Differentiation of medullary thyroid cancer by C-Raf-1 silences expression of the neural transcription factor human achaete-scute homolog-1.
BACKGROUND
Human achaete-scute homolog-1 (hASH1), a fetal neural transcription factor, is highly expressed in neuroendocrine tumors such as medullary thyroid cancer (MTC). Although hASH1 probably plays a part in the growth and development of these tumors, its precise role and mechanism are unknown.


METHODS
To further elucidate the function and regulation of hASH1 in neuroendocrine tumor differentiation, we used a model of MTC tumor differentiation mediated by the ras/raf-1 signaling pathway. The MTC TT cells alone or transduced with a beta-estradiol activatable raf-1 construct (TT: delta Raf-1:ER) were treated with beta-estradiol or carrier. Northern analysis and nuclear run-off assays were performed to determine the hASH1 messenger RNA (mRNA) levels and transcription rate, respectively.


RESULTS
The TT: delta Raf-1:ER cells treated with beta-estradiol underwent marked biochemical and morphologic changes, including cell rounding, increase in calcitonin transcription, loss of RET proto-oncogene expression, and cessation of cell growth. During this differentiation process expression of hASH1 mRNA was silenced. Nuclear run-off experiments revealed that this decrease in steady-state hASH1 mRNA by raf-1 activation resulted predominantly from transcriptional inhibition.


CONCLUSIONS
Silencing of hASH1 in parallel with loss of RET is associated with development of a mature C-cell differentiation pattern. Mechanisms leading to transcriptional silencing of hASH1 may be crucial in regulating the proliferative capacity or differentiation status of MTC. Downstream targets of hASH1 could play a role in C-cell proliferation and progression to MTC.
Chen
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H
Department of Surgery, Johns Hopkins Medical Institutions, Baltimore, Md, USA.
Carson-Walter
E B
EB
Baylin
S B
SB
Nelkin
B D
BD
Ball
D W
DW
D051792:Basic Helix-Loop-Helix Proteins; D015152:Blotting, Northern; D002454:Cell Differentiation / Q000235:genetics; D002467:Cell Nucleus / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics*; D015972:Gene Expression Regulation, Neoplastic / Q000502:physiology; D018257:Helix-Loop-Helix Motifs / Q000235:genetics; D006801:Humans; D009490:Neurosecretory Systems / Q000473:pathology; D017346:Protein Serine-Threonine Kinases / Q000235:genetics*; D000090063:Proto-Oncogene Mas; D011518:Proto-Oncogene Proteins / Q000235:genetics*; D019908:Proto-Oncogene Proteins c-raf; D012333:RNA, Messenger / Q000032:analysis; D015398:Signal Transduction / Q000502:physiology; D013964:Thyroid Neoplasms / Q000235:genetics / Q000473:pathology*; D014157:Transcription Factors / Q000235:genetics*; D014158:Transcription, Genetic / Q000235:genetics; D014407:Tumor Cells, Cultured / Q000502:physiology
D051792:Basic Helix-Loop-Helix Proteins; D004268:DNA-Binding Proteins; D017346:Protein Serine-Threonine Kinases; D000090063:Proto-Oncogene Mas; D011518:Proto-Oncogene Proteins; D019908:Proto-Oncogene Proteins c-raf; D012333:RNA, Messenger; D014157:Transcription Factors; C065500:ASCL1 protein, human; C000715470:MAS1 protein, human