Overexpression of SMYD3 in Ovarian Cancer is Associated with Ovarian Cancer Proliferation and Apoptosis via Methylating H3K4 and H4K20.

Background: Epigenetic regulation has been verified as a key mechanism in tumorigenesis. SET and MYND domain-containing protein 3 (SMYD3), a histone methyltransferase, is a promising epigenetic therapeutic target and is overexpressed in numerous human tumors. SMYD3 can promote oncogenic progression by methylating lysines to integrate cytoplasmic kinase signaling cascades or by methylating histone lysines to regulate specific gene transcription. However, the exact role of SMYD3 in the progression of ovarian cancer is still unknown. Methods: Immunohistochemistry was employed to test SMYD3 expression in ovarian cancer tissues from clinical patients. CCK-8 assay, Real-time cell analysis (RTCA), colony formation assay, cell cycle and apoptosis tested by Flow cytometer were employed to test the effects of SMYD3 on cell proliferation and apoptosis in ovarian cancer cell lines. A PCR array was used to identify the downstream targets of SMYD3. And, PCR and Western blot were used to verify their expression. The binding of SMYD3 on the promoter of target genes were tested by ChIP assays. We also use nude mice subcutaneous tumor model and patient-derived xenograft (PDX) model to investigate the tumor promotive function of SMYD3 in vivo. Results: SMYD3 expression was higher in ovarian cancer tissues and cell lines than in normal ovarian epithelial tissue and human ovarian surface epithelial cells (HOSEpiC). After silencing SMYD3, the proliferation of ovarian cancer cells was significantly inhibited in vitro. In addition, the SMYD3-specific small-molecule inhibitor BCI-121 suppressed ovarian cancer cell proliferation. Downregulation of SMYD3 led to S phase arrest and increased the cell apoptosis rate. Furthermore, a PCR array revealed that SMYD3 knockdown caused the upregulation of the cyclin-dependent kinase (CDK) inhibitors CDKN2A (p16INK4), CDKN2B (p15INK4B), CDKN3 and CDC25A, which may be responsible for the S phase arrest. In addition, the upregulation of CD40LG and downregulation of BIRC3 may explain the increased cell apoptosis rate after silencing SMYD3. We also discovered that SMYD3 bound on the promoter of CDKN2A and down-regulated its expression by triple-methylating H4K20. In addition, SMYD3 bound on the promoter of BIRC3 and up-regulated its expression by triple-methylating H3K4. Finally, knocking down SMYD3 could inhibit ovarian cancer growth in nude mice subcutaneous tumor model and PDX model. Conclusion: Our results demonstrated that SMYD3 was overexpressed in ovarian cancer and contributes to the regulation of tumor proliferation and apoptosis via SMYD3-H4K20me3-CDKN2A pathway and SMYD3-H3K4me3-BIRC3 pathway. Thus, SMYD3 is a promising epigenetic therapeutic target for ovarian cancer.