Biomedical subjects

S Chellappan

Publications and source records attributed to S Chellappan.

6 recordsLinked to original sources

PubMed1999

Prohibitin, a potential tumor suppressor, interacts with RB and regulates E2F function.

The retinoblastoma tumor suppressor protein and its family members, p107 and p130, are major regulators of the mammalian cell cycle. They exert their growth suppressive effects at least in part by binding the E2F family of transcription factors and inhibiting their transcriptional activity. Agents that disrupt the interaction between Rb family proteins and E2F promote cell proliferation. Here we describe the characterization of a novel interaction between Rb family proteins and a potential tumor suppressor protein, prohibitin. Prohibitin physically interacts with all three Rb family proteins in vitro and in vivo, and was very effective in repressing E2F-mediated transcription. Prohibitin could inhibit the activity of E2Fs 1, 2, 3, 4 and 5, but could not affect the activity of promoters lacking an E2F site. Surprisingly, prohibitin-mediated repression of E2F could not be reversed by adenovirus E1A protein. A prohibitin mutant that could not bind to Rb was impaired in its ability to repress E2F activity and inhibit cell proliferation. We believe that prohibitin is a novel regulator of E2F activity that responds to specific signaling cascades.

Animals↗

PubMed1999

Regulation of Rb and E2F by signal transduction cascades: divergent effects of JNK1 and p38 kinases.

The E2F transcription factor plays a major role in cell cycle regulation, differentiation and apoptosis, but it is not clear how it is regulated by non-mitogenic signaling cascades. Here we report that two kinases involved in signal transduction have opposite effects on E2F function: the stress-induced kinase JNK1 inhibits E2F1 activity whereas the related p38 kinase reverses Rb-mediated repression of E2F1. JNK1 phosphorylates E2F1 in vitro, and co-transfection of JNK1 reduces the DNA binding activity of E2F1; treatment of cells with TNFalpha had a similar effect. Fas stimulation of Jurkat cells is known to induce p38 kinase and we find a pronounced increase in Rb phosphorylation within 30 min of Fas stimulation. Phosphorylation of Rb correlated with a dissociation of E2F and increased transcriptional activity. The inactivation of Rb by Fas was blocked by SB203580, a p38-specific inhibitor, as well as a dominant-negative p38 construct; cyclin-dependent kinase (cdk) inhibitors as well as dominant-negative cdks had no effect. These results suggest that Fas-mediated inactivation of Rb is mediated via the p38 kinase, independent of cdks. The Rb/E2F-mediated cell cycle regulatory pathway appears to be a normal target for non-mitogenic signaling cascades and could be involved in mediating the cellular effects of such signals.

Calcium-Calmodulin-Dependent Protein Kinases↗

PubMed1999

Rb and prohibitin target distinct regions of E2F1 for repression and respond to different upstream signals.

E2F transcription factor is subject to stringent regulation by a variety of molecules. We recently observed that prohibitin, a potential tumor suppressor protein, binds to the retinoblastoma (Rb) protein and represses E2F transcriptional activity. Here we demonstrate that prohibitin requires the marked box region of E2F for repression; further, prohibitin can effectively inhibit colony formation induced by overexpression of E2F1 in T47D cells. Prohibitin was also found to interact with the signaling kinase c-Raf-1, and Raf-1 could effectively reverse prohibitin-mediated repression of E2F activity. Agents such as E1A, p38 kinase, and cyclins D and E had no effect on prohibitin-mediated repression of E2F1, but all of these molecules could reverse Rb function. Similarly, stimulation of the immunoglobulin M signaling pathway in Ramos cells could inactivate prohibitin, but this had no effect on Rb function. Serum stimulation of quiescent Ramos cells inactivated Rb and prohibitin with different kinetics; further, while the serum-dependent inactivation of Rb was dependent on cyclin-dependent kinase activity, the inactivation of prohibitin was not. We believe that prohibitin is a novel regulator of E2F function which channels specific signaling cascades to the cell cycle regulatory machinery.

Binding Sites↗

PubMed1998

E2F-1 and E2F-3 are functionally distinct in their ability to promote myeloid cell cycle progression and block granulocyte differentiation.

