Proliferation-dependent changes in arachidonic acid mobilization from phospholipids of 3T6 fibroblasts

When we stimulated 3T6 fibroblasts, we observed a mobilization of arachidonic acid (AA) from phospholipids. The magnitude of this response decreased as the cells became confluent and the change coincided with a decrease in the percentage of cells in growth phases (G2 + M); this was not a consequence of the time in culture or a factor in the growth medium. Preconfluent fibroblasts incubated with the calcium ionophore A23187 (1 microM), 4beta-phorbol-12-myristate acetate (PMA, 10 microM), bradykinin (10 microM) or fetal calf serum (20%) released 38.8%, 62.5%, 11.2% and 45.6% [3H]AA, respectively. Confluent cells stimulated under the same conditions released only 16.9%, 1.5%, 0.5% and 18.5% [3H]AA, respectively. This decreasing mobilization of AA was demonstrated using metabolic labelling and measurement of prostaglandin PGE2. The decrease was not due to a changing pool of AA. [3H]AA release from each phospholipid decreased with confluence. Conversion of confluent cells to the proliferative phenotype by mechanical wounding of the monolayer increased the release of [3H]AA. This effect is consistent with regulated, growth-dependent changes in the activity of phospholipase A2, a process regulated by changes in phosphorylation of the enzyme. The increased release of [3H]AA from preconfluent, actively dividing cells may have important physiological implications and may help elucidate mechanisms regulating release of AA.