Our Science – Colburn Website
Nancy H. Colburn, Ph.D.
In 1989, (Bernstein and Colburn, Science,) Dr. Colburn discovered that mouse cell variants for transformation response to tumor promoters were differentially able to activate AP-1 dependent transcription. This led to the discoveries in 1994 (Dong et al PNAS) that AP-1 activation is required for tumor promotion in a cell culture model and in 1999 (Young et al PNAS,) that AP-1 activation is required for skin tumor promotion in vivo and can be specifically targeted by dominant negative AP-1 in a transgenic model. In 2002, 2003 (Young et al Molec Carc, Cooper et al Molec Ca Res,) targeting AP-1 for prevention was extended to human-relevant exposures such as human papilloma virus and UVB. In 2002 (Young et al MCB,) Dr. Colburn demonstrated the importance of Fos family protein Fra-1 as a target for cancer prevention, a finding extended by others to several human models. In 1999 (Cmarik et al PNAS,) Dr. Colburn discovered and demonstrated the transformation suppressing activity of the novel suppressor Pdcd4. In 2001, 2002, (Yang et al Oncogene,) AP-1 activation emerged as a functionally significant target of Pdcd4. In 2003, 2004 and 2006 (Yang et al MCB,) and 2005 (Jansen et al Cancer Research) Dr. Colburn demonstrated that Pdcd4 functions as an inhibitor of translation initiation to consequently inhibit Jun kinase activation, AP-1 dependent transcription, transformation, tumorigenesis, and invasion. In 2004 (Jansen et al Mole Ca Ther,) Dr. Colburn demonstrated that Pdcd4 expression is predictive for and contributes to human cancer cell sensitivity to geldanamycin and tamoxifen. The recent crystallization of an MA3 domain of Pdcd4 (LaRonde-LeBlanc, Santhanam et al, MCB 2006) and the identification of Pdcd4 as a Beta TrCP-ubiquitin-proteasome degradation target (Dorrello et al Science 2006) provide insights into how Pdcd4 might be targeted for drug design.
Dr. Colburn has served as organizer of major International symposia: Keystone, FASEB, ACS, and others. She has also served on NIH RO-1 and PO-1 study sections and chaired many NIH, ACS Site Visits. She has served on numerous journal editorial boards, been an invited speaker at many conferences and universities, served on Scientific Advisory Boards at several Cancer Centers, Medical Schools, served on the Board of Directors of the AACR and as President of the WICR
Chair, Cancer Prevention Faculty 2001-present
Member, Preclinical Models Strategy Team
Member, Animal Models Initiative
Member, CCR Promotions Review Panel
Chair, NCI Frederick Distinguished Scientist Speaker Series
Endowed Professorships at UT Galveston, University of Nebraska Medical School, Yonsei University Medical School, UT MD Anderson
NIH Merit Award 2002
NCI Outstanding Mentor Award 2002
The Gene Regulation Section (Nancy Colburn, Ph.D.) investigates gene regulation events that drive carcinogenesis and can be exploited for cancer prevention. Of particular interest is the activation of oncogenic transcription factors AP-1 and NFkB and of the novel tumor suppressing translation inhibitor Pdcd4. Mice genetically engineered for resistance to carcinogenesis are used to validate known targets and to discover new targets. Human cancer cell models are used to test applicability of molecular targeting for intervention in human disease.
In collaboration with both NCI scientists and our colleagues in universities, the GRS continues its work on identification of biomarkers for response to chemoprevention of colon cancer. Dietary interventions are being used to prevent colon carcinogenesis in genetically obese and non-obese mice and to discover response biomarkers. An ongoing study is identifying early biomarkers of response to bean diet interventions in humans under conditions in which the risk of adenoma recurrence is reduced.
The Role of Transcription Factors AP-1 and NFkB in the Cause and Prevention of Carcinogenesis.
The AP-1 transcription factor is a heterodimer of Jun and Fos family proteins that binds to a specific sequence on the transcriptional promoter of certain genes and drives their transcription. Keratinocyte-specific expression of Dominant Negative Jun in transgenic mice inhibits induced AP-1 and tumorigenesis without inhibiting cell proliferation or cell survival in multiple mouse models relevant to human carcinogenesis (Young et al., PNAS, 1999). Among these are mice whose skin tumor promotion response is elevated by expression of Human Papilloma Virus E7 (Young et al Molec Carc 2002) and mice induced to form squamous carcinomas by repeated exposure to UVB (Cooper et al Molec Cancer Res 2003). Tetracycline regulated expression of TAM 67 has recently been directed to mammary epithelia in the laboratory of collaborator Powel Brown (Shen et al Dev Biol 2006) and is being tested for efficacy in preventing HER2/Neu induced mammary carcinogenesis. The transcription factor NFkappa B is coordinately regulated with AP-1, suggesting the possible importance of both factors in transformation (Li et, Cancer Res 1997). Recent observations have identified NFkB non-responsiveness as an explanation for transformation non-responsiveness in the JB6 model (Hsu et al, Cancer Res 2001, Hu et al Carcinogenesis 2004). Transformation resistant cells owe their nonresponsiveness to an inability to activate NFkappa B p65 protein. p65 phosphorylation at S536 is important both for DNA binding and for ubiquitination and degradation of inhibitor IkappaB alpha (Hu et al Molec Carcinog 2005). The observation that targeting AP-1 and NFkB elevation prevents tumor promotion and progression has been extended from the mouse JB6 model to mouse and human keratinocyte progression models, and to transgenic mouse models.
