The signaling mechanism of SHetA2 induction of G1 arrest and apoptosis in ovarian cancer

Ovarian cancer is the most lethal of all gynecologic malignancies and has no effective cure, hence there is a need for novel drugs that can prevent and/or treat the disease. One promising drug is SHetA2. However to be able to interpret outcomes of clinical trials testing, it is important to understand how SHetA2 modulates cell physiology. We demonstrated that SHetA2 prevents chemical-induced carcinogenesis, inhibits angiogenesis and cell proliferation, and induces apoptosis. It is essential to understand how the drug is exerting these effects at molecular level. A number of pathways known to be deregulated in ovarian cancer include the Akt and NF-kappaB survival pathways, which are both involved in signaling that contributes to increased proliferation and decreased apoptosis in cancer cells. A main downstream molecule regulated by both pathways is cyclin D1, a rate-limiting factor for G1 cell cycle progression, that integrates all mitogenic signaling stimuli and engages cell cycle machinery. The goals of this dissertation were to evaluate cyclin D1, Akt and NF-kappaB in the mechanism(s) of SHetA2-induced cell cycle arrest and apoptosis, and study the relationship between cell cycle arrest and apoptosis. We report that SHetA2 induces G1 cell cycle arrest and GSK-3beta kinase independent phosphorylation-mediated targeting of cyclin D1 for proteasomal degradation. SHetA2-induced loss of cyclin D1 and G1 arrest were attenuated by a proteasome inhibitor. Although SHetA2 also represses transcription from the cyclin D1 promoter, most likely through targeting NF-kappaB transcription factors, attenuation of this repression was insufficient to interfere with G1 arrest, indicating that cyclin D1 phosphorylation and degradation is the primary mechanism of SHetA2-induced G1 arrest. Attenuation of cyclin D1 loss inhibited G1 arrest, but not apoptosis, indicating that these two cellular outcomes are independent. Akt knockdown did not alter G1 arrest, but did enhance apoptosis by SHetA2, indicating that Akt interferes with SHetA2 induction of apoptosis, but not G1 arrest. These molecular events induced by SHetA2 can be used as biomarkers to study the effects of the drug in animal models and clinical trials.