Putrescine transport is greatly increased in human fibroblasts initiated to proliferate

Putrescine (diaminobutane) was previously found to stimulate proliferation of human fibroblasts in tissue culture, and a growth factor produced by these cells was identified as putrescine. In the present paper putrescine transport is studied. The rate of putrescine transport was dependent on temperature, and most of the labeled putrescine was retained by the cells after washing with excess unlabeled putrescine. The concentration of radioactivity after a [14 C]putrescine pulse was 85 times higher in the cells than in the medium, and over 95% of the radioactivity in the cells was as unchanged putrescine. Butanol treatment removed 70% of the radioactivity from the cells. The calculated Km was about the same for rapidly growing and for starved cultures, while Vmax was higher for the former than for the latter cultures. Putrescine transport was inhibited to varying degrees by other polyamines, but not by amino acids or divalent cations. Stimulation of cell proliferation by serum was followed by an 18-100-fold increase in the rate of putrescine transport, which was not inhibitable with cyclic AMP, dibutyryl cyclic AMP, or prostaglandin E1. Removal of serum resulted in a rapid decrease in the rate of putrescine transport. Insulin in low serum medium and trypsin in the absence of serum also accelerated putrescine transport. Moreover, the rate of putrescine transport was dependent on cell density. It was faster in sparsely populated than in densely populated cultures. SV40-transformed human fibroblasts responded to addition and removal of serum in the same way as the untransformed parent cell line.