Circular RNA <i>circHIPK3</i> modulates autophagy via <i>MIR124-3p</i>-STAT3-PRKAA/AMPKα signaling in STK11 mutant lung cancer

The role of circular RNA in cancer is emerging. A newly reported circular RNA <i>HIPK3</i> (<i>circHIPK3)</i> is critical in cell proliferation of various cancer types, although its role in non-small cell lung cancer (NSCLC), has yet to be elucidated. Our results provided evidence that silencing of <i>circHIPK3</i> significantly impaired cell proliferation, migration, invasion and induced macroautophagy/autophagy. Mechanistically, we uncovered that autophagy was induced upon loss of <i>circHIPK3</i> via the <i>MIR124-3p</i>-STAT3-PRKAA/AMPKa axis in STK11 mutant lung cancer cell lines (A549 and H838). STAT3 abrogation as well as transfection with a <i>MIR124-3p</i> mimic, recapitulated the induction of autophagy. We also demonstrated antagonistic regulation on autophagy between <i>circHIPK3</i> and linear <i>HIPK3</i> (<i>linHIPK3</i>). We therefore propose that the ratio between <i>circHIPK3</i> and <i>linHIPK3</i> (C:L ratio) may reflect autophagy levels in cancer cells. We observed that a high C:L ratio (&gt;0.49) was an indicator of poor survival, especially in advanced-stage NSCLC patients. These results support that <i>circHIPK3</i> is a key autophagy regulator in a subset of lung cancer and has potential clinical use as a prognostic factor. The circular RNA <i>HIPK3</i> (<i>circHIPK3)</i> functions as an oncogene and autophagy regulator may potential use as a prognostic marker and therapeutic target in lung cancer. <b>Abbreviations</b> 3-MA: 3-methyladenine; AMPK: AMP-activated protein kinase; ATG7: autophagy related 7; Baf-A: bafilomycin A₁; BECN1: beclin 1; <i>circHIPK3</i>: circular HIPK3; CQ: chloroquine; GAPDH: glyceraldehyde-3-phosphate dehydrogenase; GFP: green fluorescent protein; HIPK3: homeodomain interacting protein kinase 3; IL6R: interleukin 6 receptor; MAP1LC3B/LC3B: microtubule associated protein 1 light chain 3 beta; NSCLC: non-small cell lung cancer; RFP: red fluorescent protein; RPS6KB1/S6K: ribosomal protein S6 kinase B1; SQSTM1/p62: sequestosome 1; STAT3: signal transducer and activator of transcription 3; STK11: serine/threonine kinase 11