Cell Growth & Differentiation, Vol 9, Issue 12 983-988, Copyright © 1998 by American Association of Cancer Research
Inhibition of G1 cyclin-dependent kinase activity in cell density-dependent growth arrest in human fibroblasts
M Afrakhte, NE Heldin and B Westermark
Department of Genetics and Pathology, University Hospital, Uppsala, Sweden.
The growth of normal fibroblasts in culture ceases as the cells reach
saturation density. Although cells in dense cultures express functionally
active growth factor receptors, they are essentially refractory to the
mitogenic activity of growth factors. Northern blot analysis revealed that
immediate early genes, c-myc, c-fos and c-jun are induced by mitogen in
dense cultures. However, these cells fail to express the late G1 genes as
E2F-1, cdc25A, and cyclin A in response to mitogen stimulation.
Furthermore, because pRb-phosphorylation is a key event in G1 progression,
here we show that in dense cultures, pRb remains in its active
(hypophosphorylated) form after stimulation by mitogens. We also show that
the kinase activity of cyclin-dependent kinases that are indispensable for
the phosphorylation of pRb in late G1 phase was decreased on increasing
cell density. The reduced kinase activity may be caused by the observed
increase in cyclin-dependent kinase inhibitors and the reduction of cdc25A
expression in dense cells.