A Serum Factor Induces Insulin-Independent Translocation of GLUT4 to the Cell Surface which Is Maintained in Insulin Resistance

In response to insulin, glucose transporter GLUT4 translocates from intracellular compartments towards the plasma membrane where it enhances cellular glucose uptake. Here, we show that sera from various species contain a factor that dose-dependently induces GLUT4 translocation and glucose uptake in 3T3-L1 adipocytes, human adipocytes, myoblasts and myotubes. Notably, the effect of this factor on GLUT4 is fully maintained in insulin-resistant cells. Our studies demonstrate that the serum-induced increase in cell surface GLUT4 levels is not due to inhibition of its internalization and is not mediated by insulin, PDGF, IGF-1, or HGF. Similarly to insulin, serum also augments cell surface levels of GLUT1 and TfR. Remarkably, the acute effect of serum on GLUT4 is largely additive to that of insulin, while it also sensitizes the cells to insulin. In accordance with these findings, serum does not appear to activate the same repertoire of downstream signaling molecules that are implicated in insulin-induced GLUT4 translocation. We conclude that in addition to insulin, at least one other biological proteinaceous factor exists that contributes to GLUT4 regulation and still functions in insulin resistance. The challenge now is to identify this factor

Assessing associations between the AURKAHMMR-TPX2-TUBG1 functional module and breast cancer risk in BRCA1/2 mutation carriers

While interplay between BRCA1 and AURKA-RHAMM-TPX2-TUBG1 regulates mammary epithelial polarization, common genetic variation in HMMR (gene product RHAMM) may be associated with risk of breast cancer in BRCA1 mutation carriers. Following on these observations, we further assessed the link between the AURKA-HMMR-TPX2-TUBG1 functional module and risk of breast cancer in BRCA1 or BRCA2 mutation carriers. Forty-one single nucleotide polymorphisms (SNPs) were genotyped in 15,252 BRCA1 and 8,211 BRCA2 mutation carriers and subsequently analyzed using a retrospective likelihood appr

Assessing Associations between the AURKA-HMMR-TPX2-TUBG1 Functional Module and Breast Cancer Risk in BRCA1/2 Mutation Carriers

While interplay between BRCA1 and AURKA-RHAMM-TPX2-TUBG1 regulates mammary epithelial polarization, common genetic variation in HMMR (gene product RHAMM) may be associated with risk of breast cancer in BRCA1 mutation carriers. Following on these observations, we further assessed the link between the AURKA-HMMR-TPX2-TUBG1 functional module and risk of breast cancer in BRCA1 or BRCA2 mutation carriers. Forty-one single nucleotide polymorphisms (SNPs) were genotyped in 15,252 BRCA1 and 8,211 BRCA2 mutation carriers and subsequently analyzed using a retrospective likelihood approach. The association of HMMR rs299290 with breast cancer risk in BRCA1 mutation carriers was confirmed: per-allele hazard ratio (HR) = 1.10, 95% confidence interval (CI) 1.04 - 1.15, p = 1.9 x 10(-4) (false discovery rate (FDR)-adjusted p = 0.043). Variation in CSTF1, located next to AURKA, was also found to be associated with breast cancer risk in BRCA2 mutation carriers: rs2426618 per-allele HR = 1.10, 95% CI 1.03 - 1.16, p = 0.005 (FDR-adjusted p = 0.045). Assessment of pairwise interactions provided suggestions (FDR-adjusted p(interaction) values > 0.05) for deviations from the multiplicative model for rs299290 and CSTF1 rs6064391, and rs299290 and TUBG1 rs11649877 in both BRCA1 and BRCA2 mutation carriers. Following these suggestions, the expression of HMMR and AURKA or TUBG1 in sporadic breast tumors was found to potentially interact, influencing patients' survival. Together, the results of this study support the hypothesis of a causative link between altered function of AURKA-HMMR-TPX2-TUBG1 and breast carcinogenesis in BRCA1/2 mutation carriers.Peer reviewe

Deciphering the role of GLUT4 N-glycosylation in adipocyte and muscle cell models.

