Date

March 2012

Document Type

Thesis

Degree Name

M.S.

Institution

Oregon Health & Science University

Abstract

Axin1 is a scaffold protein that regulates multiple signaling pathways including Wntβ-Catenin, SAPK/JNK, TGFβ, and p53. Axin1 also coordinates a degradation complex for c-Myc. c-Myc is a proto oncogene that belongs to a family of basic helix loop helix transcription factors and it is overexpressed in many cancers. This thesis seeks to investigate the role of Axin1 in regulating the c-Myc oncoprotein in leukemia and also provides insight into the role of Axin1 Exon 7 in binding to c-Myc and regulating its transcriptional activity. Axin1 has been characterized as a tumor suppressor and it is mutated in hepatocellular, colorectal and other cancers. c-Myc is known to be overexpressed in leukemia. Since Axin1 coordinates a destruction complex for c-Myc, I investigated the role of Axin1 in leukemia. Here, I show that increased Axin1 protein levels are able to decrease c-Myc protein levels in the U937 leukemia cell line and I also show that the U937 leukemia cell line preferentially expresses the variant 2 isoform of Axin1, which is less effective in regulating c-Myc. I also sequenced Axin1 in 26 leukemia patient samples and found many sequence variations of Axin1 and I show that Axin1 protein sequence changes are rare in leukemia. We have previously shown that Axin1 coordinates a degradation complex for c-Myc consisting of Gsk3β, PP2A, Pin1, Axin1 and c-Myc. Preliminary protein truncation experiments suggested that Exon 7 of Axin1 is important for Axin1 binding to c-Myc. Based on these results, I investigated the role of Axin1 Exon 7 in binding and regulating c-Myc. Here, I show that Exon 7 of Axin1 contains a domain that is conserved across species and deletion of this conserved domain decreases Axin1 binding to c-Myc. Furthermore, I also show that Axin1 in which Exon 7 is deleted is specifically defective in regulating c-Myc while maintaining its activity towards β-Catenin. These findings further expand our understanding of Axin1’s role in regulating c-Myc in cancer and they provide tools that may help in discriminating Axin1’s role in different pathways.

Identifier

doi:10.6083/M4M906NH

Division

Program in Cancer Biology

School

School of Medicine

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