Armoni M, Harel C, Burvin R, Karnieli E
Metabolic Unit, Rambam Medical Center, Haifa, Israel.
Endocrinology. 1995 Aug;136(8):3292-8. doi: 10.1210/endo.136.8.7628363.
To study the contribution of glucose transporters (GLUT) to insulin resistance in aging, GLUT intrinsic activity was assessed in a cell-free system. Adipocytes were isolated from 18-month-old rats and young controls and incubated either with or without 7 nM insulin. Plasma membrane (PM) and low density microsomal fractions were prepared from the cells, and GLUT levels were assessed in these fractions before and after reconstitution into liposomes. Glucose transport rates were measured in intact cells and liposomes. Functional and intrinsic activities of GLUT were assessed from the ratio between these transport rates and GLUT levels in the respective fractions. Basal 3-O-methylglucose transport rates were unaffected by aging, which is consistent with unchanged levels of GLUT in PM. Insulin-stimulated glucose transport was 60% lower in aging, as was the extent of GLUT recruitment to PM. The effect of insulin stimulation of GLUT functional activity by 6-fold at PM was attenuated by 40% in aging. Conversely, the basal intrinsic activity of GLUT was significantly enhanced in aging (by 280% and 230% in PM and density microsomal liposomes, respectively) and was further stimulated by insulin by about 160% in PM, compared to only about 117% stimulation in controls. In conclusion, our data show that insulin stimulates the intrinsic activity of GLUT in rat adipocytes, and this activity is further enhanced in aging. Impaired glucose uptake in aging can be attributed to depleted GLUT4 levels and impaired function of GLUT at the cell surface. The discrepancy observed between impaired function and enhanced intrinsic activity of GLUT suggests the presence of additional factors that modulate the full functional expression of GLUT at the cell surface.
为研究葡萄糖转运蛋白（GLUT）在衰老过程中对胰岛素抵抗的作用，在无细胞体系中评估了GLUT的内在活性。从18月龄大鼠和年轻对照动物中分离脂肪细胞，并在有或无7 nM胰岛素的情况下进行孵育。从细胞中制备质膜（PM）和低密度微粒体组分，并在将这些组分重构成脂质体之前和之后评估其中的GLUT水平。在完整细胞和脂质体中测量葡萄糖转运速率。根据这些转运速率与各组分中GLUT水平的比值评估GLUT的功能活性和内在活性。基础3 - O - 甲基葡萄糖转运速率不受衰老影响，这与PM中GLUT水平未改变一致。衰老过程中胰岛素刺激的葡萄糖转运降低了60%，GLUT募集到PM的程度也降低了60%。胰岛素刺激PM处GLUT功能活性6倍增强的效应在衰老过程中减弱了40%。相反，衰老过程中GLUT的基础内在活性显著增强（PM和密度微粒体脂质体中分别增强280%和230%），并且与对照中仅约117%的刺激相比，PM中胰岛素进一步刺激其内在活性约160%。总之，我们的数据表明胰岛素刺激大鼠脂肪细胞中GLUT的内在活性，且该活性在衰老过程中进一步增强。衰老过程中葡萄糖摄取受损可归因于GLUT4水平降低以及细胞表面GLUT功能受损。GLUT功能受损与内在活性增强之间的差异表明存在其他调节GLUT在细胞表面完整功能表达的因素。
