Epidermal growth factor inhibits Na-P_i cotransport and mRNA in OK cells

The present study examined the effect of epidermal growth factor (EGF) on Na-P_i cotransport in a tubular epithelial cell line derived from the opossum kidney (OKP cells). EGF caused a time- and dose-dependent decrease in Na-P_i cotransport. The inhibition of Na-P_i cotransport by 10^-8 M EGF was first demonstrable after 18 h with maximal effect seen at 24 h. EGF inhibited Na-P_i cotransport by decreasing the maximal velocity (10.8 ± 0.9 in control vs. 4.9 ± 0.8 nmol ³²P_i·4 min^-1·mg protein^-1 in EGF, P < 0.001). Northern blot analysis indicated that EGF caused a significant decrease in NaPi-4 mRNA abundance. The abundance of NaPi-4 mRNA relative to β-actin and/or glyceraldehyde-3-phosphate dehydrogenase mRNA was decreased by twofold in OK cells treated with EGF for 4 h and threefold in OKP cells treated with EGF for 24 h. Thus the decrease in NaPi-4 mRNA abundance preceded the decrease in Na-P_i cotransport activity. Inhibition of transcription with actinomycin D and protein synthesis with cycloheximide prevented the inhibition of Na-P_i cotransport. Furthermore, inhibition of phospholipase C activity with U-73,122 also significantly blocked the inhibitory effect of EGF on Na-P_i cotransport. The results indicate that EGF-induced decrease in OKP Na-P_i cotransport is mediated through a decrease in NaPi-4 mRNA and activation of the phospholipase C signaling pathway.