Cell Biology and Biochemistry
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Ina L. Urbatsch, Ph.D.
University of Kaiserslautern, Germany
Department of Cell Biology and Biochemistry
Texas Tech University Health Sciences Center
3601 Fourth Street, STOP 6540
Lubbock, TX 79430
Phone: (806) 743-2700 X279
FAX: (806) 743-2990
ABC Transporters in Cancers, Cystic Fibrosis and Atherosclerosis
ABC transporter Superfamily:
The ATP-binding cassette (ABC) transporter superfamily comprises a large number of membrane spanning transport proteins that extends from bacteria to man. They are associated with many human disorders, including cystic fibrosis, immunodeficiency, retinal degeneration, and defects in lipid and cholesterol metabolic pathways. In addition, several transporters are involved in cancer drug resistance, resulting in poor treatment outcome and increased patient relapse. ABC transporter typically consist of two transmembrane domains (TMDs) and two nucleotide-binding domains (NBDs). The TMDs provide the transport pathway for a particular substrate, and the NBDs power the transport by ATP hydrolysis. Consistent with their functional diversity (e.g., transport of ions, peptides, hydrophobic drugs, lipids, cholesterol) the transmembrane domains exhibit weak similarity, whereas the NBDs are highly conserved. We recently crystallized P-glycoprotein (Pgp), an ABC transporter involved in multidrug resistance of cancers. In this nucleotide-free structure the NBDs are ~30 Å apart, exposing a central cavity formed by the TMDs to the inner leaflet of the membrane and the cytoplasm (Fig. 1A). Comparison of nucleotide-free Pgp with nucleotide-bound structures of other ABC transporters led to an alternating-access model, where the central cavity is accessible to only one side of the membrane at a time. In this model, Pgp switches between inward- and outward-facing conformations, gated by ATP binding, resulting in NBD dimerization and ATP hydrolysis that then reopens the NBD dimer. Still, many questions remain about the sequence of conformational changes (simplified as a two-step oscillation in Fig. 1) leading to the NBD dimerization and reopening of the dimer, the substrate binding and translocation sites, and the mechanisms by which ATP hydrolysis is coupled to substrate translocation.
Figure 1: X-ray structures of nucleotide-free Pgp and nucleotide-bound MsbA. In the inward-facing Pgp structure (left), the NBDs are distant from each other, and two bundles of transmembrane helices surround a cavity for drug binding that is open to the inner membrane leaflet and cytoplasm. In the outward-facing AMPPNP-bound MsbA structure (right) the transmembrane helices have rotated and opened to the outer leaflet. ICLs: intracellular loops; black lines denote the approximate boundaries of the membrane.
Role of P-glycoprotein (Pgp) in multidrug resistance of cancers:
Cancer is a major health problem with one million new cases annually and roughly 370,000 deaths worldwide. Often chemotherapy fails because the patient's tumors either were resistant at the outset of treatment or eventually develop resistance after exposure to the cancer drugs. Several ABC transporters such as Pgp, the Breast Cancer Resistance Protein (BCRP) and the Multidrug Resistance Associated Proteins (MRP) have been implicated in drug resistance. Yet, Pgp is the most prevalent and best-studied cause of multidrug resistance. Its expression in tumors is associated with poor treatment outcome and increased patient relapse. This has made Pgp a therapeutic target since its discovery more than three decades ago. Besides its relevance to cancer treatment, the pharmaceutical industry has also targeted Pgp for its role in multidrug resistance of HIV, epilepsy and psychiatric illnesses. Several inhibitors of Pgp have been tested in clinical trials, but none have been approved for clinical use because of their toxicities and unfavorable interactions with organs involved in drug clearance. Consequently, there is currently great interest in understanding the mechanism by which drugs interact with Pgp, with the aim of developing inhibitors that can be used in combination with cancer drugs to improve the response of tumors to chemotherapeutic agents. For this purpose a thorough understanding of drug/inhibitor interactions with Pgp is essential.
