microRNA expression is altered by ionizing radiation: A novel potential therapeutic target

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Introduction: MicroRNAs (miRNA) are small, well-conserved, non-coding RNA species that have recently been found to regulate protein translation by interfering with mRNA. Their role in the cellular response to external stimuli, such as radiation exposure, is unknown. Free radicals and DNA damage mediate the damage caused by ionizing radiation. Thus, it seems logical that alterations in miRNA expression may occur following exposure to several anticancer agents including ionizing radiation, the topoisomerase inhibitor Etoposide, and hydrogen peroxide (H₂O₂).
 Methods: Normal human fibroblasts in culture were exposed to several doses of radiation, or treated with either Etoposide or H₂O₂, to induce DNA damage and free radical stress. Dose of each drug was chosen based on survival curves. Total RNA was extracted and miRNA expression was determined by microarray. Altered expression was confirmed using RT-PCR for individual miRNA species.
 Results: Expression of 17 miRNA species was altered by radiation, while 23 were altered in the H₂O₂ treated cells, and 45 in the Etoposide treated cells, resulting in either increased or decreased expression. This cellular response to radiation-induced stress was noted to overlap the profile of miRNA response to H2O2, Etoposide, or both. RT-PCR confirm alterations in miRNA expression with radiation dose. Cysteine abrogated the response of microRNA to radiation.
 Conclusion: For the first time, we have shown that radiation induces miRNA alterations consistent with those observed in response to free radical and genotoxic stress. The response of miRNA to cysteine, may indicate that free radical stress maybe responsible for altering microRNA expression after radiation. Since miRNA functions to downregulate the expression of many proteins, it is logical to suggest that miRNA may be a target that can be exploited to improve the treatment of cancer.