Abstract A17: MiR-15, miR-16 and miR-21 are direct transcriptional targets of E2F1 that limit E2F-induced proliferation and apoptosis

Abstract

Abstract microRNAs (miRs) are small non-coding RNA molecules that have recently emerged as critical regulators of gene expression and are often deregulated in cancer. In particular, miRs encoded by the miR-15a, miR-16-1 cluster are frequently deleted in certain cancers and appear to act as tumor suppressors. Furthermore, miR-21 is over-expressed in a wide variety of human tumors and functions as an oncogene. Here, we provide evidence that miR-21, the miR-15a, miR-16-1 cluster and related miR-15b, miR-16-2 cluster comprise miRs regulated by E2F1, a pivotal transcription factor that can induce both proliferation and cell death. E2F1 is a critical downstream target of the tumor suppressor RB. The retinoblastoma pathway is often inactivated in human tumors resulting in deregulated E2F activity. We show that expression levels of the five mature miRs, miR-15a, miR-16-1, miR-15b, miR-16-2 and miR-21 are elevated upon activation of ectopic E2F1. Moreover, activation of endogenous E2Fs up-regulates expression of these miRs and endogenous E2F1 binds their respective promoters. Importantly, we corroborate that miR-15a/b inhibits expression of cyclin E, a key direct transcriptional target of E2F pivotal for the G1/S transition. Similarly, miR-21 inhibits the expression of the pro-apoptotic tumor suppressor gene maspin, another direct transcriptional target of E2F. Therefore, our data suggest the existence of feed forward loops that modulate E2F activity and consist of E2F1; E2F1-regulated miRs -miR-15 and miR-21; and E2F1-regulated genes-cyclin E and maspin. In support of this, ectopic expression of miR-15 inhibits G1/S transition and, conversely, inhibition of miR-15 expression enhances E2F1-induced G1/S transition and up-regulation of cyclin E levels. Also, inhibition of miR-21 expression enhances E2F1-induced up regulation of maspin, as well as E2F1-mediated apoptosis. In summary, our data identify miR-15, miR-16 and miR-21 as novel transcriptional targets of E2F that, in turn, modulate E2F activity. Citation Format: Matan Ofir, Berta Ben-Shachar, Dalia Hacohen, Doron Ginsberg. MiR-15, miR-16 and miR-21 are direct transcriptional targets of E2F1 that limit E2F-induced proliferation and apoptosis [abstract]. In: Proceedings of the AACR Special Conference on Noncoding RNAs and Cancer; 2012 Jan 8-11; Miami Beach, FL. Philadelphia (PA): AACR; Cancer Res 2012;72(2 Suppl):Abstract nr A17.