Abstract: Objective To investigate the combined effect of all⁃trans retinoic acid (ATRA) and suberoyl anilide hydroxamic acid  (SAHA) on the growth of the transplanted tumors of human breast cancer cell MCF⁃7 in nude mice and its mechanism. Methods  MCF⁃7 cells were cultured in vitro and the optimal compatibility concentration of the ATRA and SAHA combination was selected by CCK⁃8 assay. The human breast cancer cell MCF⁃7 nude mice xenograft model was established and randomly divided into control group (PBS), ATRA group (0.84 mg/kg ATRA), SAHA group (0.42 mg/kg SAHA) and ATRA+SAHA group (0.84 mg/kg ATRA+0.42 mg/kg SAHA), with 5 nude mice in each group. When the transplanted tumor grew to 100 mm3, the drug was given subcutaneously for 2 weeks administration according to the grouping, then the orbital vein blood was collected. The tumor tissues were taken from nude mice after sacrificed by cervical amputation and weighed, and tumor inhibition rate and organ index were calculated. The serum interleukin⁃6 receptor (IL⁃6R) contents of nude mice were measured by ELISA. The morphological structure of tumor tissues was observed by HE staining, and the expression levels of the tumor proliferation and metastasis related proteins Caspase⁃3, MMP⁃9, MMP⁃2, and the anti⁃tumor gene proteins p53 and EGFR in tumor tissues were detected by immunohistochemistry and Western blot. Results The CCK⁃8 assay showed that MCF⁃7 cell viability was the lowest when treated with 0.40 mg/mL ATRA+0.15 mg/mL SAHA, which was the optimal compatibility concentration for the combination of  SAHA and ATRA. Compared with the control group and each monotherapy group, the tumor inhibition rates of nude mice in the ATRA+SAHA group was significantly increased (all P<0.05), the serum IL⁃6R content was decreased (all P<0.05), and the liver organ index was significantly decreased (all P<0.01); the spleen index was also significantly lower in the ATRA+SAHA group compared with the ATRA group (P<0.05). The transplanted tumors tissues in the ATRA+SAHA group were mainly severely necrotic, and the expression levels of MMP⁃9, MMP⁃2, and EGFR in the tumor tissues were decreased significantly compared with the control group and each monotherapy groups (all P<0.05), while the expression levels of p53 and Caspase⁃3 were increased significantly (all P<0.01). Conclusions The combination of ATRA and SAHA has synergistic anti⁃tumor effect.

Key words: Breast cancer, Nude mice, All-trans retinoic acid, Suberoyl anilide hydroxamic acid, Tumor inhibition