Loss of p27(Kip1) from cyclin E/cyclin-dependent kinase (CDK) 2 but not from cyclin D1/CDK4 complexes in cells transformed by polyamine biosynthetic enzymes

Cancer cells are known to display up-regulation of ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC), the key enzymes in the biosynthesis of polyamines that are essential for cellular proliferati on. We have shown previously that overexpression of ODC or AdoMetDC alone c an induce tumorigenic transformation of rodent fibroblasts. Because the sub version of normal cell cycle control is thought to be a crucial event in ca ncer development, we examined ODC- and AdoMetDC-transformed fibroblasts for alterations in the cell cycle components. The level of cyclin D1 and cycli n D1-dependent kinase and total cyclin-dependent kinase (CDK) 4 activities were elevated in the ODC transformants and particularly in the AdoMetDC tra nsformants. Cyclin E content was not elevated, but a moderate increase in c yclin E-dependent kinase activity was seen in both cells. Total CDK2 activi ty was increased only in the ODC-transformed cells. The amount of the p27(K ip1) CDK inhibitor was greatly decreased in both transformants. Nevertheles s, p27(Kip1) was present in the active cyclin D1/CDK4 complexes in the cell s but absent from the cyclin E/CDK2 complexes. Restoration of p27(Kip1) exp ression in the ODC- and AdoMetDC-transformed cells by transfection resulted in growth inhibition, but not in morphological reversion. An elevation in the level of hyperphosphorylated retinoblastoma protein was observed mainly in the ODC-transformed cells. These results suggest that the expression of ODC or AdoMetDC may affect cell cycle regulation in many ways. However, th e largest common effect, which is therefore potentially relevant to some as pects of transformation, appears to be the constitutive down-regulation of p27(Kip1) its loss from the cyclin E/CDK2 complexes.