Development of reverse transcription-competitive polymerase chain reaction method to quantitate the expression levels of human sodium iodide symporter.

The sodium iodide symporter (NIS) is the plasma membrane protein that mediates active iodide uptake into thyroid follicular cells. To investigate whether human NIS (hNIS) mRNA levels in papillary thyroid carcinomas (PCs) correlate with the ability of tumors to concentrate radioiodide, we developed a reverse transcription-competitive polymerase chain reaction (RT-cPCR) method to quantify the hNIS mRNA levels in thyroid tissues. We studied 7 normal thyroid tissues, 8 PCs, and 1 follicular adenoma. hNIS mRNA levels in PCs were generally lower than those found in normal thyroid tissues. The reduced radioiodide concentrating activity of PCs is due, at least in part, to the reduced expression and/or the decreased stability of hNIS mRNA.

An immunohistochemical study of Na+/I- symporter in human thyroid tissues and salivary gland tissues.

The human Na+/I- symporter (hNIS) is the plasma membrane protein that mediates active iodide uptake into several tissues, such as the thyroid and salivary glands. To study the distribution and cellular localization of the hNIS protein, we have generated a polyclonal antibody that could detect the hNIS protein by immunohistochemical staining on tissue sections. In normal thyroids, hNIS expression is heterogeneous, and it is only detected in sporadic thyrocytes of a given follicle. The hNIS protein was not detected in thyroid carcinomas, yet it was detected in the majority of thyrocytes in Graves' thyroids. In salivary glands, hNIS protein was not detected in acinar cells, but it was detected in ductal cells. The hNIS proteins are clustered in the basal and lateral membranes in cells stained positive for hNIS.

Expression, exon-intron organization, and chromosome mapping of the human sodium iodide symporter.

The active iodide uptake of the thyroid gland in humans is mediated by the human sodium iodide symporter (hNIS). In this report, we show that hNIS expression was detected primarily in thyroid tissue, but also in breast, colon, and ovary tissues. Expression of hNIS is greatly reduced in thyroid tumors compared to normal thyroid tissue. Among tumor tissues, hNIS expression appears to be variable, consistent with the variable response to radioiodide treatment observed for thyroid carcinomas. The coding region of hNIS is interrupted by 14 introns, and the nucleotide sequence of each exon-intron junction is reported. Using this information, an alternatively spliced form of hNIS was identified. Finally, the chromosome location of the hNIS gene was mapped to chromosome 19p.

Cloning of the human sodium lodide symporter.

The iodide concentrating activity of the thyroid gland is essential to the production of thyroid hormone and also provides a mechanism for the treatment of thyroid cancer by radioiodine ablation. We report here the nucleotide and amino acid sequence of the human sodium iodide symporter (hNIS), which mediates the iodide uptake activity in the thyroid gland. An open reading frame of 1929 nucleotides encodes a protein of 643 amino acids with 84% identity to the rat NIS (rNIS). Transient expression of the hNIS cDNA conferred perchlorate-sensitive iodide uptake to a nonthyroid cell line, COS-7. The expression of hNIS was detected at variable levels in papillary thyroid carcinoma tissues but not in any of the thyroid carcinoma cell lines that have lost the iodide uptake activity.

Characterization of ret oncogenic activation in MEN2 inherited cancer syndromes.

Germline mutations of c-ret, encoding a receptor-type tyrosine kinase, were found to be associated with variants of multiple endocrine neoplasia type 2 (MEN2A, MEN2B), and familial medullary thyroid carcinoma. NIH/3T3 stable transfectants expressing RET with a mutation of MEN2A (MEN2A/RET) or MEN2B (MEN2B/RET) gained a transformed morphology, formed colonies in soft agar, and formed tumors in nude mice. These results confirmed that both MEN2A/RET and MEN2B/RET exert dominant transforming activities in NIH/3T3 cells. However, in contrast to their clinical manifestation, transfectants expressing MEN2A/RET exhibited a higher tumorigenicity in nude mice than transfectants expressing MEN2B/RET may depend on the presence of its ligand and/or substrates that are absent in NIH/3T3 cells. No change in the cellular localization of the mutated RET proteins was observed compared to c-RET. Interestingly, ret activation in NIT/3T3 cells appeared to be associated with up-regulation of homologous gap-junctional intercellular communication and increased expression of a gap-junctional protein, connexin43.

