Simultaneous downregulation of CDK inhibitors p18(Ink4c) and p27(Kip1) is required for MEN2A-RET-mediated mitogenesis.
Multiple endocrine neoplasia type 2A (MEN2A) is predisposed by mutations in the RET proto-oncogene. Low expression of the cyclin-dependent kinase inhibitor (CDKI) p27(Kip1) is present in thyroid tumors, and recent evidence demonstrates p27 downregulation by the active RET mutant, RET/PTC1, found in papillary thyroid carcinoma. This implicates decreased p27 activity as an important event during thyroid tumorigenesis. However, p27(-/-) mice develop MEN-like tumors only in combination with loss of another CDKI, p18(Ink4c). This suggests that p18 and p27 functionally collaborate in suppression of tumorigenesis, that loss of both is critical in the development of MEN tumors and that both p18 and p27 are regulated by RET. We report that induction of the constitutively active MEN2A-specific RET mutant, RET2A(C634R), correlates with reduced p18/p27, and elevated cyclin D protein levels, leading to increased CDK activity, increased pRb phosphorylation and proliferation under growth arrest conditions. Mechanistically, RET2A represses p18/p27 mRNA levels while elevating cyclin D1 mRNA levels. RET2A expression also correlates with decreased p27 protein stability. RET2A-mediated regulation of p18 and p27, but not of cyclins D1 and D2, requires functional mitogen-activated protein kinase signaling. Additionally, RET2A-dependent p18 repression is required and sufficient to increase cell proliferation. Perhaps most significantly, MEN2A adrenal tumors also display these changes in cell cycle expression profile, demonstrating the biological relevance of our cell culture studies. Our results demonstrate for the first time that RET2A regulates p18, and suggest that loss of not only p27 but also of p18 expression is a key step in MEN tumorigenesis.
Joshi
P P
PP
Department of Biological Sciences, Purdue Cancer Center, Purdue University, West Lafayette, IN, USA.
Kulkarni
M V
MV
Yu
B K
BK
Smith
K R
KR
Norton
D L
DL
van Veelen
W
W
Höppener
J W M
JW
Franklin
D S
DS
D000310:Adrenal Gland Neoplasms / Q000378:metabolism; D000818:Animals; D018276:Carcinoma, Medullary / Q000378:metabolism; D002453:Cell Cycle; D049109:Cell Proliferation; D050763:Cyclin-Dependent Kinase Inhibitor p18 / Q000378:metabolism*; D050760:Cyclin-Dependent Kinase Inhibitor p27 / Q000378:metabolism*; D015536:Down-Regulation; D006801:Humans; D020935:MAP Kinase Signaling System / Q000502:physiology; D051379:Mice; D008954:Models, Biological; D018813:Multiple Endocrine Neoplasia Type 2a / Q000378:metabolism*; D041681:NIH 3T3 Cells; D015513:Oncogene Proteins / Q000378:metabolism; D010673:Pheochromocytoma / Q000378:metabolism; D000090063:Proto-Oncogene Mas; D051096:Proto-Oncogene Proteins c-ret / Q000378:metabolism / Q000502:physiology*; D014162:Transfection
D050763:Cyclin-Dependent Kinase Inhibitor p18; C000715470:MAS1 protein, human; D015513:Oncogene Proteins; D000090063:Proto-Oncogene Mas; C065400:oncogene protein trk; D050760:Cyclin-Dependent Kinase Inhibitor p27; D051096:Proto-Oncogene Proteins c-ret