DNA methyltransferase 1/miR-342-3p/Forkhead box M1 signaling axis promotes self-renewal in cervical cancer stem-like cells in vitro and nude mice models.

Abstract

Background: Cervical cancer (CC) stem cell-like cells (CCSLCs), defined by the capacity of differentiation and self-renewal and proliferation, play a significant role in the progression of CC. However, the molecular mechanisms regulating their self-renewal are poorly understood. Therefore, elucidation of the epigenetic mechanisms that drive cancer stem cell self-renewal will enhance our ability to improve the effectiveness of targeted therapies for cancer stem cells.

Aim: To explore how DNA methyltransferase 1 (DNMT1)/miR-342-3p/Forkhead box M1 (FoxM1), which have been shown to have abnormal expression in CCSLCs, and their signaling pathways could stimulate self-renewal-related stemness in CCSLCs.

Methods: Sphere-forming cells derived from CC cell lines HeLa, SiHa and CaSki served as CCSLCs. Self-renewal-related stemness was identified by determining sphere and colony formation efficiency, CD133 and CD49f protein level, and SRY-box transcription factor 2 and octamer-binding transcription factor 4 mRNA level. The microRNA expression profiles between HeLa cells and HeLa-derived CCSLCs or mRNA expression profiles that HeLa-derived CCSLCs were transfected with or without miR-342-3p mimic were compared using quantitative PCR analysis. The expression levels of DNMT1 mRNA, miR-342-3p, and FoxM1 protein were examined by quantitative real-time PCR and western blotting. In vivo carcinogenicity was assessed using a mouse xenograft model. The functional effects of the DNMT1/miR-342-3p/FoxM1 axis were examined by in vivo and in vitro gain-of-activity and loss-of-activity assessments. Interplay among DNMT1, miR-342-3p, and FoxM1 was tested by methylation-specific PCR and a respective luciferase reporter assay.

Results: CCSLCs derived from the established HeLa cell lines displayed higher self-renewal-related stemness, including enhanced sphere and colony formation efficiency, increased CD133 and CD49f protein level, and heightened transcriptional quantity of stemness-related factors SRY-box transcription factor 2 and octamer-binding transcription factor 4 in vitro as well as a stronger tumorigenic potential in vivo compared to their parental cells. Moreover, quantitative PCR showed that the miR-342-3p level was downregulated in HeLa-derived CCSLCs compared to HeLa cells. Its mimic significantly decreased DNMT1 and FoxM1 mRNA expression levels in CCSLCs. Knockdown of DNMT1 or miR-342-3p mimic transfection suppressed DNMT1 expression, increased miR-342-3p quantity by promoter demethylation, and inhibited CCSLC self-renewal. Inhibition of FoxM1 by shRNA transfection also resulted in the attenuation of CCSLC self-renewal but had little effect on the DNMT1 activity and miR-342-3p expression. Furthermore, the loss of CCSLC self-renewal exerted by miR-342-3p mimic was inverted by the overexpression of DNMT1 or FoxM1. Furthermore, DNMT1 and FoxM1 were recognized as straight targets by miR-342-3p in HeLa-derived CCSLCs.

Conclusion: Our findings suggested that a novel DNMT1/miR-342-3p/FoxM1 signal axis promotes CCSLC self-renewal and presented a potential target for the treatment of CC through suppression of CCSLC self-renewal. However, this pathway has been previously implicated in CC, as evidenced by prior studies showing miR-342-3p-mediated downregulation of FoxM1 in cervical cancer cells. Additionally, research on liver cancer further supports the involvement of miR-342-3p in suppressing FoxM1 expression. While our study contributed to this body of knowledge, we did not present a completely novel axis but reinforced the therapeutic potential of targeting the DNMT1/miR-342-3p/FoxM1 axis to suppress CCSLC self-renewal in CC treatment.