Water inorganic salts sugars lipids nucleic acids
Water, inorganic salts, sugars, lipids, nucleic acids and proteins are the main basic materials forming the organism. Life activities can be controlled or regulated by a series of complicated and chain-reactive protein–protein interactions. Protein–protein interaction is considered as the bases and characteristics for different physiological and pathological cell functions or activities. The precision and specificity of these molecular interactions have ignited a strong interest in pursuing protein–protein interactions as new targets for developing drugs with reduced side effects (Arkin et al., 2014, Arkin and Wells, 2004, Guo et al., 2014, Stoilova-McPhie et al., 2013). To survive, cells must avoid excessive changes in cell volume that jeopardize structural integrity and the stability of the intracellular milieu (Lang et al., 1998). Cell volume regulation plays important roles in a wide variety of cellular functions, including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation and cell death. The strategy adopted by animal cells for coping with volume regulation upon osmotic perturbation is to activate transport pathways, including channels and transporters, mainly for inorganic osmolytes, to drive water flow (Okada, 2004). Although there has been much research on the function and properties of channel proteins, the molecular mechanisms of cooperation among channels remain unclear. A considerable amount of evidence has shown that there are close relationships between the classical AQPs and cation channels (Benfenati and Ferroni, 2010, Galizia et al., 2012, Zhang and Verkman, 2010); this suggests that there may be a relationship between the aquaglyceroporins and ion channels, which has been less well-studied. We recently reported that AQP-3 and volume-activated chloride channels are expressed in normal and carcinomatous nasopharyngeal epithelium, and AQP-3 is conjugated with the ClC-3 chloride channel to regulate cell volume (Sun et al., 2012, Zhang et al., 2014). ClC-3 gating is controlled by the membrane voltage (polarization/depolarization), and it is also modulated by factors such as phosphorylation/dephosphorylation, oxidation/deoxidation and Cy3 carboxylic acid (non-sulfonated) (Kasinathan et al., 2007, Zhang et al., 2013a, Zhang et al., 2013b, Zhu et al., 2013). Results reported by others suggest that AQP-2 can act as a sensor, leading to coordinate activation of specific ionic channels for potassium and chloride efflux (Flamenco et al., 2009). Based on these and our previous findings (Bai et al., 2010, Zhang et al., 2014), the roles of AQP-3 in the complexes composed of AQP-3 and ClC-3 was investigated. Hypotonic extracellular solutions may cause cell swelling, leading to activation of volume-regulated chloride channels. However, in vivo solid tumor cells are not likely to be exposed to major changes in extracellular osmolarity. Glycerol represents an important metabolite for the control of fat accumulation and glucose homeostasis; there is a positive correlation between the intracellular glycerol content and the cell proliferation rate (Qin et al., 2014). Metabolism of tumor cells at a high level may result in alterations in intracellular osmolarity and thereby cell volume (Cairns et al., 2011, Dang, 2012, Hamanaka and Chandel, 2012). Extracellular glycerol can rapidly enter cells via the aquaglyceroporins, resulting in an increase in intracellular osmolarity, which may be a model for changes in intracellular osmolarity that are induced by metabolism. In this work, it was found that the exposure of CNE-2Z cells to the 140mM glycerol isoosmotic solution could immediately cause cell swelling, activate volume-sensitive chloride currents and induce a regulatory volume decrease (RVD). These responses are similar to those induced by hypotonic challenges. The data suggest that the activation of the chloride channels is caused by glycerol-induced cell swelling. This notion is supported by our next experiments. When bathed in the hyperosmotic solution containing the same concentration of glycerol (140mM), the CNE-2Z cells was only slightly swollen, and the glycerol-induced chloride currents were much smaller. These data indicate that the chloride currents induced by 140mM glycerol are at least in part activated by a swelling-dependent pathway; the reduced currents activated by the 140mM glycerol hyperosmotic solution may be mainly associated with a swelling-independent activation pathway.