Germ-line point mutations of the RET gene are responsible for multiple endocrine neoplasia (MEN) type 2A and 2B that develop medullary thyroid carcinoma and pheochromocytoma. We performed a differential display analysis of gene expression using NIH 3T3 cells expressing the RET-MEN2A or RET-MEN2B mutant proteins. As a consequence, we identified 10 genes induced by both mutant proteins and eight genes repressed by them. The inducible genes include cyclin D1, cathepsins B and L, and cofilin genes that are known to be involved in cell growth, tumor progression, and invasion. In contrast, the repressed genes include type I collagen, lysyl oxidase, annexin I, and tissue inhibitor of matrix metalloproteinase 3 (TIMP3) genes that have been implicated in tumor suppression. In addition, six RET-MEN2A-and five RET-MEN2B-inducible genes were identified. Among 21 genes induced by RET-MEN2A and/or RET-MEN2B, six genes including cyclin D1, cathepsin B, cofilin, ring finger protein 11 (RNF11), integrin-␣6, and stanniocalcin 1 (STC1) genes were also induced in TGW human neuroblastoma cells in response to glial cell line-derived neurotrophic factor stimulation. Because the STC1 gene was found to be highly induced by both RET-MEN2B and glial cell line-derived neurotrophic factor stimulation, and the expression of its product was detected in medullary thyroid carcinoma with the MEN2B mutation by immunohistochemistry, this may suggest a possible role for STC1 in the development of MEN 2B phenotype. The RET proto-oncogene encodes a receptor tyrosine kinase with a cadherin-related motif and a cysteine-rich domain in the extracellular domain and is located on chromosome 10q11.2.1,2 It has been demonstrated that RET is a functional receptor for four related neurotrophic factors including glial cell line-derived neurotrophic factor (GDNF), neurturin, artemin, and persephin. These factors are known to require glycosylphosphatidylinositol-anchored co-receptors, GFR-␣s, as ligand-binding components and to promote the survival of various central and peripheral neurons in culture.1,2 In addition, gene knockout studies revealed that the GDNF/RET signaling plays a crucial role in the development of the enteric nervous system and the kidney. [3][4][5][6] Germline mutations of the RET gene cause dominant inherited cancer syndromes; multiple endocrine neoplasia (MEN) type 2A and 2B.7-10 MEN 2A is characterized by the development of medullary thyroid carcinoma (MTC), pheochromocytoma, and parathyroid hyperplasia. MEN 2B shows a more complex phenotype with association of MTC, pheochromocytoma, and developmental abnormalities such as mucosal neuroma, hyperganglionosis of the intestinal tract, and marfanoid skeletal changes. The MEN2A mutations were identified in cysteine residues of the RET extracellular domain, leading to ligand-independent RET dimerization. 11,12 The MEN2B mutations were detected in methionine at codon 918 or in alanine at codon 883 in the tyrosine kinase domain and appear to activate RET without dimerization. 12,13 A variety of si...

The nucleolus is the site of ribosomal DNA (rDNA) transcription and ribosome production. In exploring the role of nucleolar protein MCRS1 (microspherule protein1)/MSP58 (58-kDa microspherule protein), we found that Mi-2␤, a component of a nucleosome remodeling and deacetylase (NuRD) complex, RET finger protein (RFP), and upstream binding factor (UBF) were associated with MCRS1. Yeast two-hybrid assays revealed that MCRS1 bound to the ATPase/helicase region of Mi-2␤ and the coiled-coil region of RFP. Interestingly, confocal microscopic analyses revealed the co-localization of MCRS1, Mi-2␤, RFP, and the rRNA transcription factor UBF in the nucleoli. We also found that MCRS1, Mi-2␤, and RFP were associated with rDNA using a chromatin immunoprecipitation assay. Finally, we showed that MCRS1, Mi-2␤, and RFP up-regulated transcriptional activity of the rDNA promoter and that ribosomal RNA transcription was repressed when MCRS1, Mi-2␤, and RFP expression was reduced using siRNA. These results indicated that Mi-2␤ and RFP, known to be involved in transcriptional repression in the nucleus, co-localize with MCRS1 in the nucleolus and appear to activate the rRNA transcription.

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