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Mol. Cell. Biol., May 1997, 2458-2467, Vol 17, No. 5
C Sandhu, J Garbe, N Bhattacharya, J Daksis, CH Pan, P Yaswen, J Koh, JM Slingerland and MR Stampfer
The effects of transforming growth factor beta (TGF-beta) were studied in
closely related human mammary epithelial cells (HMEC), both finite-
life-span 184 cells and immortal derivatives, 184A1S, and 184A1L5R, which
differ in their cell cycle responses to TGF-beta but express type I and
type II TGF-beta receptors and retain TGF-beta induction of extracellular
matrix. The arrest-resistant phenotype was not due to loss of
cyclin-dependent kinase (cdk) inhibitors. TGF-beta was shown to regulate
p15INK4B expression at at least two levels: mRNA accumulation and protein
stability. In TGF-beta-arrested HMEC, there was not only an increase in p15
mRNA but also a major increase in p5INK4B protein stability. As cdk4- and
cdk6-associated p15INK4B increased during TGF- beta arrest of sensitive
cells, there was a loss of cyclin D1, p21Cip1, and p27Kip1 from these
kinase complexes, and cyclin E-cdk2-associated p27Kip1 increased. In HMEC,
p15INK4B complexes did not contain detectable cyclin. p15INK4B from both
sensitive and resistant cells could displace in vitro cyclin D1, p21Cip1,
and p27Kip1 from cdk4 isolated from sensitive cells. Cyclin D1 could not be
displaced from cdk4 in the resistant 184A1L5R cell lysates. Thus, in
TGF-beta arrest, p15INK4B may displace already associated cyclin D1 from
cdks and prevent new cyclin D1-cdk complexes from forming. Furthermore,
p27Kip1 binding shifts from cdk4 to cyclin E-cdk2 during TGF-beta-mediated
arrest. The importance of posttranslational regulation of p15INK4B by
TGF-beta is underlined by the observation that in TGF-beta-resistant
184A1L5R, although the p15 transcript increased, p15INK4B protein was not
stabilized and did not accumulate, and cyclin D1-cdk association and kinase
activation were not inhibited.
Copyright © 1997, American Society for Microbiology
Transforming growth factor beta stabilizes p15INK4B protein, increases p15INK4B-cdk4 complexes, and inhibits cyclin D1-cdk4 association in human mammary epithelial cells
Division of Cancer Biology Research, Toronto-Sunnybrook Regional Cancer Centre, Ontario, Canada.
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