Date of Award

Spring 5-7-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Biochemistry & Molecular Biology

First Advisor

Dr. Michael G. Brattain

Second Advisor

Dr. Sanjib Chowdhury

Abstract

There is extensive evidence for the role of aberrant cell survival signaling mechanisms in cancer progression and metastasis. Akt acts as a key signaling node that bridges oncogenic receptors to many essential pro-survival cellular functions, and is perhaps the most commonly activated signaling pathway in human cancer. Akt has three isoforms, Akt1, 2 and 3. Variable phenotypic differences are observed following the genetic inactivation and/or removal of the Akt isoforms, which suggests that the isoforms have distinct non-redundant functional characteristics despite sharing a high level of structural homology and similar mechanisms of activation.

The major goal of the work presented in this dissertation was to identify the role of Akt isoforms on cell survival, establishment and/or maintenance of metastasis in colorectal cancer (CRC). Although there is an increase in the 5-year survival rate of CRC during early stages, the progress in the survival rate during the metastatic stage is still dismal, suggesting the need to develop anti-metastatic therapy. The work presented in this dissertation has led to the identification of Akt2, among Akt isoforms, as a major player for establishment and/or maintenance of metastasis. shRNA-mediated knockdown of Akt2, not Akt1, causes reduction in metastatic burden in CRC. We show that loss of Akt2 upregulates Metastasis Suppressor 1 (MTSS1) and inhibits the expression of anti-apoptotic genes, XIAP and survivin thus inhibiting cell survival which in turn could lead to reduction in metastatic potential of the cells. It has been shown that activated Akt stabilizes XIAP by S87 phosphorylation, leading to survivin/XIAP complex formation, caspase inhibition and cytoprotection of cancer cells. Extensive drug development efforts and clinical evaluations are underway targeting the aberrant cell survival properties associated with PI3K/Akt signaling in regulating cancer progression and metastasis. Inhibition of Akt activation by small molecule kinase inhibitors is an attractive candidate for targeting aberrant cell survival associated with malignant progression and metastasis and could be effective in the treatment of CRC. In this dissertation work, we have used a kinase inhibitor of Akt, MK-2206, to inhibit phosphorylation of Akt. We provide novel mechanistic insights on MK-2206-mediated cell death. MK-2206 showed an anti-tumorigenic role and led to a dual mechanism of cell death by inhibiting XIAP and survivin and by inducing Apoptosis Inducing Factor.

Additionally, strategies to inhibit Akt2 as opposed to the other two isoforms may provide a therapeutic approach for treatment of metastases. A negative correlation between Akt2 and MTSS1 in human primary colorectal cancer samples might be useful in identification of metastatic patients.

The work presented in this dissertation may assist in understanding whether loss of Akt2 could be a mechanism of increasing cell death, thus leading to reduction in metastasis. Additionally, this work provides a new paradigm for MK-2206-mediated control of aberrant cell survival associated with IGF1R-dependent CRC that may offer new targets for enhancing cell death in cancer cells.

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