New research shows that a form of vitamin A used to treat acute promyelocytic leukemia induces changes in an unusual class of small molecules called microRNAs (miRNAs) in the leukemic cells.
The study also shows that three of these miRNAs inhibit the action of two genes important for cancer development, helping to explain how the drug works.
The drug is called all-trans-retinoic acid (ATRA) and it is considered the gold standard for treating the disease.
The study showed that ATRA raises the levels of three particular miRNAs in leukemia cells and that this rise coincides with a fall in activity of two important cancer-causing genes. The three are identified as miRNA-15b, miRNA-16-1 and let-7.
Two of these, miRNA-15b and miRNA-16-1, reduce the activity of the Bcl-2 gene, which is over-active in many kinds of cancer. The protein produced by this gene blocks the normal process of cell death and helps keep cancer cells alive long after they should have died.
The remaining miRNA molecule, let-7, lowered the activity of the Ras oncogene, an important cancer-causing gene. (Oncogenes are normal genes that when mutated lead to cancer.)
Researchers at the Ohio State University Comprehensive Cancer Center led the study, which was published in a recent issue of the journal Oncogene.
“The findings are important because they tell us that some miRNAs switch off genes that promote cancer,” says first author Ramiro Garzon, a hematologist and oncologist at Ohio State's James Cancer Hospital and Solove Research Institute.
Acute promyelocytic leukemia occurs when cells that give rise to a form of white blood cell become stuck at an immature stage. The immature cells accumulate until they crowd out healthy white cells in the blood and bone marrow.
“Our findings suggest that these three miRNAs help re-program the malignant cells to a more normal state,” Garzon says, “and that they are also important for normal differentiation.”
In this study, the researchers used leukemia cells grown in the laboratory and cells donated by patients to study how the drug ATRA affects miRNA levels and how those changes affect the cells.
The investigators exposed the leukemia cells to the drug for up to 96 hours, causing the cells to mature. The treatment increased the level of eight miRNAs and a drop in one compared with untreated cells.
Of these, the researchers focused on miRNA-15b and miRNA-16-1, which are known to regulate the activity of the Bcl-2 gene. They found that high levels of the two miRNAs were associated with low Bcl-2 activity.
Next, they showed that the two miRNAs actually caused the drop in Bcl-2 activity. They did this by adding additional amounts of the two miRNAs to leukemia cells not treated with ATRA. Restoring the miRNAs caused a strong drop in Bcl-2 levels.
The researchers then looked at miRNA let-7, a known regulator of the Ras oncogene, and likewise found that high levels of this miRNA were associated with low Ras activity.
They established a cause and effect relationship as before, by adding additional let-7 to untreated leukemia cells.
“Overall,” Garzon says, “our findings show that ATRA induces the expression of these three miRNAs, and through them regulates genes that need to be silenced for the cell to differentiate.”
Funding from the National Cancer Institute, the Paul and Mary Haas Chair in Genetics, a Lauri Strauss Discovery grant award, the Kimmel Foundation and the CLL Global Research Foundation supported this research.
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