Glucocorticoid hormones (GCs) are widely used in the therapy of lymphoma and acute lymphoblastic leukemia (ALL) owing to their apoptogenic effect on these cancerous cells. A major impediment of GC-based therapy is the gradual acquisition of apoptotic resistance to GC treatment. Therefore, a desirable goal is to decipher the mechanism of GC-sensitivity in order to develop therapeutic strategies that confer GC-sensitivity on otherwise GC-resistant lymphoma cells. Upon treatment, GC binds the glucocorticoid receptor (GR) at the cytosol, undergoes dimerization and then translocates to the nucleus, where it activates and represses multiple genes, most of which are unrelated to apoptosis. Indeed, GR alters gene expression in both GC-sensitive and GC-resistant cells. Therefore, we carried out Deep Sequencing analysis of small RNA and found that in addition to gene alteration, GCs regulate miRNA expression. Surprisingly, miRs alteration occurs only in GC-sensitive but not in GC-resistant cells, even though they both express GR. In particular, GC-sensitive cells treated with GC upregulate miR-103. Transfection experiments indicated that miR-103 sensitizes GC-resistant to GC-induced apoptosis. In a search for miR-103 target genes, we found that GC treatment of miR-103 overexpressing cells caused upregulation of the pro-apoptotic protein Bim and downregulation of the oncogene c-Myc. This pattern resembles the profile of GC-responding cells. Downregulation of Bim confered GC-resistance on miR-103 overexpressing cells. In addition, GC downregulates miR-17-92a multi-cystrone. miR-17-92a is upregulated in many cancerous cells. We demonstrated that overexpression of miR-20a (a member of miR-17-92a) downregulates apoptosis of GC-sensitive cells. Furthermore, it reverses the miR-103 effect on GC-induced apoptosis. Indeed, Bim is slightly downregulated by overexpression of miR-20a. c-Myc is a central transcription factor of miR-17-92a and it was demonstrated that this multi-cystrone is able to elevate Bim. Altogether, overexpression of miR-103 in GC-resistant cells downregulates c-Myc which is followed by miR-17-92a downregulation and Bim upregulation. Altogether, understanding the factors that mediate/interfere with GC-induced apoptosis is crucial for the success of GC based therapy and for the development new therapeutic approaches. Our data show that it is possible to overcome GC-resistance by developing drugs that specifically target factors interfering with GC-induced apoptosis.
