Epidermal growth factor stimulation of phosphatidylinositol 3-kinase during wound closure in rabbit corneal epithelial cells

To determine whether there is an association between epidermal growth factor (EGF)-induced activation of phosphatidylinositol 3-kinase (PI 3-kinase) and stimulation of wound closure in rabbit corneal epithelial cells.

Immortalized rabbit corneal epithelial cells were cultured in 24-well plates until they became confluent. Circular wounds were created in confluent cultures by cell denudation and then incubated in the absence and presence of EGF for varying intervals. Wound closure was monitored by staining the cells with Giemsa and quantifying the wound area with SigmaS can computer program. Cell proliferation during wound repair was estimated by measuring the incorporation of [3H]thymidine into nuclear DNA. Changes in PI 3-kinase activity were assessed by measuring the production of phosphatidylinositol 3,4,5-triphosphate [PI(3,4,5)P3] in 32P-labeled cells as well as by immunoprecipitating and assaying PI 3-kinase activity with phosphatidylinositol 4,5-bisphosphate and [gamma-32P]ATP as substrates. The enzyme product, PIP3, was analyzed by a combination of thin-layer and high-pressure liquid chromatography.

Addition of 10 ng/ml EGF to the wounded corneal epithelial cells stimulated wound closure in a time-dependent manner, and the wound closed completely within 48 hours. The effect of EGF was dose dependent, and maximal wound closure occurred at 10 ng/ml EGF. As the epithelial cells were undergoing EGF-stimulated wound closure, there was a time-dependent increase in PI 3-kinase activity. The enzyme activity increased maximally at 24 hours and then decreased gradually as the incubation was continued to 48 hours. When the cells were treated with wortmannin, a PI 3-kinase inhibitor, the EGF-stimulated PIP3 formation as well as the wound closure were inhibited significantly. Treatment of the cells with genistein or tyrphostin B42 also decreased both EGF-stimulated PIP3 formation and wound closure in a dose-dependent manner. Concomitant with stimulation of wound repair, the growth factor increased [3H]thymidine incorporation into nuclear DNA, and this effect was inhibited by pretreatment of the cell with wortmannin.

The data suggest a close correlation between EGF-stimulated wound closure and activation of PI 3-kinase in corneal epithelial cells. It can be concluded that PI 3-kinase might be an important component in signal transduction cascade initiated by EGF-receptor interaction, which leads to mitosis and cell proliferation during wound closure in corneal epithelial cells.