Cell shape and increased free cytosolic calcium [Ca2+]i induced by growth factors

Growth factors stimulate DNA synthesis of neoplastic cells but not of non-neoplastic cells in suspension cultures. Similarly, growth ceases in dense monolayers of non-neoplastic cells, while crowded neoplastic cells continue to grow. The mechanism of these important phenotypic changes is unknown; the block in growth stimulation could occur in early events of signal transduction at the plasma membrane or in a late step in the final steps of gene activation and induction of DNA synthesis. One particular early intracellular event, [Ca2+]i increases, is in fact necessary for the induction of DNA synthesis in attached non-neoplastic Balb/c 3T3 cells stimulated by platelet-derived growth factor (PDGF). We therefore used digital image analysis of intracellular Fura-2 fluorescence to determine whether PDGF can stimulate [Ca2+]i transients in suspension or in dense monolayer cultures of Balb/c 3T3 cells. In dense cells (greater than 8 x 10(4) cells/cm2) the basal [Ca2+]i and [Ca2+]i response to PDGF stimulation were both lower than those in sparser, more spread cells. PDGF also did not release internal stores of Ca2+ or produce Ca2+ influx in completely suspended cells. Remarkably, attachment alone, with minimal cell spreading, was enough to reinitiate the entire early signalling mechanism stimulated by PDGF. Thus, a block in PDGF-induced [Ca2+]i increases may contribute to the inability of PDGF to stimulate DNA synthesis in suspended non-neoplastic cells. This early block in signal transduction must be abrogated in neoplastic cells growing in suspension and dense monolayer cultures.