Emerging treatments in lung cancer - targeting the RLIP76 molecular transporter

A potential function of RLIP76 in the ovarian corpus luteum

Ral interacting protein of 76 kDa (RLIP76) is multifunctional protein localized and distributed in the plasma membrane, cytosol, and nucleus of the cell. In tumorigenesis, RLIP76 emerges as a common feature for the solid tumor growth. RLIP76 is ubiquitously expressed in various tissues including the ovary. Interestingly, the similar physiological events in obtaining an adequate supply of nutrient by gaining access to the host vascular system are required either for corpus luteum formation or tumor development. In addition, the identical angiogenesis modulators were found in neoplastic and normal ovaries. Our previous study involving RLIP76−/− mice implanted with melanoma or carcinoma cell conclusively demonstrated that RLIP76 is necessary for angiogenesis and neovascularization of primary solid tumors. RLIP76 plays an essential role in tumor angiogenesis through the regulation of pro-angiogenic factors such as vascular endothelial growth factor (VEGF) and hypoxia-inducible factor-1 (HIF-1). In certain previous studies, those pro-angiogenic factors were found significantly to be upregulated during the corpus luteum formation. To that, the following review will discuss the likelihood of RLIP76 role in ovarian corpus luteum.

RLIP76 Inhibition: A Promising Developmental Therapy for Neuroblastoma

Refractory and relapsed neuroblastoma (NB) present with significant challenges in clinical management. Though primary NBs largely with wild-type p53 respond well to interventions, dysfunctional signaling in the p53 pathways in a MYCN oncogene driven background is found in a number of children with NB. The p53-mutant NB is largely unresponsive to available therapies and p53-independent targeted therapeutics represents a vital need in pediatric oncology. We analyzed the findings on mercapturic acid pathway (MAP) transporter RLIP76, which has broad and critical effects on multiple pathways as essential for carcinogenesis, oxidative stress and drug-resistance, is over-expressed in NB. RLIP76 inhibition by antibodies or depletion by antisense causes apoptosis and sensitization to chemo-radiotherapy in many cancers. In addition, recent studies indicate that the interactions between p53, MYCN, and WNT regulate apoptosis resistance and protein ubiquitination. RLIP76 and p53 interact with each other and colocalize in NB cells. Targeted depletion/inhibition of RLIP76 causes apoptosis and tumor regression in NB irrespective of p53 status. In the present review, we discuss the mechanisms and the role of RLIP76 in oxidative stress, drug-resistance and clathrin-dependent endocytosis (CDE), and analyze the molecular basis for the role of RLIP76 targeted approaches in the context principal drivers of NB pathogenesis, progression and drug-resistance. The evidence from RLIP76 studies in other cancers, when taken in the context of our recent RLIP76 focused mechanistic studies in NB, provides strong basis for further characterization and development of RLIP76 targeted therapies for NB.

RLIP76 Targeted Therapy for Kidney Cancer

Despite recent improvements in chemotherapeutic approaches to treating kidney cancer, this malignancy remains deadly if not found and removed at an early stage of the disease. Kidney cancer is highly drug-resistant, which may at least partially result from high expression of transporter proteins in the cell membranes of kidney cells. Although these transporter proteins can contribute to drug-resistance, targeting proteins from the ATP-binding cassette transporter family has not been effective in reversing drug-resistance in kidney cancer. Recent studies have identified RLIP76 as a key stress-defense protein that protects normal cells from damage caused by stress conditions, including heat, ultra-violet light, X-irradiation, and oxidant/electrophilic toxic chemicals, and is crucial for protecting cancer cells from apoptosis. RLIP76 is the predominant glutathione-electrophile-conjugate (GS-E) transporter in cells, and inhibiting it with antibodies or through siRNA or antisense causes apoptosis in many cancer cell types. To date, blocking of RLIP76, either alone or in combination with chemotherapeutic drugs, as a therapeutic strategy for kidney cancer has not yet been evaluated in human clinical trials, although there is considerable potential for RLIP76 to be developed as a therapeutic agent for kidney cancer. In the present review, we discuss the mechanisms underlying apoptosis caused by RLIP76 depletion, the role of RLIP76 in clathrin-dependent endocytosis deficiency, and the feasibility of RLIP76-targeted therapy for kidney cancer.

RLIP76 regulates HIF-1 activity, VEGF expression and secretion in tumor cells, and secretome transactivation of endothelial cells

This study was undertaken to reveal the mechanisms by which RLIP76 regulates endothelial cell angiogenic responses. RLIP76 is an effector of the angiogenic modulator, R-Ras. RLIP76 is overexpressed in many tumors, required for tumor angiogenesis, and blockade of RLIP76 results in tumor regression in multiple models. We report here that RLIP76 was required for expression and secretion of vascular endothelial growth factor (VEGF) in carcinoma and melanoma cells. Conditioned medium derived from RLIP76-depleted tumor cells, but not control knockdown cells, could not stimulate proliferation, migration, or Matrigel cord formation in endothelial cell cultures, which indicates that RLIP76 regulates angiogenic components of the tumor cell secretome. Recombinant VEGF added to conditioned medium from RLIP76-knockdown tumor cells restored these endothelial cell functions. Transcriptional activity of hypoxia-inducible factor 1 (HIF-1), which drives VEGF expression, was blocked in RLIP76-depleted tumor cells. RLIP76 was required for PI3-kinase activation, known to regulate HIF-1, in these cells. However, HIF-1α expression and nuclear localization were unaffected by RLIP76 knockdown, which suggests that RLIP76 regulates HIF-1 at the functional level. Thus, RLIP76 regulates tumor cell transactivation of endothelial cells via control of VEGF expression and secretion, providing a new important link in the mechanism of tumor cell induction of angiogenesis.