**2. Structure of AQPs**

detrimental impactduring its pathogenesis [1].This forms the basis for endocrine therapy, with the application of pharmacological antagonists generally termed selective estrogen receptor modulators, such as tamoxifen. These have resulted in significant improvements in quality of life as well as improved prognosis [2] in a significant proportion of patients with clinically defined ER+ve status [3]. Unfortunately, *de novo* resistance to tamoxifen occurs in about 30– 40% of patients (those with very low level of ER expression, clinically designated as ER−ve) and even in about 50% of the clinically defined ER+ve patients. Furthermore, almost all initially responsive patients with late stage metastatic disease eventually relapse due to the develop‐ ment of *acquired* resistance to anti-estrogen therapy. These forms of endocrine resistance invariably lead to a more aggressive form of resurgent disease [4], and occur in parallel with cellular transition from epithelial to mesenchymal phenotype (EMT). There is a strong associ‐ ation between the EMT process and metastasis, which involves detachment of individual epithelial cells from neighboring cells, loss of polarity, scattering, acquisition of enhanced motility and invasion into the extracellular matrix (ECM) before entering blood and lymphat‐ ic vessels. Many phenotypic changes occur during this process which includes the loss of cellcell adhesion as a result of reduced E-cadherin and catenins expression in adherens junctions, reducedclaudins andoccludins expressionattightjunctions andreducedexpressionof various epithelial cytokeratins such as KRT8, 18 and 19 which presumably aids in disruption of cytoskeletal connections that maintain tissue architecture. These changes are also paralleled withup-regulationofmesenchymalmarkerssuchasvimentin,fibronectin,alphasmoothmuscle actin (ACTA2), N-cadherin and various matrix metalloproteinases (MMPs) [4, 5]. Attempts to overcome endocrine resistance include the use of pure estrogen antagonists such as fulves‐ trant (in place of tamoxifen, which is associated with some agonist actions with prolonged administration) or agents which inhibit peripheral extragonadal synthesis of estrogen (aroma‐ tase inhibitors such as anastrazole), which delays but does not resolve this problem [6, 7]. In addition, receptor tyrosine kinase (RTK) inhibitors have been used recently in the treatment of endocrine-resistant breast cancer [8], but they have limitations in terms of target specificity and clinical outcomes. For example, the reversible inhibitor of epidermal growth factor receptor (EGFR) erlotinib also blocks ERBB2 [9, 10], AKT (the downstream target of phosphatidylinosi‐ tide 3-kinases; PI3K) and mitogen-activated protein kinase (MAPK) phosphorylation in breast cancer cells [11]. Furthermore, imatinib inhibits the activity of the tyrosine kinase domain of various targets such as ABL, KIT and platelet-derived growth factor receptor (PDGFR) [12, 13]. The lack of specificity of these agents might increase the risk of side effects and therefore limits their clinical usage and utility. Since the current therapeutic options for endocrine insensitive breast cancer patients have various limitations (including severe side effect profile and resistance),there isaneedtofindbettertherapeutic targets tocontrolthis conditionandimprove

Aquaporins (AQPs) represent a family of 13–14 small hydrophobic integral transmembrane water channel proteins which are widely distributed in various tissues in the body. Their function is to transport mainly water (through passive transport), glycerol, solutes (such as urea, carbon dioxide, ammonia and nitric oxide) [14–20], as well as larger polar solutes (such as sugars and hydrogen peroxide) [21–23]. The first discovered family member of these proteins was initially called CHIP28, but it is now known as AQP 1 [24, 25]. AQPs are classified

its prognosis.

60 Tumor Metastasis

Unlike other types of channels, AQPs do not show gating, saturation or membrane potentialdependent behavior. AQP family members share 25–60% protein sequence homology [14, 26, 27], and are assembled on the cell membrane and cytoplasmic compartments as homotetramers [28]. Each monomer is about 28–30 kDa in size and has its own water pore. Some members of this family such as AQPs 0 and 4 have unique features in that their tetramers assemble into higher order supramolecular structures described as orthogonal arrays of particles [29, 30]. The monomeric units of AQPs consist of six transmembrane α-helices (M 1, 2, 4–7 and 8), two half helices (M 3 and 7) and five connecting loops (a–e) [31]. Both the N- and carboxyterminal domains are present in the cytoplasmic compartment. Water movement occurs through a narrow pore (<0.3 nm) in which steric and electrostatic factors prevent the transport of protons and other small molecules [32]. Several studies have also indicated that the central pore allows the rapid transport of oxygen, carbon dioxide and nitric oxide (seen in AQPs 1, 4 and 5) [19, 33]. On the other hand, the aquaglyceroporins have a less constricted pore with a larger proportion of hydrophobic residues [34, 35]. **Figure 1** illustrates a schematic arrangement of an AQP channel.

**Figure 1.** Schematic diagram of the aquaporin channel. The aquaporins are formed by two tandem repeats of three membrane-spanning helices. Two connecting loops, each containing a conserved sequence motif of Asn-Pro-Ala (NPA) on the loops, bend into molecules to pair with each other and form a channel in the plasma membrane through which water and solutes can pass between the cell and its environment.
