Ras induces anchorage-independent growth by subverting multiple adhesion-regulated cell cycle events.
Anchorage-independent growth is a hallmark of transformed cells, but little is known of the molecular mechanisms that underlie this phenomenon. We describe here studies of cell cycle control of anchorage-independent growth induced by the ras oncogene, with the use of a somatic cell mutant fibroblast line (ER-1-2) that is specifically defective in oncogene-mediated, anchorage-independent growth. Control, nontransformed PKC3-F4 cells and ER-1-2 cells cannot proliferate in semisolid medium. Three important cell cycle events are dependent on adhesion of these cells to a substratum: phosphorylation of the retinoblastoma protein, pRB; cyclin E-dependent kinase activity; and cyclin A expression. PKC3-F4 cells that express ras (PKC3-F4/ras cells) proliferate in nonadherent cultures, and each of these three events occurs in the absence of adhesion in PKC3-F4/ras cells. Thus, ras can override the adhesion requirement of cellular functions that are necessary for cell cycle progression. ER-1-2 cells that express ras (ER-1-2/ras cells) possess hyperphosphorylated forms of pRB and cyclin E-dependent kinase activity in the absence of adhesion but remain adhesion dependent for expression of cyclin A. The adhesion dependence of pRB phosphorylation and cyclin E-dependent kinase activity is therefore dissociable from the adhesion dependence of cyclin A expression. Furthermore, ectopic expression of cyclin A is sufficient to rescue anchorage-independent growth of ER-1-2/ras cells but does not induce anchorage-independent growth of PKC3-F4 or ER-1-2 cells. However, like pRB phosphorylation and cyclin E-dependent kinase activity, the kinase activity associated with ectopically expressed cyclin A is dependent on cell adhesion, and this dependence is overcome by ras. Thus, the induction of anchorage-independent growth by ras may involve multiple signals that lead to both expression of cyclin A and activation of G1 cyclin-dependent kinase activities in the absence of cell adhesion.
Kang
J S
JS
Department of Biochemistry, Mount Sinai School of Medicine, New York, New York 10029, USA.
Krauss
R S
RS
D016475:3T3 Cells; D000818:Animals; D002448:Cell Adhesion; D002453:Cell Cycle; D002455:Cell Division; D002460:Cell Line; D002999:Clone Cells; D018844:Cyclin-Dependent Kinases / Q000378:metabolism*; D016213:Cyclins / Q000096:biosynthesis / Q000378:metabolism; D005434:Flow Cytometry; D011905:Genes, ras; D006801:Humans; D007700:Kinetics; D051379:Mice; D016296:Mutagenesis; D010766:Phosphorylation; D016283:Proto-Oncogene Proteins p21(ras) / Q000096:biosynthesis; D051381:Rats; D011994:Recombinant Proteins / Q000096:biosynthesis / Q000378:metabolism; D016160:Retinoblastoma Protein / Q000378:metabolism*
D016213:Cyclins; D011994:Recombinant Proteins; D016160:Retinoblastoma Protein; D018844:Cyclin-Dependent Kinases; C501009:HRAS protein, human; D016283:Proto-Oncogene Proteins p21(ras)