Abstract 1992: Evidence for modulation of FoxM1 by p21 in ovarian cancer

Abstract

Abstract The oncogenic transcription factor forkhead box M1 (FoxM1) is overexpressed in many cancers, including 84% of ovarian cancer and plays a role in DNA repair, mitotic checkpoint, cell proliferation, and cancer drug resistance. Similar to Her2 in breast cancer, the constitutive expression of FoxM1 makes it a plausible gene target for novel anti-cancer therapies, but the regulation of FoxM1 has yet to be elucidated. Evidence suggests FoxM1 up-regulation is a result of TP53 mutations, however, no TP53 response element has been found within the FoxM1 promoter, thus there is likely another molecule mediating p53-induced FoxM1 overexpression. We hypothesize that cyclin-dependent kinase inhibitor 1a, p21, is involved in the regulation of FoxM1 by p53 in ovarian cancer. To test this hypothesis, we knocked down p21 expression with siRNA and shRNA in two wild-type p53 cancer cell lines, NCI-H23 and A2780. Knock down of p21 resulted in a mirrored increase in FoxM1 protein level in both cell lines as detected by Western blot. The addition of nutlin-3 (10 μM), a p53 stabilizer, to these p21 knocked down cell lines restored some p21 expression 24 hour post-treatment, which led to a subsequent decrease in FoxM1 levels compared to the corresponding DMSO-treated p21 knock down. These results suggest p21 is a negative regulator of FoxM1 in cancer cell lines. To further investigate this pathway, the same two cancer cell lines were treated with the FoxM1 inhibitor thiostrepton (10 μM) and protein levels were measured over a time course of 48 hours. FoxM1 protein levels were reduced following 8 hours, a time point where p21 protein levels peaked. This finding was further support by qRT-PCR analysis of A2780 ovarian cancer cell mRNA collected 0, 6, 8, 9, 10, 11, and 12 hours following thiostrepton (10 μM) treatment. In this experiment, p21 mRNA levels peaked 6 hours post-treatment, with FoxM1 mRNA levels reduced at 8 hours post-treatment, relative to control. Although thiostrepton has been shown to directly bind and inhibit the transcription of FoxM1, these results suggest that thiostrepton may also induce p21, potentiating FoxM1 inhibition. Thiostrepton has previously been shown to induce cancer cell death and reduce tumor burden, therefore further supporting FoxM1 as a potential drug target as well as encouraging the need to understand regulators involved in this pathway. Citation Format: Jill Madden, Jeremy Chien. Evidence for modulation of FoxM1 by p21 in ovarian cancer. [abstract]. In: Proceedings of the 106th Annual Meeting of the American Association for Cancer Research; 2015 Apr 18-22; Philadelphia, PA. Philadelphia (PA): AACR; Cancer Res 2015;75(15 Suppl):Abstract nr 1992. doi:10.1158/1538-7445.AM2015-1992