Studies on the mechanism of Ca2+ stimulation of plasminogen activator synthesis/release by Swiss 3T3 cells

Imbalance of total cellular nucleotide pools and mechanism of the colchicine-induced cell activation

Treatment with colchicine or vinblastine, both inhibitors of microtubule assembly, renders quiescent 3T3 cells in an "activated state" as evidenced by induction of DNA synthesis and other criteria. Microtubule disassembly caused by colchicine or vinblastine brings about a dramatic expansion of total cellular UTP pools with a concomitant diminution in total cellular ATP pools, thus resulting in a marked imbalance in total cellular nucleotide pools. Colchicine and vinblastine also stimulate total cellular RNA synthesis without enhancing uridine phosphorylation, suggesting that these drugs affect the G1 phase of the cell cycle at a point beyond the enhancement of uridine phosphorylation that usually accompanies mitogenic stimulation of quiescent mammalian cells. The markedly expanded cellular UTP pools appear to be necessary for initiation of the colchicine-stimulated DNA synthesis because decreasing cellular UTP pools by addition of D-glucosamine results in a selective inhibition of DNA synthesis in the colchicine-stimulated, but not control, cells. Furthermore, D-glucosamine exerts its inhibitory effect only when it is present in the cultures within the first 14 hr after colchicine treatment. When added at 21 hr, D-glucosamine still decreases cellular UTP pools, but it is no longer inhibitory for DNA synthesis, which commences 14-16 hr after colchicine stimulation. Taxol, an antitumor drug, prevents microtubule disassembly and also blocks such events as expansion of total cellular UTP pools and stimulation of RNA and DNA synthesis, indicating that microtubule depolymerization acts as a primary event initiating the process of cell activation induced by colchicine.

Induction of plasminogen activator in a human diploid fibroblast cell line (MRC-5) by conditions which cause induction of interferon: the role of calcium ions

The human diploid fibroblast cell line, MRC-5, derived from embryo lung tissue produced only small quantities of plasminogen activator (PA) when harvested using a standard nutrient medium (Eagle's Minimal Essential Medium, MEM). Use of a schedule designed to induce high concentrations of fibroblast interferon in these cells also resulted in production of considerably enhanced levels of PA. The kinetics of PA production differed from those of interferon production; specifically, PA was produced for at least 6 days following induction despite the toxicity of the inducers whereas interferon synthesis continued for only 1 day. Further investigation of the induction conditions for PA revealed that double-stranded RNA which was absolutely required for interferon production was not required for induction of PA. Indeed, the stimulus for enhancement of PA production appeared to be solely an elevated concentration of calcium ions in the extracellular medium. The possible physiological relevance of this induction of PA by elevated concentrations of calcium ions is discussed.