The cell cycle dependence of c‐sis gene expression: artifactual conclusions in cells prepared by chemical but not physical techniques

Abstract. The c‐sis oncogene encoding the B‐chain of platelet‐derived growth factor (PDGF) may be involved in an autocrine growth stimulation of tumours expressing the PDGF receptor, such as glioblastomas and sarcomas. To investigate whether expression of c‐sis RNA is regulated in a cell cycle dependent manner, human A172 glioblastoma cells were synchronized by either centrifugal elutriation or chemical blockage with the DNA synthesis inhibitors hydroxyurea or aphidicolin. In non‐perturbed elutriated cells, c‐sis RNA levels were lower in the S phase of the cell cycle than in the G₁ phase. In contrast, the chemically synchronized cells revealed a transient rise in c‐sis RNA shortly after drug release, in early S phase. The RNA changes occurring after release from drug inhibition represent cell recovery from drug induced metabolic disturbances rather than true cell cycle dependent effects.