Post-transcriptional silencing of RET occurs, but is not required, during raf-1 mediated differentiation of medullary thyroid carcinoma cells

Abstract

Medullary thyroid carcinoma (MTC) is a neuroendocrine tumor of the calcitonin secreting thyroid C-cells. Somatic and germline mutations in the RET proto-oncogene are associated with sporadic and inherited cases of MTC, respectively. The human MTC cell line, TT, can be di erentiated by activated raf-1. This di erentiation is characterized, in part, by down-regulation of the RET proto-oncogene. We now show that raf-1 induction is followed by activation of the downstream kinases MEK1/2 and ERK1/2 and that di erentiation is dependent on activation of MEK1/2. The concurrent down-regulation of RET appears to involve altered nuclear compartmentalization and transport of RET mRNA. Although RET is down-regulated during raf-1 mediated di erentiation, overexpression of activated RET alleles which resist down-regulation does not alter the raf-1 mediated di erentiation response. These data suggest that RET down-regulation is associated with, but not required, for raf-1 mediated MTC cell di erentiation and that the raf-1 signal transduction pathway plays a dominant role in promoting MTC cell di erentiation.

Expression and alternative splicing of c-ret RNA in papillary thyroid carcinomas

“…A recent study (Wilson et al, 1999) supports a di erentially controlled mechanism for precursor mRNA splicing and export. This would also be consistent with the post-transcriptional silencing mechanism of c-ret observed in a medullary carcinoma cell line (TT) (Carson-Walter et al, 1998).…”

The Ras/Raf/MEK/Extracellular Signal-Regulated Kinase Pathway Induces Autocrine-Paracrine Growth Inhibition via the Leukemia Inhibitory Factor/JAK/STAT Pathway

“…1A), retarded growth ( Fig. 1B and C), and G 1 cell cycle arrest (Table 1), similar to the changes caused by Raf activation (9,10). This treatment also induced phosphorylation of ERK1/2, increased expression of the calcitonin/CGRP gene, and downregulation of RET (Fig.…”

“…We have shown that in human medullary thyroid carcinoma (MTC) cells, Ras or Raf activation results in differentiation and growth arrest (9,10,31). In the present study, we report that Ras or Raf activation induces expression and secretion of a protein that can mediate differentiation and G 1 cell cycle arrest.…”

Identification of a Soluble Factor that Induces Cell Death and Growth Inhibition in Prostate Carcinoma Cells

“…This apparently straightforward mechanism is complicated by cell typedependent participation of various intermediate signaling pathways, including p38 MAPK/PRAK, Wnt/glycogen synthase kinase 3/␤-catenin, secretion of soluble factors, and modulators of cellular redox balance (30 -35). We also have shown that the Raf/MEK/ERK pathway mediates growth arrest utilizing leukemia inhibitory factor, the JAK/STAT pathway, or IFI16 in a subset of tumor cell lines (22)(23)(24)36 ERK1/2, as the focal point of the Raf/MEK/ERK pathway, would be expected to play a pivotal role in regulating these diverse growth arrest signaling networks and use of the MEK1/2-specific inhibitors, U0126 and PD98059, strongly support this notion (12,16,22,37), the necessity of ERK1/2 has not been directly addressed. Study of ERK signaling is hampered because many cell types are sensitive to the absence of ERK1/2.…”

“…First, the MEK1/2 inhibitors, U0126 and PD98059, abrogate the growth inhibitory signaling, as reported previously by us and others (12,16,22,37). These inhibitors have high specificity to MEK1/2, which is expected for an inhibitor not competitive with ATP (61,62), and were tested for various kinases (63); this makes the possibility unlikely that other kinases are also involved.…”