engleski

Immunochemical characterization of Na+-d-glucose cotransporters SGLT1 and SGLT2 in human kidneys, small intestine and liver

Introduction. In the mammalian nephron, a high affinity/low capacity SGLT1 (SLC5A1) and a low affinity/high capacity SGLT2 (SLC5A2) have been identified as the major mediators of Na+-dependent hexose reabsorption. Our immunochemical studies in rats have shown the presence of both transporters in various organs and strong female-dominant sex differences in their expression in proximal tubules (PT) (Balen et al.: Am. J. Physiol. Cell. Physiol, 295:475-489, 2008 ; and unpublished data). Both SGLTs exhibit 59% homology and some functional and localizational differences. Functional studies in renal tubules and in isolated membranes have shown that SGLT1 transports equally well galactose and glucose, and has been localized to the brush-border membrane (BBM) of PT and small intestine (SI), while SGLT2 transports glucose much better, and has been detected in the BBM of PT. Due to lack of specific antibodies, a detailed immunolocalization of SGLT1 and SGLT2 in the human kidneys (K) and other organs has not been performed. Materials and Methods. A novel, highly specific rabbit-raised polyclonal antibodies against the human SGLT1 (hSGLT1) and SGLT2 (hSGLT2) proteins were used to study their expression in kidneys (K), SI (jejunum), and liver (L) in humans of both sexes by Western blotting (WB) in isolated total cell membranes (TCM) and by immunocytochemistry (IC) in tissue cryosections. Results. In WB of the K and SI membranes, the hSGLT1 protein was identified as the 75 (K)-80 (SI) kDa band, while the hSGLT2 protein appeared as the 75 kDa band in the K membranes, which was not detected in the SI membranes. The WB studies in TCM from the K cortex (CO) and outer stripe (OS) revealed the presence of zonal (hSGLT1: CO < OS ; hSGLT2: CO > OS) but absence of sex differences in the expression of both hSGLTs. By IC in K, hSGLT1 was restricted to the BBM of PT S3 segments in the OS and medullary rays. In SI, hSGLT1 was detected in: a) BBM and subapical vesicles of absorptive cells, b) apical membrane of crypt epithelium, and c) sporadic individual cells in villous epithelium. A significant hSGLT1-positive staining was also observed in the apical domain of bile ducts in L. Prominent hSGLT2-specific staining was localized to the BBM of PT S1/S2 segments in the K CO, and to the apical domain and/or intracellular organelles in individual, enteroendocrine-type cells scattered among absorptive and crypt cells. In L, no hSGLT2-staining was detected. Conclusion. Our data confirm that hSGLT1 is critically involved in glucose and galactose absorption in SI, and for reabsorption of these monosaccharides in the renal PT S3 segments and liver bile ducts, while hSGLT2 is mainly engaged in glucose reabsorption in the renal PT S1/S2 segments. Both hSGLTs may contribute to regulation of glucose absorption via the enteroendocrine-type cells in SI.

enteroendocrine cells; immunoblotting; immunolocalization; glucose transporters; nephron; proximal tubule; sex differences

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