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About “Ina L. Urbatsch”

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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. Co-crystal structures of Pgp with the cyclopeptide inhibitors have located these inhibitor binding sites in the center of the membrane bilayer (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.

Role of P-glycoprotein (Pgp) in multidrug resistance of cancers:
Breast 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 other ABC transporters such as 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 MDR 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 is necessary 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 de novo 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 and thus, are directly involved in the regulation of cholesterol homeostasis.