microRNA-222-Mediated VHL Downregulation Facilitates Retinoblastoma Chemoresistance by Increasing HIF1α Expression

PURPOSE. Retinoblastoma (RB) is the most common primary intraocular tumor in children. Chemoresistance is the major obstacle for treatment of these tumors. This study aims to determine whether or not downregulating microRNA-222 (miR-222) could serve as a potential therapeutic target for preventing chemoresistance in RB treatment. METHODS. Differentially expressed miR-222 in RB samples and its downstream target genes were predicted using bioinformatics methods. The expression of miR-222 was altered by mimic or inhibitor to examine its role in RB cell in response to the chemotherapeutic agent vincristine (VCR). Further bioinformatic analysis predicted involvement of the stability of hypoxia-inducible factor 1 alpha (HIF1 alpha) protein in regulation of the von Hippel-Lindau (VHL) tumor suppressor, followed by characterization of the effect of VHL on the ubiquitin-proteasome degradation of HIF1 alpha. Next, VHL or HIF1 alpha was overexpressed to determine their effects on RB cell activities after VCR treatment. In vivo assays were performed on nude mice to further verify the in vitro results. RESULTS. miR-222 is highly expressed in RB tissues and cells and was found to facilitate resistance of RB cells to VCR. Of note, miR-222 specifically bound to and negatively regulated VHL. VHL could inhibit the stability of HIF1 alpha and promote the degradation of ubiquitin-proteasome, thus reducing HIF1 alpha expression to attenuate VCR resistance in RB cells. Moreover, inhibition of miR-222 in combination with VCR suppressed tumor formation in nude mice. CONCLUSIONS. miR-222 promotes the expression of HIF1 alpha by targeting VHL, thus accelerating the resistance of RB cells to the chemotherapeutic agent VCR.