Graduate Student (2008-present)
Ph.D. Candidate in Cellular and Molecular Biology
The role of genomic targets of SF-1 and Beta-catenin in adrenal homeostasis and cancer.
Studies in the Hammer laboratory, focused on adrenal gland development and homeostasis, have made significant contributions to our understanding of adrenocortical carcinoma. Genes that have been shown to be critical in adrenal development have also been shown to be perturbed in tumorigenesis. An important mediator of normal growth and homeostasis in the adrenal gland is the orphan nuclear receptor SF-1. This transcription factor regulates steroidogenesis in differentiated adrenal tissue but also is essential for adrenal development. Proper dosage of SF-1 is critical for adrenal homeostasis as loss of Sf-1 in mice results in aplasia of the adrenal gland and adrenals of Sf-1 heterozygotes are reduced in size (Bland, Fowkes et al. 2004). Excessive Sf-1 promotes adrenal dysplasia and tumorigenesis in mice (Doghman, Karpova et al.). Mouse models of adrenal cancer correlates to what is seen in human patients. Upregulation of SF-1 resulting from gene amplification and overexpression is frequently observed in childhood adrenocortical tumors (Figueiredo, Cavalli et al. 2005; Pianovski, Cavalli et al. 2006). In vitro, SF-1 overexpression in adrenocortical carcinoma increases cell proliferation by mechanisms that affect both apoptosis and cell cycle progression (Doghman, Karpova et al.). However, the identity of all the SF-1 target genes that mediate these effects is unclear. Additionally, it remains largely unknown how SF-1 promotes distinct transcriptional programs to mediate the divergent functions of proliferation vs. differentiation.
Another pathway intimately involved in both development and oncogenesis in multiple organ systems is the canonical Wnt signaling pathway. Constitutive activation of Wnt signaling is well known to be an initiator of transformation and tumorigenesis in multiple organ types (Polakis). Under normal circumstances, the absence of an extracellular Wnt ligand promotes the Axin/APC/GSK3Beta complex to target the soluble pool of Beta-catenin to the proteasome for degradation. In the presence of a canonical Wnt signal, the Axin/APC/GSK3Beta complex dissociates from Beta-catenin, allowing stabilized Beta-catenin to accumulate and translocate to the nucleus. Beta-catenin can then interact with the DNA-binding transcription factors of the TCF/LEF family to modulate the expression of genes involved in regulating cell proliferation, cell fate and cell renewal, dependent upon the cellular context.
Beta-catenin also modulates transcription of target genes through TCF/LEF-independent mechanisms, namely by interacting with other transcription factors such as members of nuclear receptor superfamily (Mulholland, Dedhar et al.). The Hammer laboratory was one of the first to detail the interaction between the nuclear receptor Sf-1, and Beta-catenin (Gummow, Winnay et al. 2003). Synergistic association of Sf-1 and Beta-catenin is known to regulate the expression of Dax1 and inhibin, two factors that regulate pluripotency and cell fate (Gummow, Winnay et al. 2003; Mizusaki, Kawabe et al. 2003). Interestingly, in the adrenal gland Beta-catenin is uniquely stabilized in the Sf-1 positive subcapsular region (Kim, Reuter et al.), the precise location of Dax1 and inhibin expression in adrenal-specific progenitor cells. This suggests a potential role for Beta-catenin in adrenal stem and progenitor cell biology. Lending further support to this hypothesis, modeling of Wnt loss/gain of function in mice has yielded the expected results of adrenal failure and tumorigenesis, respectively (Kim, Reuter et al.). Profiling studies done in collaboration with Dr. Tom Giordano have confirmed a major role of Wnt signaling in human adrenocortical carcinoma (ACC). Both familial and sporadic ACCs exhibit dysregulation of the canonical Wnt pathway. Abnormal accumulation of stabilized Beta-catenin results from loss-of-function mutations in APC or activating mutations in Beta-catenin itself (Giordano, Kuick et al.). However, the adrenal specific targets of Beta-catenin that mediate oncogenic events in humans are not well characterized.
