From an iodide transport defect to colorectal cancer: Physiology and pathophysiology of two related transporters NIS (sodium ion/iodine- transporter) and SMCT (sodium ion -dependent monocarboxylate transporter)
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The human sodium/monocarboxylate transporter (SMCT) is a plasma membrane protein that mediates active short-chain fatty acid (SCFA) transport. SMCT is the product of the SLC5A8 gene and is 70% homologous to the sodium/iodide symporter (NIS). We have cloned SMCT from a human thyroid cDNA library and generated high-affinity anti-SMCT polyclonal antibodies (Abs). Using our Abs, we have shown that SMCT is apically localized in the thyroid, kidney, colon, liver, uterus, ovaries, fallopian tubes, lactating mammary gland and in MDCK cells, stably expressing SMCT. SMCT is a glycoprotein when expressed endogenously in the FRTL-5, (a line of highly functional rat thyroid cells), human colon tissues, or stably transfected MDCK cells. In Xenopus laevis oocytes, in addition to lactate, pyruvate, propionate, and butyrate, SMCT transports beta-hydroxybutyrate but not acetate or acetoacetate and is not inhibited by high substrate concentrations. We have shown for the first time the existence of an endogenously expressed high-affinity Na+-dependent monocarboxylate transporter in the thyroid cells, not regulated by TSH. Strikingly, we have also shown for the first time that the expression of SMCT at the protein level is either completely abolished or markedly downregulated in 93% of the tested human colorectal tumors as compared to the surrounding normal tissue.;In the second part of this work we examined congenital hypothyroidism-causing mutation (R124H) in the Na+/I- symporter (NIS), an intrinsic membrane glycoprotein which mediates active transport of iodide from the bloodstream into the thyroid epithelial cells. R124H NIS mutation was recently identified in a French family of African descent. In COS-7 cells transiently transfected with NIS cDNA constructs, carrying various amino acid substitutions (A, W, K, E, D, N) at position 124, we demonstrated that these mutants have a defect in maturation and trafficking to the plasma membrane and therefore, do not exhibit any significant iodide transport. R124H NIS is sensitive to Endoglycosidase H and co-localizes with endoplasmic reticulum resident protein marker, indicating that it never completes glycosylation pathway and is retained within ER and/or Golgi compartments. In contrast, R124Q NIS is well expressed, targeted to the plasma membrane and accumulates ∼40% of the iodide as compared to wild-type NIS. The kinetic analysis of R124Q NIS indicates that the Km of this mutant protein is similar to the wild-type NIS, while the Vmax is roughly half of the wild-type. These results indicate that the presence of an NH2 group at position 124 is important for NIS function.