circ-0000512 inhibits PD-L1 ubiquitination through sponging miR-622/CMTM6 axis to promote triple-negative breast cancer and immune escape

BACKGROUND: This study reported the function and mechanism of circ-0000512 in the progression of triple-negative breast cancer (TNBC). METHODS: circ-0000512 expression in TNBC tissues and paired adjacent normal tissues and cells was examined by qRT-PCR. Moreover, circ-0000512 expression in TNBC cells was modulated by transfection. Thereafter, colony formation assay, Transwell assay and flow cytometry were conducted to observe cell proliferation, migration and apoptosis. TNBC cells were treated with cycloheximide and the protease inhibitor MG132. Later, ubiquitination assay was performed to detect programmed cell death ligand 1 (PD-L1) ubiquitination in TNBC cells. The T cell killing ability was assessed by the T cell-mediated tumor cell killing assay. IFNγ and IL-2 levels were detected by ELISA. The percentage of activated T cells was detected with a flow cytometer. In addition, dual luciferase reporter gene assay and RNA immunoprecipitation assay were carried out to evaluate the binding between two genes. In vivo study was conducted on mice. CD8+ T cells in xenograft tumors were detected by immunohistochemistry. RESULTS: circ-0000512 was upregulated in patients with TNBC. circ-0000512 knockdown attenuated the proliferation and migration of TNBC cells and enhanced their apoptosis. circ-0000512 overexpression had opposite effects. circ-0000512 knockdown enhanced the PD-L1 protein ubiquitination in TNBC cells by inhibiting CMTM6. Meanwhile, circ-0000512 promoted CMTM6 expression by sponging miR-622. circ-0000512 knockdown increased the ratio of CD8+T cells and the lethality of T cells against TNBC cells. Besides, circ-0000512 knockdown inhibited the growth of TNBC cells in immunodeficient nude mice and normal immune mice and increased the ratio of CD8+T cells in xenograft tumors of normal immune mice. CONCLUSIONS: circ-0000512 inhibited PD-L1 ubiquitination by sponging the miR-622/CMTM6 axis, thus promoting TNBC progression and immune escape.