Research Assistant Professor, Internal Medicine, College of Medicine - Phoenix
The research focus of our lab is to understand the mechanism(s) by which recepteur d’ origine nantais (RON), a tyrosine kinase receptor for macrophage stimulating protein (MSP) is regulated and promotes cancer metastasis. A comprehensive understanding of the RON kinase signaling has the potential to facilitate development of novel therapeutics for cancer metastasis in a subset of patients where RON kinase is overexpressed. Our laboratory utilizes pancreatic, colon and breast cancer model systems to delineate the role of RON tyrosine kinase receptor.
1. Regulation of RON tyrosine kinase gene expression: MSP is the only known ligand for RON which belongs to the MET gene family. Normal epithelial cells and benign lesions (adenomas and papillomas) express relatively low levels of RON. However, RON is over-expressed in tumors and plays an important role in tumor metastasis than tumor initiation. We have shown that MSP promotes invasion of RON positive cancer cells. Our work led to the identification of the basal regulatory elements that are required for RON gene expression. In collaboration with Dr. Jim Freeman’s group we have identified a cross-talk between tumor suppressor TGF-beta and oncogenic RON kinase pathway through regulation of the smad family DNA binding proteins. We are continuing these studies to further delineate the regulation of RON tyrosine kinase gene expression.
2. Analysis of pharmacological agents to block RON expression/activity: Hypoxia inducible factor-1 alpha (HIF-1α) has emerged as an important transcription factor in carcinogenesis. Multiple studies of HIF-1α and cancer have shown a significant association between HIF-1α over-expression and poor prognosis coupled to increased patient mortality. These results suggested that HIF-1α is a negative prognostic factor in cancer progression. The molecular targets of HIF-1α that contribute to tumorigenesis are under intensive investigations. We reported RON tyrosine kinase as a target of HIF-1α mediated cancer cell invasion. Using HIF-1α inhibitor, echinomycin as well as siRNA approaches we demonstrated that inhibition of HIF-1α expression/activity blocks RON mediated invasion of carcinoma cells. Rescue of RON expression in the HIF-1α knock-down cells rescued the invasive phenotype of carcinoma cells confirming the role of HIF-1α/RON kinase axis in cancer cell invasion. Our lab is currently evaluating the efficacy of LCRF-0004 and its analogs in blocking RON mediated cancer metastasis.
3. Analysis of natural products and their analogs to inhibit RON expression/activity: Natural products such as Curcumin and Sulforaphane (SFN) have been identified as potential chemoprevention agents in both in vitro and in vivo cancer model systems. Our research efforts led to the identification of NFkB p65 subunit as a regulator of RON gene expression. We have demonstrated that Curcumin inhibits p65 expression/activity and abrogates RON mediated invasion of carcinoma cells. SFN activates nuclear factor-erythroid 2-related factor 2 (Nrf2), a transcription factor that plays a key role in anti-oxidant response element (ARE)-mediated gene expression. Nrf2 is sequestered in the cytoplasm by an actin binding protein, kelch-like ECH associating protein 1 (keap1), which upon activation by SFN dissociates from keap1, trans-locates to the nucleus and activates target gene expression by binding to the ARE sites. Immuno-histochemical analysis on tumors with normal controls indicated high Nrf2/low RON expression in normal tissues and low Nrf2/high RON expression in tumors. Further, our data has shown that in SFN treated cancer cells Nrf2 is stabilized and binds a distinct cis-regulatory element not the classical ARE element and inhibits RON gene expression and invasion of carcinoma cells. The role of Nrf2 in the induction of phase I and phase II detoxifying enzymes is well-documented. However, our results identified a novel function for Nrf2 i.e. inhibition of an oncogene that plays a role in cancer cell invasion. Nrf2 inducer, Tecfidera is in clinic for the treatment of recurring multiple sclerosis. We are currently evaluating the efficacy of Tecfidera in blocking RON mediated cancer metastasis.
4. RON/HIF-1α Pathway in Cancer Cell Invasion and Metastasis: HIF-1α expression/activity is hypoxia regulated i.e. HIF-1α is stabilized under hypoxia (1-2% O2) and degraded through a proteasome-dependent pathway under normoxia (20% O2). However, carcinoma cells which exhibit invasive phenotype express steady state HIF-1α even under normoxia. We have reported co-expression of RON and HIF-1a in triple negative breast cancer cells. Treatment of RON/HIF-1α positive breast carcinoma cells with PI3K inhibitor LY 294002 or chemoprevention agent SFN reduced AKT activity and HIF-1α protein expression. Similarly, RON expression knock-down in the same invasive cancer cells led to the inhibition of AKT activation and HIF-1α protein expression. This data suggested that RON through PI3K/AKT pathway induces HIF-1α expression and we reported HIF-1α regulates RON by directly binding to HRE site on the RON promoter suggesting the existence of a positive stimulatory loop between RON and HIF-1α, thus providing continuous oncogenic signaling to carcinoma cells. We propose to identify HIF-1α molecular targets that contribute to RON kinase mediated tumorigenesis and if blocking RON/HIF-1α axis abrogates cancer cell progression.