Exosomal circ-HIPK3 Facilitates Tumor Progression and Temozolomide Resistance by Regulating miR-421/ <i>ZIC5</i> Axis in Glioma

Abstract

<b><i>Background:</i></b> The resistance of glioma patients to temozolomide (TMZ) treatment is a limiting factor in clinical treatment. Circular RNA HIPK3 (circ-HIPK3) was found to be highly expressed in glioma, however, the role and potential mechanism of exosomal circ-HIPK3 from TMZ-resistant cells remain poorly unclear. <b><i>Methods:</i></b> Exosomes were characterized by transmission electron microscopy. The levels of all protein were detected by Western blot. Expression levels of circ-HIPK3, microRNA-421 (miR-421), and zinc finger protein of the cerebellum 5 (<i>ZIC5</i>) were measured by quantitative real-time polymerase chain reaction. The cell's 50% inhibitory concentration (IC₅₀) of TMZ, apoptosis, and invasion were determined by methyl thiazolyl tetrazolium (MTT), flow cytometry, and Transwell assays, respectively. The correlation between miR-421 and circ-HIPK3 or <i>ZIC5</i> was identified by dual-reporter luciferase and RNA immunoprecipitation (RIP) assays. The xenograft model was established to explore the effect of circ-HIPK3 <i>in vivo</i>. <b><i>Results:</i></b> circ-HIPK3 was obviously increased in TMZ-resistant glioma cells and their exosomes, miR-421, was downregulated in TMZ-resistant glioma. circ-HIPK3 directly targeted miR-421 and their expressions were negatively correlated in glioma tissues. Besides, circ-HIPK3 knockdown hampered the IC₅₀ of TMZ, cell invasion, TMZ resistance, and triggered cell apoptosis, whereas these effects were reversed by transfection of anti-miR-421. <i>ZIC5</i> was the target of miR-421 and <i>ZIC5</i> overexpression weakened the inhibition effects of miR-421 on cell progression and TMZ resistance. More importantly, circ-HIPK3 depletion inhibited tumor growth by decreasing <i>ZIC5</i> through sponging miR-421 <i>in vivo</i>. <b><i>Conclusion:</i></b> Exosomal circ-HIPK3 could promote cell progression and TMZ resistance by regulating miR-421/<i>ZIC5</i> axis in TMZ-resistant glioma.