Radiation inactivation target size of rat adipocyte glucose transporters in the plasma membrane and intracellular pools

Low-dose radiation pretreatment improves survival of human ceiling culture-derived proliferative adipocytes (ccdPAs) under hypoxia via HIF-1 alpha and MMP-2 induction

Poor survival is a major problem of adipocyte transplantation. We previously reported that VEGF and MMPs secreted from transplanted adipocytes are essential for angiogenesis and adipogenesis. Pretreatment with low-dose (5 Gy) radiation (LDR) increased VEGF, MMP-2, and HIF-1 alpha mRNA expression in human ceiling culture-derived proliferative adipocytes (hccdPAs). Gene expression after LDR differed between adipose-derived stem cells (hASCs) and hccdPAs. Pretreatment with LDR improved the survival of hccdPAs under hypoxia, which is inevitable in the early stages after transplantation. Upregulation of VEGF and MMP-2 after LDR in hccdPAs is mediated by HIF-1 alpha expression. Our results suggest that pretreatment with LDR may improve adipocyte graft survival in a clinical setting through upregulation of VEGF and MMP-2 via HIF-1 alpha.

Structural analysis of the GLUT1 facilitative glucose transporter (review)

The structure of the human erythrocyte facilitative glucose transporter (GLUT1) has been intensively investigated using a wide array of chemical and biophysical approaches. Despite the lack of a crystal structure for any of the facilitative monosaccharide transport proteins, detailed information regarding primary and secondary structure, membrane topology, transport kinetics, and functionally important residues has allowed the construction of a sophisticated working model for GLUT1 tertiary structure. The existing data support the formation of a central aqueous channel formed by the juxtaposition of several amphipathic transmembrane-spanning alpha-helices. The results of extensive mutational analysis of GLUT1 have elucidated many of the structural determinants of the glucose permeation pathway. Continued application of currently available technologies will allow further refinement of this working model. In addition to providing insights into the molecular basis of both normal and disordered glucose homeostasis, this detailed understanding of structure/function relationships within GLUT1 can provide a basis for understanding transport carried out by other members of the major facilitator superfamily.

Glucose transporter gene expression: regulation of transcription and mRNA stability

The facilitated diffusion of D-glucose across the plasma membrane is carried out by a set of stereospecific transport proteins known as the glucose transporters. These integral membrane proteins are members of a gene family where tissue-specific expression of one or more members will determine in part the net rate of glucose entry into the cell. The regulation of glucose transporter gene expression is a critical feature of cellular homeostasis, as defects in specific transporter expression can lead to profound alterations in cellular physiology. In this review, we provide a brief descriptive background on the family of glucose transporters and examine in depth the regulation of the two transporters expressed in adipose tissue, GLUTI, a basal growth-related transporter and GLUT4, the insulin-responsive glucose transporter.

Insulin-regulated sorting of glucose transporters in 3T3-L1 adipocytes

Two glucose transporters (GLUT-4 and GLUT-1) move from within the cell to the plasma membrane (PM) when 3T3-L1 adipocytes are stimulated with insulin. To study the sorting of these two molecules, vesicles containing GLUT-4 and GLUT-1 were immunoadsorbed from basal and insulin-treated cells. Two different vesicle populations were isolated as follows: 1) a compartment that contained the majority of intracellular GLUT-4 and GLUT-1 and 2) a subpopulation of vesicles containing 43% of the intracellular GLUT-4 that was highly insulin regulatable and that contained relatively low levels of GLUT-1. After incubation at 19 degrees C, basal glucose transport was slightly increased, whereas insulin-dependent transport was blocked. Consistent with these observations, cell surface GLUT-1 levels were increased in the basal state, whereas insulin-dependent translocation of GLUT-4 to the PM was blocked at 19 degrees C. However, insulin-dependent sorting of GLUT-4 within the intracellular compartment was still evident at 19 degrees C. These data indicate that GLUT-1 and GLUT-4 are heterogeneously distributed throughout the same intracellular compartment in 3T3-L1 adipocytes. Furthermore, we have uncoupled two distinct steps in the insulin-dependent movement of GLUT-4 to the cell surface. These include movement of GLUT-4 out of its storage compartment and accumulation of GLUT-4 at the cell surface. Only the former step occurs in cells preincubated at 19 degrees C.