My initial project in the Hammer Lab continued an ongoing characterization of a mouse model studying the consequence of Beta-catenin stabilization in Sf-1-positive cells through targeted loss of APC, a member of the complex that is responsible for Beta-catenin turnover. This APC knockout mouse progresses from adrenal dysplasia to adenoma and ultimately adrenocortical carcinoma. My thesis is aimed at determining novel whole genome targets of Wnt signaling involved in both adrenocortical homeostasis and cancer. We are primarily interested in determining specific transcriptional targets that are engaged by SF-1 and Beta-catenin. We will utilize two approaches that include both mouse and human studies. Subsequently, we aim to define the role of a subset of these genes in adrenocortical tumorigenicity in vitro, using a variety of molecular and cellular based approaches in cell culture systems. Ultimately, mouse xenografts models will be employed to evaluate the role of selected genes in tumorigenesis in vivo.
In parallel to adrenal cancer studies, we will elucidate Beta-catenin and Sf-1 target genes involved in adrenal specific stem and progenitor cell biology. In addition to Dax1 and inhibin, we predict classic TCF/LEF-mediated Beta-catenin and unique Sf-1 target genes to be involved in pluripotency and fate decisions of subcapsular cell populations. We will evaluate the role of target genes obtained from this study in the context of stem and progenitor cell biology and assess whether they are perturbed in adrenal cancer. Such studies will employ methods described above, as well as utilizing targeted gene knockouts in mouse models to investigate their role in adrenal homeostasis and adrenal tumorigenesis in vivo.
These studies are expected to yield a wealth of information on the gene expression programs driven by Beta-catenin and SF-1 that dictate adrenal homeostasis and will identify drivers of adrenal tumorigenesis. This will not only enrich our understanding of stem and progenitor cell biology, but will also inform future development of therapeutics for the treatment of adrenal cancer.
- Bland, M. L., R. C. Fowkes, et al. (2004). "Differential requirement for steroidogenic factor-1 gene dosage in adrenal development versus endocrine function." Mol Endocrinol 18(4): 941-952.
- Doghman, M., T. Karpova, et al. (2007). "Increased steroidogenic factor-1 dosage triggers adrenocortical cell proliferation and cancer." Mol Endocrinol 21(12): 2968-2987.
- Figueiredo, B. C., L. R. Cavalli, et al. (2005). "Amplification of the steroidogenic factor 1 gene in childhood adrenocortical tumors." J Clin Endocrinol Metab 90(2): 615-619.
- Giordano, T. J., R. Kuick, et al. (2009). "Molecular classification and prognostication of adrenocortical tumors by transcriptome profiling." Clin Cancer Res 15(2): 668-676.
- Gummow, B. M., J. N. Winnay, et al. (2003). "Convergence of Wnt signaling and steroidogenic factor-1 (SF-1) on transcription of the rat inhibin alpha gene." J Biol Chem 278(29): 26572-26579.
- Kim, A. C., A. L. Reuter, et al. (2008). "Targeted disruption of beta-catenin in Sf1-expressing cells impairs development and maintenance of the adrenal cortex." Development 135(15): 2593-2602.
- Mizusaki, H., K. Kawabe, et al. (2003). "Dax-1 (dosage-sensitive sex reversal-adrenal hypoplasia congenita critical region on the X chromosome, gene 1) gene transcription is regulated by wnt4 in the female developing gonad." Mol Endocrinol 17(4): 507-519.
- Mulholland, D. J., S. Dedhar, et al. (2005). "Interaction of nuclear receptors with the Wnt/beta-catenin/Tcf signaling axis: Wnt you like to know?" Endocr Rev 26(7): 898-915.
- Pianovski, M. A., L. R. Cavalli, et al. (2006). "SF-1 overexpression in childhood adrenocortical tumours." Eur J Cancer 42(8): 1040-1043. Polakis, P. (2007). "The many ways of Wnt in cancer." Curr Opin Genet Dev 17(1): 45-51.