Role of CFTR in Cystic Fibrosis
Cystic Fibrosis (CF) is perhaps the most common fatal genetic disease in the Western world. CF results from deficiencies in the chloride ion transport of epithelial cells that line the lungs, intestines, and other organs. In the lungs this leads to accumulation of thick dehydrated mucus and eventually permanent lung damage and death. Presently, only the symptoms of CF can be treated, and although the average life expectancy for CF patients has improved dramatically to ~40 years, there is no cure yet. CF is caused by mutations in the Cystic Fibrosis Transmembrane conductance Regulator (CFTR) gene. This gene encodes an ABC transporter with two non-identical nucleotide binding domains (NBDs) and two transmembrane domains that harbor the chloride ion channel pore. Opening and closing of the channel are controlled by ATP-driven events at the NBDs and are somehow strictly regulated by phosphorylation of a regulatory (R-) domain that is specific to CFTR. Much remains to be learned about the interactions between the two NBDs, the channel pore, and the R-domain, and how they regulate gating. Such knowledge may provide insight into strategies for increasing channel activity, and thus for modulating ion flux and water movement in cases of disease. The critical enabling information to study assembly and function at the molecular level is the production of purified, highly active protein in sufficient quantities for biophysical studies.
Role of ABCG5 and ABCG8 in Sterol and Cholesterol Transport:
Hypercholesterolemia is a major risk factor for cardiovascular disease. It develops from an imbalance in cholesterol homeostasis, a finely tuned process involving many steps including synthesis, absorption from the diet, trafficking through the body via lipoproteins and excretion through the liver and intestines. Sitosterolemia is a hypercholesterolemia-like disease characterized by significantly elevated plasma levels of plant and shellfish sterols. This genetic disorder also causes hyperabsorption and insufficient excretion of cholesterol, although plasma levels appear normal in half of the cases because de novo biosynthesis of cholesterol is downregulated. The clinical syndromes include the development of tendon and tuberous xanthomas and premature coronary arteriosclerosis due to significantly increased absorption from the diet and decreased excretion of sterols into bile. Sitosterolemia is caused by mutations in the ABCG5 and ABCG8 genes, suggesting that these gene products play a role in the efficient removal of plant sterols and cholesterol (Fig. 2) and thus, are directly involved in the regulation of cholesterol homeostasis.
Figure 2: Putative topology of ABCG5 and ABCG8 with mutations known to cause sitosterolemia. ABCG5 and ABCG8 are half-size ABC transporter each containing six transmembrane a-helices (green and blue cylinders) and a cytoplasmic nucleotide binding domain (NBD). Mutations that cause sitosterolemia are indicated by a red asterix (stop, premature termination; ins, insertion; Δ, deletion). The conserved Walker A, Walker B, and C motifs are shown in cyan circles, and N-glycosylation sites in brown diamonds. Based on Swiss-Prot entries Q9H222 and Q9H221.
The biomechanics of the sterol transport mechanism by ABCG5/G8 is still largely unknown, but can be elucidated from studies of their biochemical properties and their molecular structures. Such information is critical to understand how ABCG5 and ABCG8 regulate cholesterol in the body, and will provide insights into strategies for modulating sterol uptake and excretion in cases of sitosterolemia and more importantly, arteriosclerosis.
1. Kiehlkopf C, Bauer W, and Urbatsch IL. Expressing cloned genes for protein production, purification and analysis. Invited chapter in Green M and Sambrook J, Molecular cloning, 4th edition CSHL Press, July 2012. (This is the Laboratory Manual originally published by Sambrook and Maniatis)
2. Bai J, Swartz DJ, Protasevich II, Brouillette CG, Harrell PM, Hildebrandt E, Gasser B, Mattanovich D, Ward A, Chang G, Urbatsch IL. A gene optimization strategy that enhances production of fully functional P-glycoprotein in Pichia pastoris. PLoS One 2011; 6(8):e22577.
3. Schölz C, Parcej D, Robenek H, Urbatsch IL, Tampé R. Transporter associated with antigen processing (TAP) is modulated by lipids. J. Biol. Chem. 2011; 286(15):13346-56.
4. Tao H, Weng Y, Zhuo R, Chang G, Urbatsch IL, and Zhang Q. Design and Synthesis of Selenazole-Containing Peptides for Co-crystallization with P-Glycoprotein. ChemBioChem 2011; 12(6):863-73.
