Epidermal growth factor: Modulator of murine embryonic palate mesenchymal cell proliferation, polyamine biosynthesis, and polyamine transport

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Abstract
Polyamines (putrescine, spermidine, and spermine) are normal cellular constituents able to modulate cellular proliferation and differentiation in a number of tissues and cell types. This investigation explores the response of murine embryonic palate mesenchymal (MEPM) cells to epidermal growth factor (EGF) in terms of biosynthesis of putrescine and its transport across the plasma membrane and tests the hypothesis that polyamine transport can serve as an alternative mechanism (other than biosynthesis) for elevating intracellular polyamines during stimulation of MEPM cellular proliferation. MEPM cells treated with EGF were stimulated to proliferate and showed a dose‐ and time‐dependent stimulation of ornithine decarboxylase (ODC) which was maximal at 4‐6 hours. EGF also stimulated the initial rate of putrescine transport in dose‐ and time‐dependent manner. This stimulation was found to be maximal 3 hours after treatment and specific for the putrescine transport system. The kinetic parameters of putrescine transport shifted from 2.52 μM (Km) and 23.6 nmol/mg protein/15 minutes (Vmax) in nonstimulated cells to 4.48 μM (Km) and 39.8 nmol/mg protein/15 minutes (Vmax) in EGF‐treated cells. This kinetic shift did not require de novo protein or RNA synthesis, as cycloheximide (10 μg/ml) and actinomycin D (50 μg/ml) had little effect on the ability of EGF to stimulate the initial rate of putrescine uptake. The rate of transport, however, was found to be inversely related to cell density. The addition of exogenous putrescine concomitantly with EGF blocked the induction of ODC, while in the presence of difluoromethylornithine (DFMO) (irreversible inhibitor of ODC) the initial rate of putrescine transport remained elevated throughout the time course studied. This stimulation of putrescine uptake caused by polyamine deprivation was reversed by exogenous putrescine and Ca+ + while α‐aminoisobutryic acid (AIB) further stimulated the rate of uptake. EGF's ability to stimulate cellular DNA synthesis was inhibited by DFMO. If DFMO‐treated cells were stimulated with EGF in the presence of exogenous putrescine, this stimulatory effect was preserved. These studies indicate that the rate of polyamine transportation is highly responsive to a signal which initiates biosynthesis of polyamines. Further, this transportation system provides a compensatory mechanism allowing the cell to increase intracellular levels of polyamines when environmental conditions inhibit biosynthesis or when polyamines are abundant.

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