Transgenic mice expressing AP-1/ NFkB inhibitor TAM 67 present a valuable opportunity to identify AP-1 or NFkB target genes whose expression is critical to neoplastic transformation. Expression microarray analysis has revealed TAM67 target genes that are being queried for functional significance in driving carcinogenesis. Such target genes may be promising new molecular targets for cancer prevention (Young et al Trends in Molec Medicine 2003). Recent studies have established the importance of chromatin architectural protein HMGA1 (Dhar et al Oncogene 2004), COX-2, and osteopontin (Matthews et al submitted) as functionally significant TAM67 targets. A promising drug discovery project in collaboration with the Molecular Targets Development Program aims to identify compounds that mimic the specificity of TAM67, i.e. that prevent tumorigenesis without inhibiting cell proliferation or cell survival. The primary high throughput screen of 300,000 synthetic and natural products coupled to a cell proliferation (XTT) assay has yielded a small set of 'hits' (Ruocco et al in press). The secondary assay will assess inhibition of NFkB.
Identification of Differentially Expressed Genes that Suppress Tumorigenesis: Pdcd4 and TIMP-3.
Differential display of mRNA analysis identified Pdcd4 (Cmarik et al., PNAS 1999) as a novel suppressor of transformation. Antisense expression of the novel pdcd4 gene converts transformation resistant (P-) to sensitive (P+) cells and pdcd4 sense expression (Yang et al Oncogene 2001) converts P+ to P- cells, thus establishing a causal relationship to prevention of tumor promoter induced transformation. Furthermore pdcd4 expression suppresses tumor phenotype in transformed JB6 cells (Yang et al Oncogene 2003). A surprising discovery is that Pdcd4 expression in human cancer cell lines is predictive for sensitivity to tamoxifen and geldanamycin. Moreover, expression of Pdcd4 actually confers sensitivity to these drugs (Jansen et al Molec Cancer Ther 2004). Examination of the possible inhibitory effect of pdcd4 on molecular events known to be required for tumor promotion revealed that pdcd4 expression inhibited the activation of transcription factor AP-1 but not of NFkappaB or of ornithine decarboxylase(Yang et al Oncogene 2001). The AP-1 inhibiting activity of Pdcd4 appears to be attributable to blocking the transactivation of cJun and cFos (Yang et al Oncogene 2003). Although expression of Pdcd4 protein blocks AP-1 activation, Pdcd4 does not interact directly with Jun or Fos proteins. Analysis of Pdcd4 binding partners by a yeast two-hybrid assay and co-immunoprecipitation revealed the translation initiation factors RNA helicase eIF4A and scaffold eIF4G as major binding partners (Yang et al Molec Cell Biol 2003, MCB 2004). Binding of Pdcd4 to eIF4A is required for Pdcd4 to inhibit eIF4A RNA helicase, to inhibit translation, and to inhibit the activation of AP-1 dependent transcription required for neoplastic transformation. Mutational analysis defines two helical MA-3 domains as required for binding to eIF4A and for inhibiting translation (Yang et al Molec Cell Biol 2004). Residues on eIF4A required for binding Pdcd4 have also been characterized (Zakowicz RNA 2005). In collaboration with the laboratory of Alexander Wlodawer, the crystal structure of the C-terminal MA3 domain has been solved (LaRonde-LeBlanc, Santhanam et al MCB in press 2006). Analysis of the crystal structure predicts a mechanism by which Pdcd4 acts to inhibit translation initiation, a prediction that has been experimentally confirmed. Pdcd4 expression is downregulated in a number of human cancers. Pdcd4 overexpression inhibits invasion by human cancer cells. The mechanism involves targeting expression of a kinase upstream of cJun N-terminal kinase to consequently inhibit AP-1 dependent transcription (Yang et al MCB 2006).
Recent investigation of the mechanism by which tumor suppressor Pdcd4 is down regulated during carcinogenesis revealed tumor promoter induced destabilization of the protein by a mechanism involving signaling through Akt, S6kinase and MEK/ERK (Schmid, Jansen et al, submitted). In collaboration with Michele Pagano at NYU, Pdcd4 has emerged as a target for degradation by the ubiquitin ligase betaTRCP (Dorrello et al Science in press). Current research is focused on identifying specific transformation relevant mRNAs whose translation is inhibited by Pdcd4 expression. Tools for drug discovery targeting translation initiation are currently being generated in collaboration with Bruce Shapiro, Stuart LeGrice, Nahum Sonenberg (McGill Univ) and Jim McMahon.
This page was last updated on 2/21/2013.