**4. Physiological role of AQPs**

**3. Expression profile of AQPs**

These channel proteins exhibit a wide tissue distribution. Several AQPs (1–4) play a role in kidney function [36, 37]. For example, AQP 2 translocates from the intracellular vesicles to the apical plasma membrane of the collecting duct in response to vasopressin stimulation leading to water reabsorption by the kidney [37, 38]. AQP 1 allows carbon dioxide transport in the proximal tubules, for regulation of arterial pH during metabolic acidosis [39]. In the brain, AQP 4 is expressed in the perivascular astrocyte foot process region and plays a role in solute clearance from the interstitial fluid [40] and the neuro-excitatory processes [41]. In the skin, AQP 3 is expressed in the stratum corneum (SC) and plays a role in maintaining skin hydration and elasticity, and epidermal proliferation [42]. In the adipocytes, AQP 7 is involved in glycerol movement across the cell [36]. Several AQPs are expressed in various regions of the eye and play a role in ocular surface hydration, intraocular pressure regulation and visual signal transduction [43]. Other AQPs are expressed elsewhere but their physiological functions remain to be determined. For example, AQP 4 is expressed in the basolateral region of gastric parietal cells but its deletion in mice does not alter acid secretion [36, 44]. Furthermore, tissuespecific expression of AQP 4 in skeletal muscle [45], AQP 5 in sweat glands [46] and AQP 8 in

various tissues [47] have not yet been linked with any specific physiological role.

There is accumulating evidence to suggest a role for several AQPs in cancer pathogenesis through their modulated expression profile in several tumors. It is speculated that AQPs facilitate water penetration into the growing tumor leading to its expansion through edema formation [48, 49]. They also appear to be involved in angiogenesis, tumor proliferation and migration/invasion [50–53]. About twenty types of tumors have been shown to express AQPs *in vivo*. For example, the expression level of AQPs 1, 4 and 9 are increased in astrocytoma [48, 54–57], while the level of AQP 1 was shown to be either increased [58] or decreased [59] in cholangiocarcinoma. Increased levels of AQPs 1, 3 and 5 [60–62] and decreased level of AQP 8 [63, 64] have been reported in colorectal cancer. In lung cancer, AQPs 1, 3, 4 and 5 were shown to be overexpressed [65–67]. Increased levels of AQPs 1, 3 and 5 were observed in cervical cancer [68, 69]. AQP 5 was increased in chronic myelogenous leukemia [70] and esophageal cancer [71]. In liver cancer, high levels of AQPs 3 and 5 [72] and low levels of AQPs 8 and 9

There is a direct correlation between the expression level of several AQPs and tumor grade. High levels of AQPs 1, 4 and 9 were observed in astrocytoma correlating with advanced disease stage [48, 54–57]. Enhanced AQP 9 expression was evident in malignant compared to benign ovarian tissues and was positively correlated with tumor grade [74]. Furthermore, enhanced expression of AQP 1 was seen in lung adenocarcinoma and its inhibition reduced cell invasion

**3.1. Normal tissues**

62 Tumor Metastasis

**3.2. Tumors**

were observed [73].

[66].

#### **4.1. Fluid transport and osmotic equilibrium**

It has been suggested that at least eight (of the known 13) AQPs transport water, while others such as AQPs 3, 7, 9 and 10 are also able to transport glycerol (termed aquaglyceroporins) [44, 75]. Their expression in various organs such as the kidney tubules, lung and alveoli facilitate active fluid absorption and secretion by the creation of an osmotic gradient across the cell membrane and subsequent fluid movement through these channels. Genetic knockout of AQP 5 in mice resulted in impaired salivary [76, 77] and airway submucosal gland secretion [78]. In addition, tissue-specific knockout of AQP 1 in mice leads to impaired secretion of the cerebrospinal fluid [79] and ocular aqueous fluid [80], and inappropriate hypertonic fluid absorption in the proximal kidney tubules [81]. It should be noted, however, that other data suggest that knockout of various AQPs does not lead to impaired fluid absorption or secretion [82–86], suggesting that the requirement of AQPs to facilitate active fluid transport depends on the rate of such transport in each compartment. AQPs (specifically 1–4 and 7) are also involved in maintaining the osmotic equilibrium across the kidney tubules and the formation of concentrated urine. Marked polyuria and low urine osmolality was seen in AQPs 1 and 3 knockout mice, which led to severe dehydration [87, 88]. Reduced expression of AQP 2 also leads to acquired forms of nephrogenic diabetes insipidus (NDI) due to the inability of the kidneys to concentrate urine owing to the insensitivity of the distal nephron to the antidiuretic hormone arginine vasopressin [89]. AQP 4 is expressed in the glial cells of the brain and spinal cord, and plays an important role in water balance in the brain. A significant reduction in osmotic water permeability in glial cells was demonstrated in AQP-4-deficient mice which led to brain edema and swelling [90, 91]. In addition, several AQPs (0, 1, 3, 4 and 5) are expressed in various compartments of the eye and play an important role in the regulation of fluid movement and intraocular pressure [92–95].

#### **4.2. CNS functions**

AQP 4 was shown to be expressed in the glial cells in the brain particularly at astrocyte endfeet at the blood-brain barrier and the ependymal-cerebrospinal fluid barrier [96]. AQP 4 deficiency in mice resulted in reduced seizure susceptibility in response to pentylenetetrazol treatment [97], as well as in electrically-induced seizure following hippocampal stimulation [98]. Delayed potassium uptake from the brain extracellular space (ECS) [98, 99], and expanded ECS which dilutes the released potassium levels [100, 101], has been suggested to be respon‐ sible for the reduced seizure susceptibility in AQP-4-deficient mice. AQP 4 also increases water exit from the brain in vasogenic edema, as AQP-4-deficient mice show greater water accumu‐ lation in various models of brain edema [102–105]. Also, AQP 1 was shown to be expressed in the dorsal root ganglion neurons and nociceptive C-fibers, and AQP 1 deficiency in mice leads to reduced pain perception in response to thermal inflammatory pain in part through modu‐ lation of voltage gated sodium channel Nav 1.8 activity [105–107].
