[circ_BACH2 affects the malignant biological behavior of papillary thyroid cancer by regulating miR-370-3p].

To investigate the influence of circ_BACH2 on the malignant biological behavior of papillary thyroid cancer and its molecular mechanism. Cancer tissues and paracancer tissues of 51 patients with papillary thyroid carcinoma from the Fourth Central Hospital of Tianjin between 2017 and 2019 were collected. Reverse transcription-quantitative real-time polymerase chain reaction (RT-qPCR) was used to detect the expressions of circ_BACH2, miR-370-3p and G protein coupled receptor kinase interacting factor 1 (GIT1) mRNA in tissues and cells; flow cytometry to detect cell apoptosis and cell cycle; plate clone formation experiment to detect the number of cell clones; cell counting kit 8 (CCK-8) to detect cell proliferation; Transwell array to detect cell migration and invasion; western blot to detect protein expressions; dual luciferase report experiment to detect the targeting relationship between circ_BACH2, miR-370-3p and GIT1; the nude mouse tumor formation experiment to detect the effect of circ_BACH2 on tumors in mice. Compared with adjacent tissues, the expressions of circ_BACH2 and GIT1 in papillary thyroid cancer tissues was increased, while the expression of miR-370-3p was decreased. Compared with Nthy-ori3-1 cells, the expressions of circ_BACH2 in papillary thyroid cancer cells TPC-1 and SW579 were increased, the mRNA and protein levels of GIT1 were increased, miR-370-3p expression was decreased. The expression level of GIT1 mRNA was negatively correlated with that of miR-370-3p (=-0.634), and the expression level of circ_BACH2 was positively correlated with that of GIT1 (=0.635). The expression level of circ_BACH2 was negatively correlated with that of miR-370-3p (=-0.394, ＜0.05). Circ_BACH2 and miR-370-3p has a binding site at the 3' UTR of GIT1. After knocking down circ_BACH2, the proportion of G/G cells in papillary thyroid cancer cells TPC-1 and SW579 was increased, the proportion of S-phase cells was decreased and the proportion of G/M-phase cells did not change significantly. The cell absorbance value was lower than that in si-NC group. The number of cell clone formation was decreased (43±5 vs 100±6, 54±8 vs 100±9); the cell apoptosis rate was increased [(19.60±2.40)% vs (4.30±0.20)%, (18.10±2.10)% vs (5.10±0.23)%]; cell migration number was decreased (61±7 vs 134±15, 58±6 vs 112±11), the invasion number was also decreased (45±6 vs 113±11, 47±4 vs 92±9); the expressions of Snail and Twist1 were decreased, and the expression of E-cadherin was increased (＜0.000). Inhibition of miR-370-3p expression reversed the effect of circ_BACH2 knockdown on proliferation, migration, invasion and apoptosis of thyroid papillary cancer cells. Overexpression of GIT1 reversed the effects of overexpression of miR-370-3p on proliferation, migration, invasion and apoptosis of thyroid papillary cancer cells. Mice injected with TPC-1 cells stably transfected with sh-circ_BACH2 showed a reduction in tumor volume [(535±91) mm vs (857±114) mm] after 35 days of culture; tumor weight was decreased [(0.62±0.13) mg vs (1.06±0.15) mg, ＜0.05]; the expressions of circ_BACH2 and GIT1 were decreased, and the expression of miR-370-3p was increased in nude mouse tumor tissue. Silencing circ_BACH2 may inhibit the proliferation, migration and invasion of papillary thyroid cancer cells in vitro, promote cell apoptosis, and inhibit tumor growth in vivo through targeted regulation of miR-370-3p/GIT1.