Inhibition of G1 cyclin-dependent kinase activity in cell density-dependent growth arrest in human fibroblasts.
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.
Afrakhte
M
M
Department of Genetics and Pathology, University Hospital, Uppsala, Sweden.
Heldin
N E
NE
Westermark
B
B
D017209:Apoptosis / Q000235:genetics*; D000077214:Becaplermin; D018797:Cell Cycle Proteins / Q000235:genetics / Q000378:metabolism; D056746:Cyclin G; D056766:Cyclin G1; D018844:Cyclin-Dependent Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D016213:Cyclins / Q000235:genetics / Q000378:metabolism*; D005347:Fibroblasts; D016193:G1 Phase / Q000235:genetics*; D005786:Gene Expression Regulation / Q000235:genetics; D016762:Genes, fos / Q000235:genetics; D016758:Genes, jun / Q000235:genetics; D016259:Genes, myc / Q000235:genetics; D006801:Humans; D010766:Phosphorylation; D010982:Platelet-Derived Growth Factor / Q000494:pharmacology; D017027:Protein Tyrosine Phosphatases / Q000235:genetics; D020574:Proto-Oncogene Proteins c-sis; D012333:RNA, Messenger / Q000235:genetics; D016160:Retinoblastoma Protein / Q000378:metabolism; D020687:cdc25 Phosphatases
C534702:CCNG1 protein, human; D018797:Cell Cycle Proteins; D056746:Cyclin G; D056766:Cyclin G1; D016213:Cyclins; D010982:Platelet-Derived Growth Factor; D020574:Proto-Oncogene Proteins c-sis; D012333:RNA, Messenger; D016160:Retinoblastoma Protein; D000077214:Becaplermin; D018844:Cyclin-Dependent Kinases; C517004:CDC25A protein, human; D017027:Protein Tyrosine Phosphatases; D020687:cdc25 Phosphatases