Current Research
Thioredoxin (Trx) Signaling in Cancer
Cancer cells in a stressed environment depend on Trx for protection against stress-disregulated redox signaling. The biological properties of thioredoxin (Trx1) rely largely on reduction-oxidation (redox) activity, which is the ability to transfer 'reducing equivalents' to disulfide groups in target proteins. Expressed ubiquitously in cytosol, Trx1, under conditions of oxidative stress, either translocates to the nucleus or is secreted extra-cellularly where it is thought to mediate some inflammatory effects. Trx-induced growth includes the provision of reducing equivalents for DNA synthesis, activation of transcription factors that regulate cell growth, increasing sensitivity of cells to other cytokines, growth factors and inhibition of apoptosis. Thus, key biological activities of Trx1 that are applicable to human disease can be categorized as antioxidant, growth promoting, anti-apoptotic and inflammation modulating. It is, therefore, not surprising that elevated expression of Trx1 is associated with increased proliferation of tumor cells, inhibition of apoptosis, aggressive tumor growth and decreased patient survival. Processes that contribute to the cancer promoting effects of Trx1 include: (i) promoting growth factor expression in the tumor, (ii) increasing the concentration and activity of hypoxia-inducible factor 1α(HIF-1α), which leads to increased synthesis of vascular endothelial growth factor (VEGF), (iii) inhibiting apoptosis and (iv) inhibiting tumor-suppressor proteins such as PTEN [phosphatase and tensin homologue (mutated in multiple advanced cancers)]. Moreover, because many anti-cancer agents act by triggering apoptosis, the anti-apoptotic effects of Trx1 reduce the effectiveness of chemotherapy strategies.
We have shown that Trx1 is overexpressed in many human primary tumors where it is associated with aggressive tumor growth, decreased spontaneous apoptosis and, in colon cancer and non-small cell lung cancer, with decreased patient survival. Our lab focuses its research on thioredoxin signaling and regulation in cancer and the development of therapeutics that will target thioredoxin with a high degree of specificity.
Redox Signaling in Drosophila melanogaster as a Model of Cancer Cell Survival Pathways
One of our research goals is to identify genes responsible for redox-dependent signaling in the fruit fly, Drosophila melanogaster, with the ultimate goal of identifying novel drug targets in human cancer. To accomplish this goal, we are undertaking a genetic screen that, when disrupted, increases or decreases the survival of flies with compromised thioredoxin reductase function. This classical "forward" genetic approach will allow us to survey most of the Drosophila genome and "let the animal tell us what is important" in this pathway. Once a target gene is identified in the fly, and confirmed to be involved in redox signaling, we confirm its role in redox signaling in mammalian systems.
Regulation in Cancer
The heterodimeric transcription factor, hypoxia-inducible factor 1 (HIF-1), has been described as the 'master regulator' of the response to low oxygen or hypoxia. HIF-1α enables the adaptation of tumor cell to hypoxia by increasing the transcription of genes that promote tumor growth, vascularization and anaerobic glycolysis. HIF-1α, the oxygen-regulated subunit of HIF-1, is believed to be a positive factor in tumor growth and its increased/constitutive expression has been correlated to poor patient prognosis. Thus, inhibiting HIF-1α offers an attractive strategy for cancer therapy.
The levels of HIF-1α protein are controlled by an oxygen-dependent mechanism and, in the presence of oxygen, the levels of HIF-1α are kept low by proteolytic degradation. In hypoxia, however, the levels of HIF-1α increase due to inhibition of proteolysis and continued HIF-1α synthesis.
Mechanisms of HIF-1α Regulation through Degradation
The hypoxia-inducible factor (HIF-1) transcription factor is the master regulator of the hypoxia response. Our research interests lie in understanding the regulation of the hypoxic response including, but not limited to, the proteolytic regulation of HIF-1/2α. Under aerobic conditions, HIF-1/2α is degraded by the pVHL E3 ligase complex through a well-characterized mechanism. However, there has been increasing evidence of pVHL-independent pathways of HIF-1/2α degradation.
Note: Image links to larger image in a pdf.
We recently described the hypoxia-associated factor (HAF) as an E3 ligase for HIF-1a that does not degrade HIF-2α. HAF mediates an oxygen-independent HIF-1α degradation pathway that is analogous to that of the oxygen-dependent pathway mediated by pVHL. However, the HAF pathway functions under conditions in which pVHL is inactive or insufficient -- such as during hypoxia or persistent growth factor activation. Intriguingly, HAF is overexpressed in many cancer types. Currently, we are delineating this fascinating new mechanism of HIF-α isoform-specific degradation by addressing particular issues such as the role of HAF under normal growth conditions and in cancer, and the mechanisms by which HAF activity may be regulated.
Mechanisms of HIF-1α Regulation through Synthesis
The cellular response to hypoxia involves the slowdown of processes that require oxygen and energy while simultaneously inducing the expression of specific genes that promote adaptation and survival. One universal response of cells to hypoxia is a rapid and substantial decrease in the rate of global protein synthesis mediated primarily by two mechanisms -- the unfolded protein response (UPR) and the mammalian target of rapamycin (mTOR).
Note: Image links to larger image in a pdf.
The mechanism(s) responsible for the maintenance or increase in HIF-1α levels in hypoxia is entirely unknown, despite the global inhibition of protein translation. We plan to perform a genome-wide siRNA screen to identify components of the new pathway regulating HIF-1α levels. Understanding the mechanisms that contribute to tumor growth in hypoxia, involving both HIF-1α and other stress proteins, may provide new drug targets and therapeutic strategies for treating solid tumors.
Development of Inhibitors to the PI3K/Akt/mTOR Signaling Pathway
The PI3K/Akt/mTOR pathway has been implicated in the growth, survival and resistance of tumors to conventional chemo and radiation therapy. Treatments in the laboratory with agents specifically targeted to the PI3K and Akt enzymes have stopped the growth of tumors and augment traditional cancer treatments. Over the last five years, we have been involved in the development of a highly selective inhibitor of PI3K, PX-866, currently in clinical trials. We are investigating issues involved with late stage development of PX-866, such as defining and managing on-target effects in normal physiology resulting from inhibition of the PI3K and identifying molecular markers that serve to predict the best response to the drug. Additionally, we are screening newly-developed molecules for activity against the PI3K/Akt/mTOR pathway to find new agents that may be developed into future cancer therapeutics.
Note: Image links to larger image in a pdf.
