Role of protein kinase a pathway in epidermal growth factor-induced liver cell repair

To gain a better understanding of the mechanisms that control the repair process in the injured liver, the actions of epidermal growth factor (EGF) and protein kinase A (PKA) were studied. Normal rat liver cells (clone 9) were grown to confluence. Standardized excisional wounds were made with a razor blade. The extent of hepatocyte migration into the wound was measured and determined at specific time intervals using a computerized digital analyzing system. Immunostaining of F-actin was performed with a fluorescein-labeled phalloidin. EGF significantly stimulated liver cell migration, whereas specific EGF-neutralizing antibody inhibited the EGF-induced migration. Agents that activate PKA at different stages of the PKA activation pathway, including 3-isobutyl-1-methylxanthine (IBMX), forskolin, and cholera toxin, inhibited EGF-induced migration. EGF triggered formation of actin stress fibers. PKA-activating agents inhibited actin stress fiber formation and stretching of cells at the wound margin. The following conclusions were drawn: (1) In excisional wounds of hepatocyte monolayers, both EGF and PKA exert action on actin microfilaments, which are stretched by EGF and inhibited by PKA; (2) the enhanced repair of wounded hepatocyte monolayers by EGF is blocked by activation of the PKA pathway at various levels; and (3) these actions of EGF and PKA indicate their important regulatory roles in controlling the rate of hepatocyte migration and restitution following the creation of excisional wounds.