Intracellular processing of epidermal growth factor by early wound healing cells

Epidermal growth factor (EGF) is a potent 53-amino-acid residue polypeptide that has been implicated in normal wound healing. Although past studies have shown that locally applied EGF accelerates wound healing, these studies have not examined intracellular events related to the processing of the growth factor. The objective of this study was to characterize both initial and later postbinding intracellular processing of EGF by a responsive cell line (osteoblasts) that is important in the healing of wounds. Cloned mouse calvarial osteoblasts (MC-3TC-E1) were incubated with radiolabeled EGF, with and without preincubation with nonlabeled EGF, for specific time intervals. Cell-associated radioactivity was characterized by nondenaturing polyacrylamide gel electrophoresis. Results showed that EGF is processed as three distinct species and that the relative proportions of these species are altered at later time periods when compared with initial processing. The patterns, similar to those reported for human fibroblasts, indicate a possible common pathway for the mitogenic signal in cells associated with the early events of wound healing. In addition, these data represent the first direct evidence that preexposure of cells to nonlabeled EGF alters the processing of radiolabeled EGF. This is significant, because cells must be exposed to EGF for 5 to 8 hours to elicit a growth response. Such data may help to explain the "lag phase" of wound healing.