Laboratory of Lactation and Metabolic Physiology, Department of Animal Science; and.
Department of Pharmacology, University of Vermont, Burlington, Vermont.
Am J Physiol Regul Integr Comp Physiol. 2014 Aug 1;307(3):R237-47. doi: 10.1152/ajpregu.00093.2014. Epub 2014 Jun 11.
Glucose is a major substrate for milk synthesis and is taken up from the blood by mammary epithelial cells (MECs) through facilitative glucose transporters (GLUTs). The expression levels of GLUT1 and GLUT8 are upregulated dramatically in the mammary gland from late pregnancy through early lactation stages. This study aimed to test the hypothesis that this increase in GLUT1 and GLUT8 expression involves hypoxia signaling through hypoxia inducible factor-1α (HIF-1α) in MECs. Mouse mammary glands showed significantly more hypoxia in midpregnancy through early lactation stages compared with in the virgin stage, as stained by the hypoxia marker pimonidazole HCl. Treatment with hypoxia (2% O2) significantly stimulated glucose uptake and GLUT1 mRNA and protein expression, but decreased GLUT8 mRNA expression in bovine MECs. In MECs, hypoxia also increased the levels of HIF-1α protein in the nuclei, and siRNA against HIF-1α completely abolished the hypoxia-induced upregulation of GLUT1, while having no effect on GLUT8 expression. A 5'-RCGTG-3' core HIF-1α binding sequence was identified 3.7 kb upstream of the bovine GLUT1 gene, and HIF-1α binding to this site was increased during hypoxia. In conclusion, the mammary glands in pregnant and lactating animals are hypoxic, and MECs respond to this hypoxia by increasing GLUT1 expression and glucose uptake through a HIF-1α-dependent mechanism. GLUT8 expression, however, is negatively regulated by hypoxia through a HIF-1α-independent pathway. The regulation of glucose transporters through hypoxia-mediated gene transcription in the mammary gland may provide an important physiological mechanism for MECs to meet the metabolic demands of mammary development and lactation.
葡萄糖是乳汁合成的主要底物，通过易化葡萄糖转运蛋白（GLUTs）从血液中被乳腺上皮细胞（MECs）摄取。GLUT1 和 GLUT8 的表达水平在妊娠后期至泌乳早期阶段在乳腺中显著上调。本研究旨在测试以下假设：在 MECs 中，通过缺氧诱导因子-1α（HIF-1α）的缺氧信号增加 GLUT1 和 GLUT8 的表达。通过缺氧标志物 pimonidazole HCl 染色，与处女期相比，从中孕期到泌乳早期，小鼠乳腺显示出明显更多的缺氧。用缺氧（2%O2）处理显著刺激了牛 MECs 的葡萄糖摄取和 GLUT1 mRNA 和蛋白表达，但降低了 GLUT8 mRNA 的表达。在 MECs 中，缺氧还增加了核内 HIF-1α 蛋白的水平，针对 HIF-1α 的 siRNA 完全消除了缺氧诱导的 GLUT1 上调，而对 GLUT8 表达没有影响。在牛 GLUT1 基因的上游 3.7kb 处鉴定出一个 5'-RCGTG-3'核心 HIF-1α 结合序列，在缺氧期间该位点的 HIF-1α 结合增加。总之，怀孕和哺乳期动物的乳腺是缺氧的，MECs 通过增加 GLUT1 表达和葡萄糖摄取来应对这种缺氧，这是通过 HIF-1α 依赖的机制。然而，GLUT8 的表达通过 HIF-1α 非依赖途径受到缺氧的负调控。通过乳腺中缺氧介导的基因转录来调节葡萄糖转运蛋白，可能为 MECs 满足乳腺发育和泌乳的代谢需求提供了一个重要的生理机制。
