*1.2. Mammalian ABC Multidrug Transporters*

Most, if not all, eukaryotic ABC transporters function as unidirectional exporters and use ATP consumption to drive transport. Some have been implicated in uptake processes as well, although this remains controversial [39,99]. Importantly, ectopic or dysregulated overexpression of certain ABC transporters often contributes to or promotes MDR phenomena in several but not all human cancer types [2,10,11,100–102]. Based on sequence similarity and domain arrangement, mammalian ABCs fall into two major groups, referred to as type I and type II exporters [12], although recently, a new classification has been proposed [18]. At least three MDR exporters have been linked to MDR in human tumors, including P-glycoprotein (P-gp/MDR1/ABCB1) [103–106], MRP1 (MDR-associated protein 1/ABCC1) [107] and BCRP (Breast Cancer Resistance/ABCG2) [108–110]. All three share rather broad and partially overlapping drug specificity [14,111]. Most substrates of ABCB1 and ABCG2 are cationic hydrophobic compounds [112], which may probably be expelled directly from the lipid phase as originally proposed by the "hydrophobic vacuum cleaner" model [105,113,114] or from the outer membrane leaflet by a floppase-like

function [5,14,19,109,111,115,116]. A similar mechanism has been proposed in ABCA1 for the cholesterol loading of apolipoprotein A-I (apoA-I) [5,116,117]. P-gp, MRP1 and ABCG2 are normally residing in the plasma membrane of epithelial organ linings (such as liver, intestine, blood–brain barrier, placenta and mammary epithelium) [118,119]. Their physiological tasks include vital roles in cellular detoxification and in organ protection by excretion of toxic compounds or xenobiotic molecules [120]. Substrates are amphipathic, lipid-soluble compounds of extremely diverse chemical spaces, ranging from small molecules to bulky lipophilic cations and conjugated organic anions [14,112,121].

ABCB1 or P-glycoprotein (P-gp) or MDR1 (encoded by the *MDR1* gene) was identified from a multidrug-resistant KB carcinoma cell line [103,122,123] as the first mammalian type I exporter class. P-gp is expressed on apical membranes of epithelial cells in colon, small intestine, liver, placenta, kidney, gut, pancreatic, bile duct and blood–brain barrier [124,125]. Homozygous P-gp knock-out mice showed a 100-fold increase in drug (ivormectin) permeability at the blood–brain barrier, which led to the discovery of its physiological role in organ protection [126–128]. ABCB1 transports a diverse array of substances, including chemotherapeutic drugs, steroids, several phospholipids, fluorescent dyes, peptides and ionophores [113]. Furthermore, ABCB1 is believed to function as a floppase-like lipid transporter [43,129]. Despite huge therapeutic promises, numerous clinical studies on ABCB1 inhibitors or reversal agents [130–138] showed marginal to no benefits, thus rendering attempts to translate P-gp inhibitors into the clinic so far futile efforts [111,139]. Of note, despite an almost highly conserved primary sequence identity, the closest P-gp homologue, ABCB4/MDR2, has not been implicated in drug transport or cancer MDR [140], as it resides in the canalicular and appears to have a restricted substrate spectrum limited to phosphatidycholine-related phospholipids in the canalicular membrane.

ABCC1 or MRP1 or multidrug resistance-associated protein1 (encoded by the *ABCC1* gene) was discovered as the second member of MDR exporters, cloned from a multidrugresistant P-gp-negative human lung cancer cell line with doxorubicin tolerance [107]. ABCC1 is mostly on the basolateral surface of polarized epithelial cells, with moderate to high abundance in the gastrointestinal tract, kidney, bladder, testis, ovary, endometrium, adipose tissues, appendix and tonsils. Low-level expression is found in brain, lung, liver, gall bladder, pancreas, bone marrow and skin [141–143] to excrete a variety of endogenous substances, including glutathione, prostaglandins, C4-leukotrienes glucuronide conjugates, sulfate conjugates, heavy metal oxyanions and, most importantly, conjugated metabolites of otherwise hydrophobic compounds [14,112,144,145].

ABCG2 or BCRP or Breast Cancer Resistance Protein (encoded by the *ABCG2* gene) was originally isolated from P-gp-negative multidrug-resistance breast cancer cell lines [109,110]. ABCG2 homes to the apical membranes in many epithelial cells and tissues, including lung, gut, intestine, liver, breast, placenta, hematopoietic stem cells and especially in the blood–brain barrier [109,110,119,146]. ABCG2 is a half-transporter carrying a TMD at the C-terminus, requiring homo-dimerization to form a full functional molecule. ABCG2 is overexpressed in many solid tumors as well as acute myeloid leukemia (AML) and acute lymphocytic leukemia (ALL). Dysregulated ABCG2 overexpression is linked with poor prognosis in several cancer types [139,147], with particularly low survival in AML patients [134,137,148–151]. Like P-gp, ABCG2, as well as PDRs such as Yor1, Pdr5, Cdr1 or Snq2, show extremely broad substrate specificity (Table 1), all in all transporting hundreds of diverse compounds, including dietary xenobiotics, toxins, metabolites, vitamins, lipids, steroids, antibiotics and antifungal as well as anticancer drugs [152–155]. Of note, the many exceptions seen for each transporter make a generalization of substrate preferences for a given ABCG or PDR transporter challenging without experimental evidence. Remarkably, however, despite their pronounced structural conservation, additional ABCG family members such as ABCG1, ABCG4 and the heterodimeric ABCG5/G8 transporter have not been associated with MDR phenotypes in cancer [15,115,156–162].


**Table 1.** Known ABC transporters in non-pathogenic yeasts.

\* All transporters located in the plasma membrane, except for Atm1, Mlt1 and Ybt1, Vmr1, Ycf1 and Hmt1, residing in the mitochondrial and vacuolar membrane. GS: glutathione, PC: phosphatidylcholine, PE: phosphatidylethanolamine, PS: phosphatidylserine.

Collectively, P-gp, MRP1 and ABCG2 act as brothers in arms to ensure the physiological detoxification of endogenous metabolites as well as exogenous xenobiotics across most epithelial barriers, including placenta, testis, mammary epithelium, liver and GI tract as well as the blood–brain barrier [119,139,146,152,206]. However, how, and sometimes even if, they actually cause clinical MDR in cancer has remained a highly controversial issue, often subject to intense discussions in the field [102,207]. As for microbial anti-infective MDR, it has been generally accepted though that bacterial ABC transporters [208–212] and fungal PDR transporters [124,213–216] are key causes for clinical MDR, often setting an unsurmountable roadblock in antimicrobial treatments [10,101,217–219].
