Glucocorticoid-regulated microRNAs and mirtrons in acute lymphoblastic leukemia

Glucocorticoids (GCs) induce apoptosis in lymphoid lineage cells and are therefore used in the therapy of acute lymphoblastic leukemia (ALL) and related malignancies. MicroRNAs (miRNAs) and the related mirtrons are similar to 22 nucleotide RNAs derived from polymerase-II transcripts and implicated in the control of essential biological functions, including apoptosis. Whether GCs regulate miRNA-encoding transcription units is unknown. We investigated miRNA/mirtron expression and GC regulation in 8 leukemia/lymphoma in vitro models and 13 ALL children undergoing systemic GC monotherapy using a combination of expression profiling techniques, real time reverse transcription (RT)-PCR and northern blotting to detect mature miRNAs and/or their precursors. We found that mature miRNA regulations can be inferred from expression data of their host genes. Although a simple miRNA-initiated canonical pathway to GC-induced apoptosis or cell cycle arrest did not emerge, we identified several miRNAs/mirtrons that were regulated by GC in patients and cell lines, including the myeloid-specific miR-223 and the apoptosis and cell cycle arrest-inducing miR15 similar to 16 clusters. In an in vitro model, overexpression of miR15b similar to 16 mimics increased and silencing by miR15b similar to 16 inhibitors decreased GC sensitivity. Thus, the observed complex changes in miRNA/mirtron expression during GC treatment might contribute to the anti-leukemic GC effects in a cell context-dependent manner.