Transport of glutathione conjugates and chemotherapeutic drugs by RLIP76 (RALBP1): A novel link between G‐protein and tyrosine kinase signaling and drug resistance

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Abstract
Our studies have shown that RLIP76 (RALBP1), a 76 kDa Ral‐binding, Rho/Rac‐GAP and Ral effector protein, is a novel multispecific transporter of xenobiotics as well as GS‐Es. Like previously characterized ABC transporters, it mediates ATP‐dependent transport of structurally unrelated amphiphilic xenobiotics and displays inherent ATPase activity, which is stimulated by its substrate allocrites. It does not have significant sequence homology with ABC transporters and differs from the ABC transporters in several other important aspects, including (i) lack of any close homologs in humans, (ii) lack of a classical Walker domain, (iii) integral membrane association without clearly defined transmembrane domains and (iv) its role as a direct link to Ras/Ral/Rho and EGF‐R signaling through its multifunctional nature, including GAP activity, regulation of exocytosis as well as clathrin‐coated pit–mediated receptor endocytosis. Its multifunctional nature derives from the presence of multiple motifs, including a Rho/Rac GAP domain, a Ral effector domain binding motif, 2 distinct ATP‐binding domains, a H+‐ATPase domain, PKC and tyrosine kinase phosphorylation sites and the ability to undergo fragmentation into multiple smaller peptides which participate as components of macromolecular functional complexes. One of the physiologic functions of RLIP76 is regulation of intracellular concentration of the electrophilic intermediates of oxidative lipid metabolism by mediating efflux of GS‐E formed from oxidative degradation of arachidonic acid, including leukotrienes and the 4HNE‐GSH conjugate. RLIP76‐mediated transport of amphiphilic chemotherapeutic agents such as anthracyclines and vinca alkaloids as well as GS‐E produced during oxidative metabolism places this multifunctional protein in a central role as a resistance mechanism for preventing apoptosis caused by chemotherapeutic agents and a variety of external/internal stressors, including oxidative stress, heat shock and radiation.