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J. Cell Biol.,
Volume 140, Number 6, March 23, 1998 1427-1439



* Institute of Interdisciplinary Research, Université Libre de Bruxelles, Campus Erasme, B-1070 Brussels, Belgium; and In different systems, cyclic adenosine monophosphate (cAMP) either blocks or promotes cell cycle
progression in mid to late G1 phase. Dog thyroid epithelial cells in primary culture constitute a model of
positive control of DNA synthesis initiation and G0-S prereplicative phase progression by cAMP as a second
messenger for thyrotropin (TSH). The cAMP-dependent mitogenic pathway is unique as it is independent
of mitogen-activated protein kinase activation and
differs from growth factor-dependent pathways at the
level of the expression of several protooncogenes/transcription factors. This study examined the involvement
of D-type G1 cyclins and their associated cyclin-dependent kinase (cdk4) in the cAMP-dependent G1 phase
progression of dog thyroid cells. Unlike epidermal
growth factor (EGF)+serum and other cAMP-independent mitogens, TSH did not induce the accumulation of cyclins D1 and D2 and partially inhibited the
basal expression of the most abundant cyclin D3. However, TSH stimulation enhanced the nuclear detection
of cyclin D3. This effect correlated with G1 and S phase
progression. It was found to reflect both the unmasking
of an epitope of cyclin D3 close to its domain of interaction with cdk4, and the nuclear translocation of cyclin
D3. TSH and EGF+serum also induced a previously undescribed nuclear translocation of cdk4, the assembly of precipitable cyclin D3-cdk4 complexes, and the
Rb kinase activity of these complexes. Previously, cdk4
activity was found to be required in the cAMP-dependent mitogenic pathway of dog thyrocytes, as in growth
factor pathways. Here, microinjections of a cyclin D3 antibody showed that cyclin D3 is essential in the TSH/
cAMP-dependent mitogenesis, but not in the pathway
of growth factors that induce cyclins D1 and D2. The
present study (a) provides the first example in a normal
cell of a stimulation of G1 phase progression occurring independently of an enhanced accumulation of cyclins
D, (b) identifies the activation of cyclin D3 and cdk4
through their enhanced assembly and/or nuclear translocation, as first convergence steps of the parallel
cAMP-dependent and growth factor mitogenic pathways, and (c) strongly suggests that this new mechanism
is essential in the cAMP-dependent mitogenesis, which
provides the first direct demonstration of the requirement for cyclin D3 in a G1 phase progression.
Division of Cancer Biology, Danish Cancer Society, DK-2100 Copenhagen, Denmark
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