cancer patients with integrin ?7 mutations experienced a recurrence of their cancer after radical prostatectomy versus only one of eight prostate cancer patients without such mutations. There were five recurrences among eight hepatocellular carcinoma patients with integrin ?7 mutations versus only one recurrence of cancer among 16 patients without such mutations.
To examine the effect of alterations in the level of integrin ?7 on tumor formation, the researchers assessed the ability of cancer cells to form colonies in a standard growth medium after increasing or decreasing the level of normal integrin ?7 in the cell lines. In this experiment, control cancer cells formed large colonies with up to 100 cells each. Cancer cells with normal levels of integrin ?7 expression formed fewer and smaller colonies. When the investigators decreased the level of integrin ?7 in two cancer cells lines using siRNAs, or silencing RNAs, both cell lines formed more colonies and grew better than corresponding control cell lines.
?When we increased levels of normal integrin ?7 in cancer cells, they grew at a much slower rate. This suggests that this protein is a fairly potent tumor suppressor,? said Dr. Luo.
Dr. Luo and his coworkers then investigated the role of integrin ?7 in metastasis by examining the relationship between the level of integrin ?7 expression and cell migration by increasing the expression of normal integrin ?7 in three cell lines. The migration rate was significantly reduced in all of the cells compared to those in which the expression of integrin ?7 remained deficient, suggesting that the level of normal integrin ?7 expression is inversely associated with tumor cell migration.
Finally, to investigate whether normal integrin ?7 possesses tumor suppressor activity, the researchers implanted human cancer cells into immune deficient mice. Some mice received tumor cells in which levels of integrin ?7 were increased, others received tumor cells in which the levels of normal integrin ?7 were decreased. Six weeks after mice were implanted with cancer cells in which levels of normal integrin ?7 were deficient, they had tumors with an average volume about four times as large as mice with implanted cancer cells in which normal integrin ?7 levels were increased. Similarly, the researchers found no visible metastasis in mice with tumors in which levels of normal integrin ?7 had been increased. On the other hand, they did find evidence of metastasis in three of 12 mice with one type of tumor deficient in normal integrin ?7 and in four of the 12 mice with another type of tumor deficient in normal integrin ?7. Furthermore, the six-week survival of mice bearing tumors with increased levels of normal integrin ?7 was higher than that of mice bearing tumors in which normal integrin ?7 had not been experimentally increased. Thus, increasing the level of normal integrin ?7 in tumors was associated with decreased tumor growth and metastasis in this animal model.
According to Dr. Luo and his coworkers, these findings suggest that not only is integrin ?7 an important tumor suppressor, but it is potentially a critical new target for cancer treatment.
?Our study shows rather definitively that when we experimentally decreased the level of integrin ?7 protein or the protein was naturally mutated in cells, those cells lost their inhibitory signals for both cell migration and proliferation. This suggests that the loss of integrin ?7 activity may lead to unchecked tumor cell proliferation and a significantly increased risk of tumor metastasis. More importantly, it suggests that if we can somehow restore normal integrin ?7 levels in tumor cells in vivo, we may be able to reduce the risk of them spreading to other sites, which would be a significant achievement in cancer therapy,? explained Dr. Luo.