9.A.1 The Non ABC Multidrug Exporter (N-MDE) Family
The RalA-binding protein 1, RalBP1 or RLIP76 can activate hydrolysis of GTP bound to RAC1 and CDC42, but not RALA. It mediates ATP-dependent transport of S-(2,4-dinitrophenyl)-glutathione (DNP-SG) and doxorubicin (DOX) and is the major ATP-dependent transporter of glutathione conjugates of electrophiles (GS-E) and DOX in erythrocytes. It can cataylze transport of glutathione conjugates and xenobiotics, and may contribute to multidrug resistance. It serves as a scaffold protein that brings together proteins forming an endocytotic complex during interphase and also with CDC2 to switch off endocytosis. It is not clear that drug transport is mediated by RLIP76 alone. Other proteins might play a role.
RalBP1 is identical to the DNP-SG ATPase, a GS-E transporter characterized in erythrocyte membranes (Awasthi et al., 2000). The multidrug resistance-associated protein (MRP1) belonging to the family of the ABC-transporters has also been suggested to be a GS-E transporter in human erythrocytes. Strong expression of RLIP76 is observed in erythrocyte membranes while neither MRP1 nor Pgp were detected in erythrocytes, when the antibodies against MRP1 or Pgp were used (Sharma et al., 2001). In erythrocyte inside-out vesicles coated with antibodies against RLIP76, a dose-dependent inhibition of the ATP-dependent transport of DOX and GS-E, including S-(dinitrophenyl)glutathione (DNP-SG), leukotriene C(4), and the GSH conjugate of 4-hydroxynonenal, was observed with a maximal inhibition of about 70%.
RLIP76 or the Ral binding protein (RalBP-1) was initially proposed as a link between the Ral and Ras pathways. This protein is found ubiquitously from Drosophila to humans (Awasthi et al., 2007). RLIP76 displays inhibitory GTPase activity toward Rho/Rac class G-protein cdc42 which is involved in regulation of cytoskeletal organization, lamellipodia, cell migration and apoptosis via Ras. RLIP76 is also a multispecific transporter of chemotherapeutic agents and glutathione conjugates (GS-E). In human cells RLIP76 accounts for more than two thirds of the transport activity for GS-E and drugs as opposed to the ABC-transporters including MRP1, which account for less than one third of this activity. Evidence is mounting that RLIP76 is a stress-responsive multi-specific, non-ABC transporter which represents an entirely novel link between stress-inducible G-protein signaling, receptor tyrosine-kinase signaling, endocytosis, heat-shock and stress defense pathways, and transport mediated drug-resistance. The expression of RLIP76 is significantly greater in human cancer cells of diverse origin as compared to the non-malignant cells. Inhibition of RLIP76, using antibodies towards a cell surface epitope, or depletion of RLIP76 using either siRNA or anti-sense phosphorothioate oligonucleotides preferentially causes apoptosis in malignant cells. Administration of RLIP76 antibodies, siRNA, or anti-sense oligonucleotides to mice bearing syngeneic B16 mouse melanoma tumors causes rapid and complete regression of tumors.
The reaction believed to be catalyzed by RLIP76 is:
drugs (in) → drugs (out)