Down-regulation of Sp1 suppresses cell proliferation, clonogenicity and the expressions of stem cell markers in nasopharyngeal carcinoma

Background: Transcription factor Sp1 is multifaceted, with the ability to function as an oncogene or a tumor suppressor, depending on the cellular context. We previously reported that Sp1 is required for the transcriptional activation of the key oncogenes in nasopharyngeal carcinoma (NPC), including B-lymphoma mouse Moloney leukemia virus insertion region 1 (Bmi1) and centromere protein H (CENPH), but the role of Sp1 and its underlying mechanisms in NPC remained largely unexplored. The objective of this study was to investigate the cellular function of Sp1 and to verify the clinical significance of Sp1 as a potential therapeutic target in NPC. Methods: The levels of Sp1 in the normal primary nasopharyngeal epithelial cells (NPECs) and NPC cell lines were analyzed by Quantitative Real-time RT-PCR (qRT-PCR) and Western blot. The location and expression of Sp1 in the NPC tissues were detected by immunohistochemistry staining (IHC). The effect of Sp1 knockdown on the cell proliferation, clonogenicity, anchorage-independent growth and the stem-cell like phenotype in NPC cells were evaluated by MTT, flow cytometry, clonogenicity analysis and sphere formation assay. Results: The mRNA and protein levels of Sp1 were elevated in NPC cell lines than in the normal primary NPECs. Higher expression of Sp1 was found in NPC tissues with advanced clinical stage (P = 0.00036). Either inhibition of Sp1 activity by mithramycin A, the FDA-approved chemotherapeutic anticancer drug or Sp1 silencing by two distinct siRNA against Sp1 suppressed the growth of NPC cells. Mechanism analysis revealed that Sp1 silencing may suppress cell proliferation, clonogenicity, anchorage-independent growth and the stem-cell like phenotype through inducing the expression of p27 and p21, and impairing the expressions of the critical stem cell transcription factors (SCTFs), including Bmi1, c-Myc and KLF4 in NPC cells. Conclusions: Sp1 was enriched in advanced NPC tissues and silencing of Sp1 significantly inhibited cell proliferation, clonogenicity, anchorage-independent growth and the stem-cell like phenotype of NPC cells, suggesting Sp1 may serve as an appealing drug target for NPC.