Bose S, Seetharam S, Hammond T G, Seetharam B
Department of Medicine, Medical College of Wisconsin, Milwaukee 53226, USA.
Biochem J. 1995 Sep 15;310 ( Pt 3)(Pt 3):923-9. doi: 10.1042/bj3100923.
Surface and intracellular membrane distribution and hormonal regulation of transcobalamin II receptor (TC II-R) activity and protein levels have been studied in an effort to understand its regulation of expression in the rat. TC II-R activity and the levels of the 62 kDa monomeric and 124 kDa dimeric forms of TC II-R were highest in the rat kidney and intestine, and in these tissues the receptor expression was not dependent upon the postnatal development of the rat. TC II-R expression was uniform in the various regions of the gut. Surface membrane distribution of TC II-R in the kidney revealed the expression of the 124 kDa dimer form of TC II-R in the apical and basolateral membranes in the ratio of 1:10. Further subcellular distribution of TC II-R in the kidney revealed the expression of the 124 kDa dimer in the intermicrovillar clefts and clathrin-coated vesicles and the 62 kDa monomer in the microsomes. Neither the monomer nor the dimer could be detected in the early endosomes or lysosomes. Membrane TC II-R activity and TC II-R protein levels and cobalamin (Cbl; vitamin B12) transport in vivo were inhibited by about 90% in adrenalectomized rats and all three returned to normal levels by oral treatment of these animals with cortisone acetate. In contrast, thyroidectomy or experimentally induced diabetes had no effect on TC II-R activity or Cbl transport. Based on these observations, we suggest that TC II-R expression is not developmentally or regionally regulated in rat renal and intestinal membranes and its expression in the kidney is asymmetrically distributed between the apical (10%) and basolateral (90%) membranes. In addition, our results also show that the dimerization of TC II-R is a post-microsomal event and that the expression of TC II-R and plasma Cbl transport is regulated by cortisone.
为了解转钴胺素II受体（TC II-R）活性和蛋白水平在大鼠体内的表达调控机制，研究人员对其在细胞表面和细胞内膜的分布以及激素调节进行了研究。大鼠肾脏和肠道中TC II-R活性以及62 kDa单体和124 kDa二聚体形式的TC II-R水平最高，且在这些组织中，受体表达不依赖于大鼠的出生后发育。TC II-R在肠道各区域的表达较为均匀。肾脏中TC II-R的细胞膜分布显示，124 kDa二聚体形式的TC II-R在顶端膜和基底外侧膜中的表达比例为1:10。肾脏中TC II-R的进一步亚细胞分布显示，124 kDa二聚体在微绒毛间裂和网格蛋白包被小泡中表达，62 kDa单体在微粒体中表达。在早期内体或溶酶体中均未检测到单体或二聚体。肾上腺切除的大鼠体内，膜TC II-R活性、TC II-R蛋白水平和钴胺素（Cbl；维生素B12）转运受到约90%的抑制，对这些动物口服醋酸可的松后，三者均恢复到正常水平。相比之下，甲状腺切除或实验性诱导糖尿病对TC II-R活性或Cbl转运没有影响。基于这些观察结果，我们认为，TC II-R在大鼠肾膜和肠膜中的表达不受发育或区域调控，其在肾脏中的表达在顶端膜（10%）和基底外侧膜（90%）之间呈不对称分布。此外，我们的结果还表明，TC II-R的二聚化是微粒体后事件，且TC II-R的表达和血浆Cbl转运受可的松调节。
