Serum stimulation of NIH 3T3 cells induces the production of lipids able to inhibit GTPase-activating protein activity.

Quiescent NIH 3T3 cells were stimulated with serum prior to the extraction of total cellular lipids. These lipids were fractionated on thin-layer chromatography plates, and individual fractions were tested for the ability to inhibit GTPase-activating protein (GAP) activity. Two separate GAP inhibitory lipids were produced. One behaved similarly to arachidonic acid during silica gel chromatography, whereas the other was related to a phosphoinositide. Further study of the arachidonic acid-related material indicated that it was produced between 1 and 5 min after serum addition but was never observed in high-density, contact-inhibited cultures. The identity of these lipids is under investigation. The possibility raised by these results, that a metabolite of arachidonic acid is involved in mitogenic signaling, was supported by the finding that several lipoxygenase products of arachidonic acid efficiently inhibited GAP activity. These results provide further support for the hypothesis that lipids, GAP, and ras activity function together in the control of cellular proliferation.