Background/Aim: The long noncoding RNA OIP5 antisense RNA 1 (OIP5-AS1) is overexpressed in various cancer types, such as lung cancer, hepatoblastoma and cervical cancer, and functions to accelerate cell proliferation, invasion and migration. Here, we investigated the roIe of OIP5-AS1 in cell-cycle progression of H1299 and A549 non-small cell lung cancer cells, and FaDu and CAL27 head and neck squamous cell carcinoma cells. Materials and Methods: The cells were transfected with small interfering RNA and subjected to cell-cycle analysis and reverse-transcription quantitative polymerase chain reaction (RT-qPCR). Results: Silencing of OIP5-AS1 suppressed the proliferation of H1299, A549, FaDu and CAL27 cells. RT-qPCR and cell-cycle analysis revealed that silencing OIP5-AS1 increased the expression of CDK inhibitors, such as p15, p16, p18 and p19, resulting in G1phase arrest. Conclusion: OIP5-AS1 regulates G1-phase progression by repressing CDK inhibitors and, thus, promotes the proliferation of H1299, A549, FaDu and CAL27 cells.OPA-interacting protein 5 antisense transcript 1 (OIP5-AS1) is a long noncoding RNA (lncRNA) identified as the mammalian homolog of cyrano in zebrafish. In zebrafish, cyrano is essential for embryonic development (1). We previously showed that silencing human OIP5-AS1 inhibited the proliferation of HeLa cells by causing cell-cycle arrest at the G 2 /M phase, implying that human OIP5-AS1 functions to promote cell proliferation (2). Recent studies with clinical cancer specimens clarified that OIP5-AS1 is overexpressed in various cancer types such as of the breast, lung, cervix and bladder, as well as glioma, osteosarcoma and hepatoblastoma (3). OIP5-AS1 also participates in both positive and negative regulation of cell proliferation in cervical cancer (2, 4). Moreover, in many different human cancer cell types, including those of lung cancer (5), osteosarcoma (6), glioma (7) and hepatoblastoma (8), OIP5-AS1 functions to accelerate cell proliferation, invasion, migration and prevent apoptosis, so it is thought to function as an oncogene in these kinds of cancer. OIP5-AS1 acts as a sponge for many microRNAs (miRNAs) thereby inhibiting them, resulting in the regulation of WNT/β-catenin, PI3K/AKT serine/threonine kinase 1 and NOTCH signalling pathways (9).Cell proliferation is strictly regulated by the cell cycle. The progression of the cell cycle is accelerated by a series of complexes consisting of cyclins and cyclin-dependent kinases (CDKs) and is repressed by CDK inhibitors (CKIs) (10). CKIs comprise two groups, the INK4 family including p15, p16, p18 and p19 and the Cip/Kip family including p21, p27 and p57 (11). Forced expression of these CKIs results in G 1 phase arrest in the cell cycle. Regulation of the G 1 phase by cyclin/CDK complexes and CKIs is thought to be important for the regulation of cell proliferation. Recently, we revealed that a lncRNA, antisense non-coding RNA in the INK4 locus (ANRIL), functions as a positive regulator of G 1 -phase progression in head and neck squamous cell...
