ANALYSIS OF HUMAN SODIUM-IODIDE SYMPORTER GENE-EXPRESSION IN EXTRATHYROIDAL TISSUES AND CLONING OF ITS COMPLEMENTARY DEOXYRIBONUCLEIC ACIDSFROM SALIVARY-GLAND, MAMMARY-GLAND, AND GASTRIC-MUCOSA

The ability to concentrate iodide is a fundamental property of normall y functioning thyroid tissue and represents the first step in the prod uction of thyroid hormones. Iodide uptake has been demonstrated in var ious extrathyroidal tissues, including salivary gland, gastric mucosa, and lactating mammary gland. Recently, cloning and molecular characte rization of the human sodium iodide symporter (hNIS) have been reporte d; however, the patterns of hNIS gene expression in human tissues have remained unidentified. To examine the profiles of human hNIS gene exp ression in various normal human tissues, we performed high-stringency Northern blot analysis using a P-32-labeled hNIS-specific complementar y DNA (cDNA) probe (nucleotides 1184-1667). To detect rare hNIS transc ripts in small tissue samples, RT-PCR was performed with a pair of hNI S-specific oligonucleotide primers designed to amplify a portion (nucl eotides 1184-1667) of the hNIS gene. hNIS-specific transcripts were co nfirmed by Southern hybridization using a digoxigenin-labeled internal hNIS-specific oligonucleotide probe (nucleotides 1460-1477). To monit or cDNA integrity and quantity, and to rule out DNA contamination and illegitimate transcription, all samples were coamplified with two pair s of intron-spanning primers designed to amplify fragments of the huma n beta-actin and thyroglobulin genes, respectively. Using Northern blo t analysis, hNIS transcripts of approximately 4 kb were detected in th yroid gland and parotid gland but not in a broad range of endocrine an d nonendocrine tissues. RT-PCR and Southern hybridization revealed hNI S gene expression in thyroid gland, salivary gland, parotid gland, sub mandibular gland, pituitary gland, pancreas, testis, mammary gland, ga stric mucosa, prostate and ovary, adrenal gland, heart, thymus, and lu ng. By contrast, hNIS transcripts were not detected in normal orbital fibroblasts, colon, and nasopharyngeal mucosa. To further analyze hNIS gene sequences in parotid gland, mammary gland, and gastric mucosa, t he EXPAND High Fidelity PCR System and three sets of overlapping NIS o ligonucleotide primers were used for amplification and cloning. The re sulting PCR products were subcloned into pBluescript-SK II(-)vector, a nd at least two independent cDNA clones derived from each tissue were subjected to automated sequencing. The nucleotide sequences of hNIS cD NA derived from parotid gland, mammary gland, and gastric mucosa revea led full identity with the recently published human thyroid-derived NI S cDNA sequence. In conclusion, our results demonstrate markedly varia ble levels of hNIS gene expression in several extrathyroidal tissues. Although the physiological role of hNIS in these tissues awaits furthe r study, our results suggest that the capacity to actively transport i odine may be a feature common to several secretory and endocrine tissu es. The diminished capacity to transport and concentrate iodide in ext rathyroidal tissues (such as parotid gland, mammary gland, and gastric mucosa), compared with thyroid gland, does not seem to be caused by a n altered primary structure of the hNIS cDNA. Variability of NIS gene expression levels in normal extrathyroidal tissues may rather be cause d by differences in NIS gene transcriptional activity. Further studies will address this hypothesis and examine the mechanisms of tissue-spe cific regulation of NIS gene expression.