Cyclin D1 and
cyclin E are overexpressed in approximately 45% and 30% of breast
cancers, respectively, and adverse associations with patient outcome have been reported. The potential roles of
cyclin D1 and
cyclin E expression as markers of therapeutic responsiveness to the pure steroidal
antiestrogen ICI 182780 were investigated using T-47D
breast cancer cell lines constitutively overexpressing
cyclin D1 or
cyclin E. Measurement of S phase fraction, phosphorylation states of the
retinoblastoma protein, and
cyclin E-
cyclin-dependent kinase (Cdk) 2 activity demonstrated that overexpression of
cyclin D1 decreased sensitivity to
antiestrogen inhibition at 24 and 48 h. Overexpression of
cyclin E produced a less pronounced early cell cycle effect indicating only partial resistance to
antiestrogen inhibition in the short-term. In ICI 182780-treated
cyclin D1-overexpressing cells, sufficient Cdk activity was retained to allow
retinoblastoma protein phosphorylation and cell proliferation, despite an increase in the association of p21 and p27 with
cyclin D1-Cdk4/6 and
cyclin E-Cdk2 complexes. After longer-term (>7 days) treatment,
antiestrogens inhibited colony growth in
cyclin D1- or
cyclin E-overexpressing
breast cancer cells, but with an approximately 2-2.5-fold decrease in dose sensitivity. This was associated with a fall in
cyclin D1 levels, a reduction in the half-life of
cyclin D1 protein and a decline in
cyclin E-Cdk2 activity in
cyclin D1-overexpressing cells, and the maintenance of
cyclin E-p27 association in the
cyclin E-overexpressing cells. These data confirm that
cyclin D1 expression and
cyclin E-p27 association play important roles in
antiestrogen action, and suggest that
cyclin D1 or
cyclin E overexpression has subtle effects on
antiestrogen sensitivity. Additional studies to elucidate the contribution of alterations in
cyclin D1 stability to
antiestrogen action and to assess the relationship between
antiestrogen sensitivity and expression of
cyclin D1,
cyclin E, or p27 in a clinical setting are required.