Interleukin 3 (IL-3)-dependent 32D.3 myeloid cells are an attractive model system for the analysis of hematopoietic cell growth, differentiation, and apoptosis. In these cells, E2F-3, E2F-4, and DP-1 are regulated by both IL-3 and granulocyte colony-stimulating factor (G-CSF), whereas E2F-1 was expressed at low levels and was not regulated by either cytokine. E2F-2 and E2F-5 were not detectable. To examine phenotypes associated with the loss of normal cell cycle regulation by pRb, we established E2F-1- and E2F-3-overexpressing cell lines. In contrast to E2F-1, E2F-3 overexpression did not accelerate apoptosis or promote S-phase entry in the absence of IL-3, demonstrating that they are not functionally redundant. In addition, when cells were cultured in G-CSF to stimulate granulocytic differentiation, E2F-1 overexpression overrode survival functions provided by G-CSF and serum and induced apoptosis. In contrast, cells overexpressing E2F-3 exhibited normal granulocytic differentiation. Bcl-2 coexpression blocked E2F-1-induced apoptosis in the presence of G-CSF. However, these cells were blocked in the granulocytic differentiation program at the metamyelocyte stage and remained dependent on G-CSF for continuous culture. Cells overexpressing both E2F-1 and Bcl-2 exhibited slowed but continuous cell cycling in the absence of IL-3 until they eventually succumbed to apoptosis. Therefore, E2F-1, but not E2F-3, can temporally replace the requirement for growth factors to promote cell cycle progression, and in terminally differentiating cells, this leads to a block in differentiation and induction of apoptosis.

Apoptosis↗

PubMed1995

v-Abl activates c-myc transcription through the E2F site.

The v-abl oncogene of Abelson murine leukemia virus encodes a deregulated form of the cellular nonreceptor tyrosine kinase. v-Abl activates c-myc transcription, and c-Myc is an essential downstream component in the v-Abl transformation program. To explore the mechanism by which v-Abl activates c-myc transcription, a cotransfection assay was developed. We show that transactivation of a c-myc promoter by v-Abl requires the SH1 (tyrosine kinase) and SH2 domains of v-Abl; the C-terminal domains are not required for transactivation. The assay also identified the E2F site in the c-myc promoter as a v-Abl-responsive element. In addition, multimerized E2F sites were shown to be sufficient to confer v-Abl-dependent activation on a minimal promoter. This is the first identification of a v-Abl response element for transcriptional activation. v-Abl tyrosine kinase-dependent changes in proteins binding the c-myc E2F site were also demonstrated, including induction of a complex containing DP1, p107, cyclin A, and cdk2. Identification of v-Abl-dependent changes in E2F-binding proteins provides an important link between v-Abl, transcription, cell cycle regulation, and control of cellular growth.

Abelson murine leukemia virus↗

PubMed1992

Adenovirus E1A, simian virus 40 tumor antigen, and human papillomavirus E7 protein share the capacity to disrupt the interaction between transcription factor E2F and the retinoblastoma gene product.

The adenovirus E1A gene product, the simian virus 40 large tumor antigen, and the human papillomavirus E7 protein share a short amino acid sequence that constitutes a domain required for the transforming activity of these proteins. These sequences are also required for these proteins to bind to the retinoblastoma gene product (pRb). Recent experiments have shown that E1A can dissociate complexes containing the transcription factor E2F bound to pRb, dependent on this conserved sequence element. We now show that the E7 protein and the simian virus 40 large tumor antigen can dissociate the E2F-pRb complex, dependent on this conserved sequence element. We also find that the E2F-pRb complex is absent in various human cervical carcinoma cell lines that either express the E7 protein or harbor an RB1 mutation, suggesting that the loss of the E2F-pRb interaction may be an important aspect in human cervical carcinogenesis. We suggest that the ability of E1A, the simian virus 40 large tumor antigen, and E7 to dissociate the E2F-pRb complex may be a common activity of these viral proteins that has evolved to stimulate quiescent cells into a proliferating state so that viral replication can proceed efficiently. In circumstances in which a lytic infection does not proceed, the consequence of this action may be to initiate the oncogenic process in a manner analogous to the mutation of the RB1 gene.

Adenovirus Early Proteins↗