Search

Glucose transport plays an important role in maintaining low sugar concentration in airway surface liquid (ASL), which is critical for mucociliary clearance and bacterial colonization. Experimental evidence indicates that glucose/hexose uptake in lung/airway cells occurs by means of two structurally distinct glucose transporter pathways: the Na+-dependent glucose transporters (SGLT family) and the facilitative glucose transporters (GLUT family). In this study, we examined the expression of the major glucose transporters of the intestine, GLUT2, GLUT5, SGLT1 and T1R3 taste receptor subunit, in the trachea of rats using immunohistochemistry and immunoelectron microscopy, and compared them using double-labeled confocal microscopy. We found that GLUT2, GLUT5, SGLT1 and T1R3 are selectively expressed in different cell types. T1R3 and GLUT2 are predominantly expressed in subsets of solitary chemoreceptor cells (SCCs) and ciliated cells, GLUT5 is present in subsets of SCCs and in secretory cells, and SGLT1 is exclusively expressed in a unique cell type, SCCs. Furthermore, we demonstrated that T1R3 is colocalized with SGLT1 in SCCs and with GLUT2 transporter in ciliated cells. In conclusion, these findings reveal that different cell types are associated with the uptake of glucose in ASL and that, due to their T1R3 expression, SCCs and ciliated cells are most likely to participate in the chemosensory process in ASL. [ABSTRACT FROM AUTHOR]

In the intestine, changes of sugar concentration generated in the lumen during digestion induce adaptive responses of glucose transporters in the epithelium. A close matching between the intestinal expression of glucose transporters and the composition and amount of the diet has been provided by several experiments. Functional evidence has demonstrated that the regulation of glucose transporters into enterocytes is induced by the sensing of sugar of the enteroendocrine cells through activation of sweet taste receptors (T1R2 and T1R3) and their associated elements of G-protein-linked signaling pathways (e.g. α-gustducin, phospholipase C β type 2 and transient receptor potential channel M5), which are signaling molecules also involved in the perception of sweet substances in the taste receptor cells (TRCs) of the tongue. Considering this phenotypical similarity between the intestinal cells and TRCs, we evaluated whether the TRCs themselves possess proteins of the glucose transport mechanism. Therefore, we investigated the expression of the typical intestinal glucose transporters (i.e. GLUT2, GLUT5 and SGLT1) in rat circumvallate papillae, using immunohistochemistry, double-labeling immunofluorescence, immunoelectron microscopy and reverse transcriptase-polymerase chain reaction analysis. The results showed that GLUT2, GLUT5 and SGLT1 are expressed in TRCs; their immunoreactivity was also observed in cells that displayed staining for a-gustducin and T1R3 receptor. The immunoelectron microscopic results confirmed that GLUT2, GLUT5 and SGLT1 were predominantly expressed in cells with ultrastructural characteristics of chemoreceptor cells. The presence of glucose transporters in TRCs adds a further link between chemosensory information and cellular responses to sweet stimuli that may have important roles in glucose homeostasis, contributing to a better understanding of the pathways implicated in glucose metabolism. [ABSTRACT FROM AUTHOR]

The oral cavity is continuously bathed by saliva secreted by the major and minor salivary glands. Saliva is the first biological medium to confront external materials that are taken into the body as part of food or drink or inhaled volatile substances, and it contributes to the first line of oral defence. In humans, it has been shown that sputum and a variety of biological fluids contain Clara cell secretory proteins (CC10–CC26). Various studies of the respiratory apparatus have suggested their protective effect against inflammatory response and oxidative stress. Recently, CC10 deficiency has been related to the protein Annexin-1 (ANXA1), which has immunomodulatory and anti-inflammatory properties. Considering the defensive role of both Clara cell secretory proteins and ANXA1 in the respiratory apparatus, and the importance of salivary gland secretion in the first line of oral defence, we decided to evaluate the expression of CC10, CC26 and ANXA1 proteins in rat major salivary glands using immunohistochemistry. CC10 expression was found only in the ductal component of the sublingual gland. Parotid and submandibular glands consistently lacked CC10 immunoreactivity. In the parotid gland, both acinar and ductal cells were always CC26-negative, whereas in the submandibular gland, immunostaining was localized in the ductal component and in the periodic acid Schiff (PAS)-positive area. In the sublingual gland, ductal cells were always positive. Acinar cells were not immunostained at all. ANXA1 was expressed in ductal cells in all three major glands. In parotid and sublingual glands, acinar cells were negative. In submandibular glands, immunostaining was present in the mucous PAS-positive portion, whereas serous acinar cells were consistently negative. The existence of some CC10-CC26–ANXA1-positive cells in rat salivary glandular tissue is an interesting preliminary finding which could support the hypothesis, suggested for airway tissue, that these proteins have a defensive and protective role. Protein expression heterogeneity in the different portions of the glands could be an important clue in further investigations of their role. [ABSTRACT FROM AUTHOR]