International audienceGLUT4 (glucose transporter 4) is responsible for the insulin-induced uptake of glucose by muscle and fat cells. In non-stimulated (basal) cells, GLUT4 is retained intracellularly, whereas insulin stimulation leads to its translocation from storage compartments towards the cell surface. How GLUT4 is retained intracellularly is largely unknown. Previously, aberrant GLUT4 N-glycosylation has been linked to increased basal cell-surface levels, while N-glycosylation-deficient GLUT4 was found to be quickly degraded. As recycling and degradation of GLUT4 are positively correlated, we hypothesized that incorrect N-glycosylation of GLUT4 might reduce its intracellular retention, resulting in an increased cell-surface recycling, in increased basal cell-surface levels, and in enhanced GLUT4 degradation. In the present study, we have investigated N-glycosylation-deficient GLUT4 in detail in 3T3-L1 preadipocytes, 3T3-L1 adipocytes and L6 myoblasts. We have found no alterations in retention, insulin response, internalization or glucose transport activity. Degradation of the mutant molecule was increased, although once present at the cell surface, its degradation was identical with that of wild-type GLUT4. Our findings indicate that N-glycosylation is important for efficient trafficking of GLUT4 to its proper compartments, but once the transporter has arrived there, N-glycosylation plays no further major role in its intracellular trafficking, nor in its functional activity

GLUT4 molecules are recruited at random for insertion within the plasma membrane upon insulin stimulation.

International audienceGlucose transporter 4 (GLUT4) is efficiently retained intracellularly. Here, we investigated the insulin-induced reduction of retention. While increasing insulin concentrations led to gradual increases in both the amount of recycling GLUT4 molecules and cell surface GLUT4 levels, the kinetics of the increase in time was independent of insulin concentration. To determine whether there are GLUT4 subpools that have a distinct insulin sensitivity, adipocytes were consecutively stimulated twice with a low concentration of insulin while recycling GLUT4 molecules were continuously labeled. This revealed that not the same pool of GLUT4 molecules was mobilized twice and thus that upon insulin stimulation, GLUT4 is likely to be recruited at random for insertion within the plasma membrane

GLUT4 molecules are recruited at random for insertion within the plasma membrane upon insulin stimulation.

International audienceGlucose transporter 4 (GLUT4) is efficiently retained intracellularly. Here, we investigated the insulin-induced reduction of retention. While increasing insulin concentrations led to gradual increases in both the amount of recycling GLUT4 molecules and cell surface GLUT4 levels, the kinetics of the increase in time was independent of insulin concentration. To determine whether there are GLUT4 subpools that have a distinct insulin sensitivity, adipocytes were consecutively stimulated twice with a low concentration of insulin while recycling GLUT4 molecules were continuously labeled. This revealed that not the same pool of GLUT4 molecules was mobilized twice and thus that upon insulin stimulation, GLUT4 is likely to be recruited at random for insertion within the plasma membrane

Rosiglitazone increases cell surface GLUT4 levels in 3T3-L1 adipocytes through an enhancement of endosomal recycling

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GLUT4 molecules are recruited at random for insertion within the plasma membrane upon insulin stimulation.

International audienceGlucose transporter 4 (GLUT4) is efficiently retained intracellularly. Here, we investigated the insulin-induced reduction of retention. While increasing insulin concentrations led to gradual increases in both the amount of recycling GLUT4 molecules and cell surface GLUT4 levels, the kinetics of the increase in time was independent of insulin concentration. To determine whether there are GLUT4 subpools that have a distinct insulin sensitivity, adipocytes were consecutively stimulated twice with a low concentration of insulin while recycling GLUT4 molecules were continuously labeled. This revealed that not the same pool of GLUT4 molecules was mobilized twice and thus that upon insulin stimulation, GLUT4 is likely to be recruited at random for insertion within the plasma membrane