Putting on the Brakes: Blocking the Growth of Metastases
Most of the suffering and death caused by cancer is due, not to the primary tumor, but to the ability of cancer cells to spread throughout the body and to form metastases in other organs. Breast and prostate cancers often have periods of dormancy, which can extend up to 30 years, between the identification and treatment of a primary tumor and the growth of overt metastases. What induces or inhibits metastatic dormancy is unknown, but prolonging this period may improve the survival of patients with these types of cancer.
In prior studies Patricia Steeg, Ph.D., and her colleagues identified a human gene, Nm23-H1, that inhibits metastasis without affecting the growth of the primary tumor. Subsequent studies showed that the expression of Nm23-H1 was inversely associated with the lysophosphatidic acid receptor 1 (LPA1). Since increased expression of LPA1 could counteract the metastasis suppressing activity of Nm23-H1, Steeg and her collaborators in CCR’s Laboratory of Molecular Pharmacology decided to investigate whether inhibiting LPA1 with the drug Debio-0719 or a small hairpin RNA (shRNA) could prevent the growth of metastases in two mouse models. The results of these studies were recently published in the Journal of the National Cancer Institute.
In the first model, 4T1 mouse breast cancer cells, which form spontaneous metastases, were implanted into the tissue in which breast cells normally mature, known as the mammary fat pad, of female mice. Two days later the researchers began treating half the animals with Debio-0719 and half with a saline solution. Ten days after implanting the cells, the breast tumors were removed, and the researchers noted that the tumors from both the control-treated and LPA1 inhibitor-treated animals were the same size. Normally, these results would prevent further studies of anti-cancer drugs.
However, the investigators continued their study and, 60 days later, saw that more than half the mice that received the saline solution had enlarged lymph nodes, an early sign of tumor metastasis, compared to only 15 percent of the mice that received Debio-0719. In addition, pathologic examination of tissues from the animals showed significantly fewer metastases in the livers and lungs of the Debio-0719-treated mice, suggesting that LPA1 inhibition can block tumor metastasis in live animals.
The researchers wondered whether the primary tumor needed to be treated with Debio-0719 in order to prevent the growth of metastases. To test this idea, they implanted 4T1 cells and removed the primary tumors 15 days later. That same day they began treating half the animals with Debio-0719 and half with the saline solution. After 41 days of treatment, the number of metastases observed in the livers of mice receiving Debio-0719 was reduced by more than 50 percent and in the lungs by 75 percent. These results suggest that LPA1 inhibition may be beneficial even after a patient’s tumor is removed, a scenario that closely resembles the adjuvant clinical trial setting.
It is possible that Debio-0719 could be affecting other proteins in addition to LPA1. To show that metastasis suppression is due specifically to LPA1 inhibition, the researchers suppressed the expression of LPA1 using shRNA. Loss of LPA1 decreased the motility of 4T1 cells in culture and halved the formation of metastases in the lungs of mice injected with the LPA1 knock-down cells. Fewer liver metastases also formed in the mice that received the LPA1 knock-down cells rather than the wild type 4T1 cells, supporting the idea that LPA1 inhibition is responsible for the anti-metastasis activity of Debio-0719.
The scientists then used a second model system to confirm the importance of LPA1 activity in tumor metastasis. For these studies, they used human MDA-MB-231T cells, a model of the aggressive, triple-negative subtype of breast cancer, which lacks estrogen, progesterone, and HER2 receptors. Cells were injected into mouse fat pads to generate primary tumors or into tail veins to produce lung metastases. Animals were then treated with Debio-0719 or saline solution. Again, treatment with Debio-0719 had no effect on the growth of tumors in the mouse fat pads. In contrast, all of the mice treated with the salt solution succumbed to breathing difficulties in the lung metastasis model compared with only one animal in the Debio-0719-treated group. Pathologic examination of the animals’ lungs revealed more, larger metastases in the control group, supporting the results of the first model.
To understand how inhibiting LPA1 affects metastasis, the researchers examined the primary tumors and metastases from both mouse models. They found that Ki67, a marker of cell proliferation, was unchanged in primary tumors regardless of treatment but was selectively reduced in metastases treated with Debio-0719, suggesting that the drug induces metastatic dormancy. Additional studies confirmed this finding and showed that Debio-0719-mediated dormancy relies on activation of the p38 MAPK signaling pathway specifically in the metastatic sites. Debio-0719 stands as one of the first compounds to induce metastatic dormancy and holds promise for eventual testing in patients with cancer.Summary Posted: 10/2012
Effect of Inhibition of the Lysophosphatidic Acid Receptor 1 on Metastasis and Metastatic Dormancy in Breast Cancer. Marshall J-CA, Collins JW, Nakayama J, Horak CE, Liewehr DJ, Steinberg SM, Albaugh M, Vidal-Vanaclocha F, Palmieri D, Barbier M, Murone M, Steeg PS. Journal of the National Cancer Institute. September 5, 2012. PubMed Link