5. Hoffman AD, Urbatsch IL, Vogel PD. Nucleotide binding to the human multidrug resistance protein 3, MRP3. Protein J. 2010; 29(5):373-9.
6. Johnson BJ, Lee JY, Pickert A, Urbatsch IL. Bile acids stimulate ATP hydrolysis in the purified cholesterol transporter ABCG5/G8. Biochemistry 2010; 49(16):3403-11.
7. Gutmann DAP, Ward A, Urbatsch IL, Chang G and van Veen HW. Understanding polyspecificity of multidrug ABC transporters: closing in on the gap in ABCB1. Trends Biochem Sci. 2010; 49(16):3403-11.
8. Aller SG, Yu J, Ward A, Weng Y, Chittaboina S, Zhuo R, Harrell PM, Trinh YT, Zhang Q, UrbatschIL, and Chang G. Structure of P-glycoprotein Reveals a Molecular Basis for Poly-Specific Drug Binding. Science 2009 Mar 27;323(5922):1718-22.
9. Zehnpfennig B, Urbatsch IL and Galla HJ. Functional reconstitution of human ABCC3 into proteoliposomes reveals a transport mechanism with positive cooperativity. Biochemistry 2009 May 26; 48(20):4423-30.
10. Harvey BH, Lee JY and Urbatsch IL. Sitosterolemia, book chapter in Encyclopedia of Molecular Mechanisms of Disease, Lang, F. (Ed.), ISBN 978-3-540-33445-3, Springer, pp1941-1943 (2009).
11. Lee JY, Urbatsch IL, Senior AE, and Wilkens S. Nucleotide-induced Structural Changes in P-glycoprotein observed by Electron Microscopy. J. Biol. Chem., 2008; 283(9):5769-79.
12. Chloupkova M, Pickert A, Lee, JY, Souza S, Trinh Y, Connelly SM, Dumont ME, Dean M, and Urbatsch IL. Expression of 25 human ABC transporters in the yeast Pichia pastoris and Characterization of the Purified ABCC3 ATPase Activity. Biochemistry 2007; 46(27):7992-8003.
13. Carrier I, Urbatsch IL, Senior AE, Gros P. Mutational analysis of conserved aromatic residues in the A-loop of the ABC transporter ABCB1A (mouse Mdr3). FEBS Lett. 2007; 581(2): 301-8.
14. Wang Z, Stalcup LD, Harvey BJ, Weber J, Chloupkova M, Dumont ME, Dean M, Urbatsch IL. Purification and ATP Hydrolysis of the Putative Cholesterol Transporters ABCG5 and ABCG8. Biochemistry 2006; 45(32), 9929-9939.
15. Tombline G, Urbatsch IL, Virk N, Muharemagic A, White LB, Senior AE. Expression, purification, and characterization of cysteine-free mouse P-glycoprotein. Arch Biochem Biophys. 2006; 445(1):124-8.
16. Delannoy S, Urbatsch IL, Tombline G, Senior AE, Vogel PD. Nucleotide Binding to the Multidrug Resistance P-Glycoprotein as Studied by ESR Spectroscopy. Biochemistry 2005; 44(42):14010-14019.
17. Tombline, G., Bartholomew, L.A., Tyndall G.A., Gimi, K.,Urbatsch, I.L., and Senior, A.E. Properties of P-glycoprotein with mutations in the “catalytic carboxylate” glutamate residues. J. Biol. Chem., 2004; 279: 46518-46526.
18. Tombline G, Bartholomew LA, Urbatsch IL, Senior AE. Combined mutation of catalytic glutamate residues in the two nucleotide binding domains of P-glycoprotein generates a conformation that binds ATP and ADP tightly. J Biol Chem. 2004; 279(30):31212-20.
19. Urbatsch, I.L., Tyndall G.A., Tombline, G., and Senior, A.E. P-glycoprotein catalytic mechanism. Studies of the ADP-vanadate inhibited state. J. Biol. Chem. 2003; 278, 23171 - 23179.
20. Lee, J.-Y., Urbatsch, I.L., Senior, A.E., and Wilkens, S. Projection Structure of P-glycoprotein by electron microscopy: evidence for a “closed” conformation of the nucleotide binding domains. J. Biol. Chem. 2003; 277, 40125-31.