The RBP1-CKAP4 Axis Activates Oncogenic Autophagy and Promotes Cancer Progression in Oral Squamous Cell Carcinoma

Background: Retinol-binding protein 1 (RBP1) is involved in several physiological functions including the regulation of the metabolism and retinol transport. RBP1 has been implicated as an important element in the development of several types of cancer. However, the roles of RBP1 and its correlation with autophagy in oral squamous cell carcinoma (OSCC) pathogenesis remain unknown. Methods: In this study, RBP1 was identified as the most significant up-regulated DEPs with fold change > 2 in OSCC samples versus normal tissues through iTRAQ-based proteomics analysis coupled with 2D LC-MS/MS and was significantly associated with malignant phenotypes (differentiation, TNM stage, lymphatic metastasis) of OSCC. Immunohistochemistry was used to measure protein expression in tissues. Cell proliferation was evaluated by clonogenic and MTT assay. Cell cycle was analyzed by flow cytometer. Cell migration and invasion potentials were assessed by wound healing assay and transwell assay as well as confirmed by in vivo xenograft mice model. QRT-PCR, western blot and immunofluorescence were performed to detect the expression of ATG5, Beclin 1 and LC3B. Transmission electron microscopy was utilized to identify autophagsomes. Immunofluorescent staining and coimmunoprecipitation were used to determine the relationship between RBP1 and CKAP4. Findings: In vitro experiments demonstrated that RBP1 overexpression promoted cell proliferation, migration and invasion as well as accelerated G1 phase progression. Silencing of RBP1 suppressed tumor formation in xenografted mice. We further showed that RBP1-CKAP4 axis was a critical regulator of the autophagic machinery, a process essential for maintaining metabolic homeostasis in OSCC. Interpretation: We discovered that RBP1 could mediate autophagy through the CKAP4 axis and regulate the proliferation, migration and invasion of OSCC cells. Therefore, we propose that RBP1 induces autophagy to promote cancer by regulating CKAP4. Funding Statement: This work was supported by the National Science Foundation of China (81502354, 81472520, 81600880), Qingdao/China Postdoctoral Science Foundation (2017M622145, 2017M612217), Source Innovation Planning of Qingdao (17-1-1-42-jch, 17-1-1-45-jch, 18-2-2-77-jch) and Natural Science Foundation of Shaanxi/Shandong Province (2015JM8396, 2016JQ8028, 2017WS215, ZR2018BH021). Declaration of Interests: The authors declare no potential conflicts of interest with respect to the authorship and/or publication of this article. Ethics Approval Statement: Ethical approval was ratified by the local Ethics Committee, and all patients provided written informed consent for the utilization of tissue samples in research. All animal experiments were approved by the Animal Care and Use Committee of the Affiliated Hospital of Qingdao University.