To proliferate, eukaryotic cells have to complete an ordered series of events called the ‘cell cycle’, which include the faithful replication of their genome and the correct segregation of the two copies generated into two daughter cells upon cell division. A disruption of these events may lead to cell death or oncogenic transformation. The processes of cell cycle, therefore, are carefully regulated.
Elucidating the role of effector proteins in G1 to S phase progression
A key step in the eukaryotic cell cycle is the G1 to S phase transition and this step is tightly coupled to the transcriptional control of genes involved in growth and DNA replication.
Figure 1. Proposed model for activation of S-phase promoters by E2F1. During G1/S phase transition, HCF-1 and its associated H3K4 HMTs are recruited to E2F-responsive promoters, replacing the pRb repressor complex. The E2F-HCF-H3K4HMT complex then activates transcription of S-phase genes.
In mammalian cells, the E2F family of transcription factors primarily controls this temporal gene expression regulation. We have shown that HCF-1 is an important regulator of G1 to S-phase transition and plays a direct role in the activation of E2F-responsive promoters through the cell-cycle-specific recruitment of the MLL-family of H3K4 histone methyltransferases (Figure 1). While this work has added new effectors to G1 to S-phase transition, how E2Fs affect passage into S phase is still poorly understood. In our laboratory, we aim to discover new effector proteins involved in regulation of E2F-responsive promoters and better understand how these effectors influence the chromatin modulation during G1 to S phase progression.
Cell cycle and cancer:
Key regulators of cell cycle play a central role in tumor development as well. For example, deregulation of the Rb-E2F pathway is one of the hallmarks of human cancers. Many genes involved in basic cell cycle processes are also highly expressed in more proliferative tumors. Characterization of the genome-wide transcriptional program of the cell cycle in mammalian cells, therefore, is a critical step in understanding the basic cell cycle processes and their role in cancer.
Building architecture of transcription factor network during the cell cycle
To build transcriptional-factor networks involved in regulation of cell-cycle progression we plan to start with cell cycle master regulators like E2Fs. Using large scale ChIP-sequencing and expression arrays during different stages of cell cycle, we hope to reveal the continuous map of coordinated events like promoter binding and gene expression during the cell cycle and set of intermediary regulators involved in the activation of G2/M or early G1 genes, which were themselves regulated by E2Fs (Figure 2). This approach can be generalized later for constructing other regulatory network.
Cancer networks for drug targets:
Our understanding of cancer genes has improved tremendously over the past three decades, but this has not translated into equivalent benefits to cancer patients. We plan to integrate transcriptional networks proposed above with existing gene expression data on proliferative tumors to identify new nodal proteins for novel cancer therapeutics.
Therefore we aim to use global approaches to build networks, and at the same time use focused approaches to understand the molecular events involved in cell cycle regulation. The overall goal is to understand the transcriptional regulation of gene expression in growth and disease by integrating cell biology, proteomics, molecular biology and genomics approaches.
Figure 2. Model for how E2F1 activates genes beyond S-phase.E2F1 regulates the transcription of gene x. Protein X in turn regulates gene y which in turn regulates gene z. In this manner a cascade of time-specific transcription regulates cell cycle.