Breakpoint characterization of the ret/PTC oncogene in human papillary thyroid carcinoma.

The ret/PTC oncogene, rearranged form of the ret proto-oncogene (c-ret), has been detected specifically in a minority of papillary thyroid carcinomas. Three forms of the ret/PTC oncogene have been identified; the two most common forms, ret/PTC-1 and ret/PTC-3, both result from a paracentric inversion, of the long arm of chromosome 10. In this study, we have successfully amplified the chimeric introns resulting from these inversions, ranging from 1.4 to 10 kb, from four of five tumors known to contain the ret/PTC-1 oncogene (where c-ret rearranges with the H4 gene), and from 1/1 tumors containing the ret/PTC-3 oncogene (where c-ret rearranges with the ele1 gene). We localized the breakpoints within the chimeric introns using nested PCR, and determined the exact nucleotide sequence at the breakpoint for each tumor. Our results indicate that the breakpoints in c-ret occur at sites distributed across intron 11, where breaks in H4 intron 1 appear more frequently at the 5'- end of the intron. Interestingly, in all tumors that we investigated, the breakpoints occurred at sits of two or three nucleotide matches between the contributing germline sequences. In summary, we describe a simple, convenient way to investigate the ret/PTC breakpoints, and have revealed several common features of the breakpoints which warrant further investigations.

Characterization of the promoter region and oligomerization domain of H4 (D10S170), a gene frequently rearranged with the ret proto-oncogene.

PTC-1, the predominant form of PTC oncogene in human papillary thyroid carcinoma, encodes a fusion protein containing the N-terminus of H4 (D10S170) fused 5' to the ret tyrosine kinase domain. Accordingly, the PTC-1 expression is driven by the H4 gene promoter. Our study showed that H4 is expressed in various human tissues, including thyroid. Furthermore, we have localized the transcriptional start sites of H4 to a region 100 to 190 bp upstream of the translation initiation site (ATG) by primer extension assay, and the H4 promoter to a region within 259 bp upstream of the ATG site by luciferase assay. Interestingly, protein sequence analysis indicated a potential coiled-coil domain in the N-terminal region of H4. Indeed, oligomerization was demonstrated by an in vitro assay with recombinant proteins containing this region. As dimerization is considered to be a crucial step for receptor tyrosine kinase activation, we hypothesize that both unscheduled expression of ret tyrosine kinase and constitutive oligomerization of PTC-1 proteins are responsible for PTC-1 transforming activity in thyroid.

Development of a single-step duplex RT-PCR detecting different forms of ret activation, and identification of the third form of in vivo ret activation in human papillary thyroid carcinoma.

In up to 25% of human papillary thyroid carcinomas (PCs), the oncogenic activation of ret results from the fusion of its tyrosine kinase domain with different unlinked amino-terminal sequences. Activation of this oncogene may be of prognostic significance in patients with PC. To screen for ret activation in archival paraffin-embedded tumors, we have developed a single-step duplex RT-PCR to detect different forms of ret activation despite different chimeric transcripts being expressed. Furthermore, we report a third type of ret oncogenic activation, named ret/PTC3, identified by using this novel method followed by rapid amplification of cDNA ends (5'-RACE) cloning.

Thyroid peroxidase expression and DNA polymorphisms in thyroid cancer.

