E2F-1 RNomics is critical for reprogramming of cancer cells to quiescent state Aanchal Mehrotra 1 , Kusum Joshi 2 and Deepak Kaul 1 1 Department of Experimental Medicine and Biotechnology, Post Graduate Institute of Medical Education and Research, Chandigarh 160012, India 2 Department of Histopathology, Post Graduate Institute of Medical Education and Research, Chandigarh 160012, India The discovery of cooperativity between pRB and E2F greatly prompted various investigators to find how E2F biology contributes to oncogenesis. Although E2F family of transcription factors have been linked to proliferation, apoptosis and differentiation, yet no heed has been paid to understand the role of E2F biology in cellular quiescence. To understand the functional RNomics (regulation of gene transcription through RNA interference) of E2F-1 gene, 2 cancer cell lines, such as Jurkat exhibiting E2F-1 gene overamplification and Hela-229 exhibiting intrinsic downregulation of E2F-1 gene expression, were used in our study. E2F-1 gene knockdown via siRNA within Jurkat cells resulted in upregulation of genes characteristic of quiescence both translationally and transcriptionally, which was accompanied by downregulation of genes at both translational and transcriptional level involved in cell cycle progression and apoptosis. This genomic phenomenon also translated into ultrastructural and phenotypic features typical of quiescent state. These observed results in Jurkat cells were simulated by upregulation of E2F-1 gene in Hela-229 cells through the downregulation of miR 17-5p. This E2F-1-regulated pathway explained as to how Jurkat cells entered exclusively into quiescent state when E2F-1 was downregulated in these cells and how Hela-229 cells proliferate vigorously when E2F-1 was upregulated in such cells. Here, we propose a gene- regulatory pathway which the cell might be using in its entry into either quiescent or proliferative states. Furthermore, this pathway may be useful in designing strategies for the treatment of cancer in general and acute lymphoblastic leukemia in particular. The destiny of a cell, whether it undergoes division, differen- tiation or death, results from an intricate balance of many regulators, including proto-oncoproteins, tumor-suppressor proteins and cell cycle-associated proteins. Abnormalities in any of these molecular circuits regulating the duplication of the genetic material and the subsequent division of a cell into 2 daughter cells lead to aberrations in the regulation of the cell cycle, which forms the hallmark of malignant transfor- mation, leading in turn to the development of tumors. The size of any cellular compartment is maintained by a finely orchestrated balance between input (proliferation) and output (differentiation þ apoptosis) processes. 1 These 3 proc- esses along with one more important process named quies- cence or G0 state form the basic intricate modalities of a healthy living cell. Quiescence is defined as a reversible cell cycle arrest in which the cells are poised to re-enter the cell cycle. Most eukaryotic cells spend a majority of their lifespan in this state. 2 It is the counterpart to proliferation: a reversi- ble, nondividing state. Although it is a common state for many somatic cells, including the stem cells, we have remark- ably little idea about the regulation of this very important cellular state. This ubiquity of quiescence as a central feature of cell life suggests that its regulation may be critical to nor- mal development, degenerative diseases and cancer. 3 Immemorial cell cycle regulatory transcription factors, e.g., Retinoblastoma (Rb) and E2F family, have been found to have important roles in cellular proliferation, differentiation and apoptotic processes, and therefore it becomes pertinent to look into their roles if any in regulating cellular quiescence as well. Most researchers agree that the pathways involved in quiescence and apoptosis share regulatory mechanism with the cell cycle control. 4 At the genomic level, the E2F tran- scription factors are crucial for regulating cell cycle progres- sion and tumorigenesis. 5 To understand these biological processes, it is imperative to understand how E2F transcrip- tion factors exert their control on the genes recognized to play crucial role in the control of cellular proliferation, apo- ptosis and quiescence. Recently, we have proposed a pathway that links cell cycle with apoptosis based on the interaction between AATF, c- Myc, Bcl-2, Rb and E2F. 6 The physiological role of E2F in apoptosis is suggested by the observation that mice deficient in E2F-1 have defects in thymocyte apoptosis. 7 However, no Key words: E2F-1 RNomics, effector genes, quiescence, apoptosis, T- cell leukemia, miR 17-5p Grant sponsor: Indian Council of Medical Research (ICMR), New Delhi, India DOI: 10.1002/ijc.25109 History: Received 31 July 2009; Accepted 19 Nov 2009; Online 9 Dec 2009 Correspondence to: Deepak Kaul, Department of Experimental Medicine and Biotechnology, Post Graduate Institute of Medical Education and Research, Chandigarh 160012, India, Tel.: þ91-0172-2755233, Fax: þ91-0172-2744401, E-mail: dkaul_24@hotmail.com Cancer Cell Biology Int. J. Cancer: 127, 849–858 (2010) V C 2009 UICC International Journal of Cancer IJC