J. Saito et al., Regulation of FRTL-5 thyroid cell growth by phosphatidylinositol (OH) 3 kinase-dependent Akt-mediated signaling, THYROID, 11(4), 2001, pp. 339-351
Thyrotropin (TSH)-initiated cell cycle progression from G(1) to S phase in
FRTL-5 thyroid cells requires serum, insulin, or insulin-like growth factor
1 (IGF-1) and involves activation of 3-hydroxSr-3-methylglutaryl-CoA reduc
tase, geranylgeranylation of RhoA, p27(Kip1) degradation, and activation of
cyclin-dependent kinase (cdk) 2. In the present report, we show that the s
erine-threonine kinase Akt is an important mediator of insulin/ IGF-1/serum
effects on cell cycle progression in FRTL-5 thyroid cells. The phosphoinos
itol (OH) 3 kinase inhibitors, Wortmannin (WM) and Ly294002 (LY), block the
ability of insulin/IGF-1 to reduce p27 expression, to induce expression of
cyclins E, D1, and A as well as cdk 2 and 4, and to phosphorylate retinobl
astoma protein. They also inhibit insulin/IGF-1-increased DNA synthesis and
cell cycle entrance (S+G(2)/M). Insulin/IGF-1 rapidly induced activation o
f Akt1 in a PI3 kinase-dependent manner, and increased Akt1 RNA levels. Mos
t importantly, FRTL-5 cells transfected with a constitutively active form o
f Akt1 have higher basal rates of DNA synthesis and no longer require exoge
nous insulin/IGF-1 or serum for TSH-induced growth. In sum, Akt1 appears to
have an important role in insulin/IGF-1 regulation of FRTL-5 thyroid cell
growth and cell cycle progression.