Radiation target sizes of the Na,K-ATPase and p-aminohippurate transport system in the basolateral membrane of renal proximal tubule

Basolateral membrane vesicles made from rabbit kidney proximal tubules were frozen and irradiated with a high energy electron beam and the effects of irradiation on Na,K-ATPase activity, p-aminohippurate (PAH) transport, the membrane diffusion barrier and vesicle volume were measured. The vesicle volume and diffusion barrier were not significantly changed by radiation exposure. Na,K-ATPase activity was inactivated as a simple exponential function of radiation dose. Target size analysis of the data yielded a molecular size of 267 +/- 17 kDa, consistent with its existence as a (alpha beta)2 dimer. The carrier-mediated PAH uptake by basolateral membrane vesicles was also inactivated as a function of radiation dose. A target molecular size of 74 +/- 16 kDa was calculated for the PAH transport system. This study is the first measurement of the functional size of the organic acid transport system based directly on flux measurements.

Differential sensitivity to guanine nucleotides of basal and insulin-stimulated glucose transporter activity reconstituted from adipocyte membrane fractions

The effects of GTP gamma S on glucose transport activity reconstituted from adipocyte membrane fractions were studied in order to test the hypothesis that intrinsic activity changes of the insulin-sensitive glucose transporter may be mediated by guanine nucleotide-dependent mechanisms. GTP gamma S and GTP inhibited reconstituted glucose transport activity by 50% in membrane fractions from insulin-treated cells in a concentration-dependent manner; no inhibitory effect was observed in membrane fractions obtained from basal cells. GDP, GMP and guanosine were less effective than GTP, whereas the adenine nucleotides ATP gamma S and AMP failed to reduce the reconstituted transport activity. The data indicate that guanine nucleotides may modulate the activity of the adipocyte glucose transporter. Since the effect is dependent on treatment of cells with insulin, the hormone appears to induce a specific functional alteration of the glucose transporter.

Alterations of glucose transporter systems in insulin-resistant uremic rats

To further define the cellular alteration(s) involved in the impaired glucose transport associated with chronic uremia, we examined the concentration and translocation of glucose transport systems in adipocytes isolated from partially nephrectomized uremic rats. Uremic animals, compared with matched controls, had increased blood urea nitrogen and serum insulin, whereas serum glucose was unchanged. In agreement with previous work, 125I-insulin binding to its receptor was unaltered and transport of 2-deoxy-D-glucose was decreased in both the absence (basal) and presence of a maximal (7 nM) insulin concentration by 44 and 35%, respectively. To assess the movement and concentration of glucose transport systems in various membrane fractions prepared from basal and insulin-treated (20 nM) uremic fat cells, the technique of D-glucose-inhibitable cytochalasin B binding was utilized. In plasma membranes isolated from these cells the concentration of glucose transporters was decreased by 16 (P less than 0.01) and 30% (P less than 0.005) in basal and insulin-treated cells, respectively. Concomitantly, microsomal membranes prepared from uremic cells treated in the absence and presence of insulin had a 28 (P less than 0.01) and 15% (P less than 0.05) decrease in concentration of glucose transport systems, respectively. Additionally, glucose transporter concentration was significantly decreased by 17% (P less than 0.025) in total membranes prepared from uremic cells. Thus, impairment of glucose transport in uremic fat cells can be attributed to a postbinding defect that, at least in part, results from a decrease in the total concentration of glucose transporters.(ABSTRACT TRUNCATED AT 250 WORDS)

Kinetic characterization and radiation-target sizing of the glucose transporter in cardiac sarcolemmal vesicles

Stereospecific glucose transport was assayed and characterized in bovine cardiac sarcolemmal vesicles. Sarcolemmal vesicles were incubated with D-[3H]glucose or L-[3H]glucose at 25 degrees C. The reaction was terminated by rapid addition of 4 mM HgCl2 and vesicles were immediately collected on glass fiber filters for quantification of accumulated [3H]glucose. Non-specific diffusion of L-[3H]glucose was never more than 11% of total D-[3H]glucose transport into the vesicles. Stereospecific uptake of D-[3H]glucose reached a maximum level by 20 s. Cytochalasin B (50 microM) inhibited specific transport of D-[3H]glucose to the level of that for non-specific diffusion. The vesicles exhibited saturable transport (Km = 9.3 mM; Vmax = 2.6 nmol/mg per s) and the transporter turnover number was 197 glucose molecules per transporter per s. The molecular sizes of the cytochalasin B binding protein and the D-glucose transport protein in sarcolemmal vesicles were estimated by radiation inactivation. These values were 77 and 101 kDa, respectively, and by the Wilcoxen Rank Sum Test were not significantly different from each other.