Using non-quantitative reverse-transcription polymerase chain reaction (RT-PCR), we found that thyroid peroxidase (TPO) is expressed in all differentiated thyroid carcinomas examined, although the ratio of the shorter to longer transcript is decreased in tumors that had lost the iodide concentrating capacity. TPO expression is lost in several thyroid carcinoma cell lines (TPC-1, 8305C, 8505C) and altered in another (TC-80). Nucleotide sequencing of the PCR products revealed missense polymorphisms in the TPO gene. Four out of five samples tested are heterozygous for TPO alleles in exon 15, showing both C and T at nucleotide 2612 (GTG coding for Val, GCG for Ala). One tumor is homozygous for T at this position. In exon 8, three samples show T at nucleotide 1189 (TCG, Ser) and C at 1265 (ACC, Thr), while most published sequences report G at both positions (GCG coding for Ala at 1189 and AGC coding for Ser at 1265).

Altered affinity of insulin-like growth factor II (IGF-II) for receptors and IGF-binding proteins, resulting from limited modifications of the IGF-II molecule.

The binding affinities of seven analogues of recombinant human insulin-like growth factor II (hIGF-II) were characterized for the IGF type-I and type-II receptors and insulin receptors, as well as for IGF-binding protein (IGFBP)-1, IGFBP-2, IGFPB-3 and human serum IGFBPs. A switch of two of the three cysteine bridges in hIGF-II, 9-47 and 46-51 to 9-46 and 47-51, severely impaired the binding of this analogue to all receptors and to the IGFBPs. The affinities for the IGF type-I receptor and the IGFBPs were decreased over 100-fold, while the binding to the insulin receptor and the IGF type-II receptor was less affected, with a 6-10-fold decrease in affinity. Slight modifications of the N-terminus had only minor effects upon the binding of hIGF-II to the IGFBPs or to the receptors. Deletion of both the N-terminal amino acid and the two C-terminal amino acids resulted in moderate decreases in affinity, with a 60% decrease in affinity for IGFBP-1 and the IGF type-I receptor. Acetylation of the N-terminus of Ala1 and the epsilon-nitrogen of Lys65 decreased the affinity, by 60-90%, of hIGF-II for all of the IGFBPs and receptors. The experiments involving acetylation of IGF-II or switching of its cysteine bridges indicated that these modifications (no substitution, deletion or addition of any of the 67 amino acids of hIGF-II) may lead to a severe impairment of the binding affinity of IGF-II for both the IGFBPs and the receptors. Acetylation of the epsilon-nitrogen of Lys65, which causes a charge change, or alteration of the three-dimensional structure, as shown by the cysteine bridge switch, lead to a severe impairment of the binding affinity for the binding proteins and for the receptors. In general, care should be taken with the synthesis of analogues and the interpretation of resulting binding data, since affinity alterations ascribed to amino acid changes may instead be caused by alterations of the charge or the three-dimensional structure of the protein.

Aspartokinase-homoserine dehydrogenase I from Escherichia coli: pH and chemical modification studies of the kinase activity.

The pH variation of the kinetic parameters was examined for the kinase activity of the bifunctional enzyme aspartokinase--homoserine dehydrogenase I isolated from Escherichia coli. The V/K profile for L-aspartic acid indicates the loss of activity upon protonation of a cationic acid type group with a pK value near neutrality. Incubation of the enzyme with diethyl pyrocarbonate at pH 6.0 results in a loss of enzymic activity. The reversal of this reaction by neutral hydroxylamine, the appearance of a peak at 242 nm for the inactivated enzyme, and the observation of a pK value of 7.0 obtained from variation of the inactivation rate with pH all suggest that enzyme inactivation occurs by modification of histidine residues. The substrate L-aspartic acid protects one residue against inactivation, which implies that this histidine may participate in substrate binding or catalysis. Activity loss was also observed at high pH due to the ionization of a neutral acid group with a pK value of 9.8. The reactions of AK-HSD I with N-acetylimidazole and tetranitromethane have been investigated to obtain information about the functional role of tyrosyl residues in the enzyme. The acylation of tyrosines leads to inactivation of the enzyme, which can then be fully reversed by treatment with hydroxylamine. Incubation of the enzyme with tetranitromethane at pH 9.5 also leads to rapid inactivation, and the substrates of the kinase reaction provide substantial protection against inactivation. However, three tyrosines are protected by substrates, implying a structural role for these amino acids.