Internalization and processing of epidermal growth factor in aging human fibroblasts in culture

Tissue culture lines established from newborn human skin were used as a model system to study the effects of the mitogenic hormone epidermal growth factor (EGF) on the aging process. These cells demonstrated a finite life span in culture which is presumed to be related to the in vivo aging process. Fibroblasts aged in vitro demonstrated a reduction in their ability to respond to the mitogenic effects of EGF as compared to these cells at an earlier population doubling level (PDL). This decreased responsiveness was not due to a decrease in the number of EGF receptors/cells, as cells at late PDL possessed either the same or more EGF receptors than cells at an earlier PDL. In Rat-1 fibroblasts, ¹²⁵I-labeled EGF is internalized following binding to the surface receptor, and is transported through intracellular organelles where it undergoes a series of modifications which result in acidification of the EGF (Matrisian et. al., 1984, J. Biol. Chem. 259, 3047). It is not known whether this acidic processing of EGF is necessary for mitogenic activity. EGF internalization and processing were therefore examined in aging human fibroblasts to determine if the decrease in EGF responsiveness is due to an alteration in EGF processing. Human fibroblasts internalized and processed pI 4.55 ¹²⁵I-labeled EGF to the more acidic pI 4.2, 4.35, and 4.0 species in a manner similar to Rat-1 fibroblasts. The nature of the processed product and the time course of processing was the same in cells at early and late passages. We therefore conclude that the decreased responsiveness of aged cells to EGF is not due to a defect in the EGF-processing mechanism.