Cross-modulatory Actions of Cell Cycle Machinery on Estrogen Receptor-alpha Level and Transcriptional Activity in Breast Cancer Cells

Bhatt, Shweta

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Description

Title

Cross-modulatory Actions of Cell Cycle Machinery on Estrogen Receptor-alpha Level and Transcriptional Activity in Breast Cancer Cells

Author(s)

Bhatt, Shweta

Issue Date

2010-08-31T20:30:06Z

Director of Research (if dissertation) or Advisor (if thesis)

Katzenellenbogen, Benita S.

Doctoral Committee Chair(s)

Katzenellenbogen, Benita S.

Committee Member(s)

• Shapiro, David J.
• Bagchi, Milan K.
• Hergenrother, Paul J.

Department of Study

Biochemistry

Discipline

Biochemistry

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Estrogen Receptor signaling
• Breast Cancer
• Skp2 mediated Ubiquitination
• p38MAPK signaling
• Cell cycle regulation in Breast cancer cells.

Abstract

Breast cancer is one of the most highly diagnosed cancers in women and the second largest cause of death of women in United States. The anti-estrogen tamoxifen which blocks gene expression through estradiol bound ERα, and hence the growth stimulatory effects of estradiol, has been widely used for decades for treating patients with ERα positive or hormone dependent breast cancer. Despite its obvious benefits, in as high as 40% of the patients receiving tamoxifen therapy there is an eventual relapse of the disease largely due to acquired resistance to the drug, underlying mechanism for which is rather poorly understood. Elucidating the molecular basis underlying “acquired tamoxifen resistance” and agonistic effects of tamoxifen on cellular growth was the primary focus of my doctoral research. We addressed this by two approaches, one being studying the molecular mechanism for the regulation of cellular levels of ERα so as to prevent its loss in ERα positive or restore its levels in ERα negative breast cancers and second to investigate the role of tamoxifen in modulating the expression of ERα target genes independent of estradiol as a function of its stimulatory or estrogenic effects on breast cancer cell growth. To this end we found a novel mechanism involving the E-3 ubiquitin ligase and major regulator of cell cycle progression, Skp2, in the proteasomal degradation of ERα upon activation by p38MAPK. Interestingly, this regulation of ERα by sequential actions of p38MAPK and Skp2 that was observed specifically during the G1/S transition and S-phases of the cells cycle was found to be critical for the progression of cells through S-phase. The underlying mechanism of increased cell cycle entry due to concerted actions of ERα and Skp2 was later identified to be due to positive regulation of the trans-activation function of ERα by Skp2, most likely due to efficient cycling of the receptor on target gene promoter upon Skp2 driven turnover. The impact of Skp2 on ERα mediated growth of tamoxifen resistant breast cancer cells was far more pronounced in the presence of tamoxifen than estradiol. In order to find the likely mechanism driving Skp2 mediated growth through ERα, in the presence of tamoxifen, we attempted to identify co-regulators unique to tamoxifen bound ERα and identified Oct-3/4, a transcription factor with well-known role in embryonic stem cell growth and differentiation, to be a specific regulator of tamoxifen, not estradiol, mediated gene regulation through ERα. Our work highlights a novel mechanism for the induction of Oct-3/4 in response to tamoxifen, not estradiol, largely due to p38MAPK initiated and Skp2 mediated degradation of Nkx3-1, a co-repressor of Oct-3/4 expression through ERα. Interestingly, further investigation revealed Oct-3/4 transcription factor binding sites were enriched in genes that recruited ERα preferentially or exclusively in the presence of tamoxifen (by >5 fold compared to estradiol). Oct-3/4 induction by tamoxifen appeared to be followed by recruitment of Oct-3/4 to a subset of genes that were targets of tamoxifen bound ERα, a phenomenon that facilitated the recruitment of ERα to its binding sites in close proximity. One such candidate target, NFATC4, was shown to be induced by ERα-Oct-3/4 axis through involvement of Skp2 and p38MAPK, in the presence of tamoxifen, not estradiol. Interestingly, Oct-3/4 was found to significantly impact on tamoxifen driven growth of breast cancer cells, largely through its actions on ERα target gene expression in response to tamoxifen. Consistent with this, Oct-3/4 protein expression was much elevated in MCF-7-Tamr cells (tamoxifen resistant derivatives) compared to parental MCF-7 cells. Taken together, these findings reveal a unique mechanism underlying the regulation of cellular levels of ERα in breast cancer cells as well as the specific role of tamoxifen in regulating ERα target gene expression independent of estradiol thereby promoting growth of breast cancer cells. These pathways we believe offer new targets for drug discovery to circumvent tamoxifen resistance in breast cancer thereby making tam-resistant tumors amenable to therapy.

Graduation Semester

2010-08

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Copyright 2010 Shweta Bhatt

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