A regulatory circuit of circ-MTO1/miR-17/QKI-5 inhibits the proliferation of lung adenocarcinoma.

Circular RNA (circRNA) is a new class of non-coding RNA that plays a pivotal role in carcinogenesis. Recently, circ-MTO1 (hsa_circ_0007874) was shown to be a cancer-related circRNA. However, its role in lung adenocarcinoma (LUAD) has not been reported. Here, we found that circ-MTO1 was significantly down-regulated in LUAD, which was closely associated with malignant features and dismal prognosis. Enforced expression of circ-MTO1 suppressed the growth of LUAD cells both in vitro and in vivo. Subsequent mechanism experiments showed that circ-MTO1 served as a sponge of oncogenic miR-17 to increase the expression of RNA-binding protein QKI-5, leading to the inactivation of Notch signaling pathway, thereby restraining the growth of LUAD. Importantly, increased QKI-5 expression caused by circ-MTO1 overexpression in turn promoted circ-MTO1 expression. Clinically, circ-MTO1 expression was strongly positively correlated with QKI-5 expression, but negatively correlated with miR-17 expression. Taken together, our data suggest that circ-MTO1 is a critical negative regulator of LUAD and elucidate the potential molecular mechanism of a novel circ-MTO1/miR-17/QKI-5 feedback loop in inhibiting LUAD progression.