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

The present study examined the effect of epidermal growth factor (EGF) on Na-Pi cotransport in a tubular epithelial cell line derived from the opossum kidney (OKP cells). EGF caused a time- and dose-dependent decrease in Na-Pi cotransport. The inhibition of Na-Pi cotransport by 10(-8) M EGF was first demonstrable after 18 h with maximal effect seen at 24 h. EGF inhibited Na-Pi cotransport by decreasing the maximal velocity (10.8 +/- 0.9 in control vs. 4.9 +/- 0.8 nmol 32Pi.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 beta-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-Pi cotransport activity. Inhibition of transcription with actinomycin D and protein synthesis with cycloheximide prevented the inhibition of Na-Pi cotransport. Furthermore, inhibition of phospholipase C activity with U-73,122 also significantly blocked the inhibitory effect of EGF on Na-Pi cotransport. The results indicate that EGF-induced decrease in OKP Na-Pi cotransport is mediated through a decrease in NaPi-4 mRNA and activation of the phospholipase C signaling pathway.