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Epidermal growth factor inhibits Na-Pi cotransport in weaned and suckling rats
M Arar1, H K Zajicek, I Elshihabi
1Department of Pediatrics, University of Texas Health Science Center at San Antonio, San Antonio 78284, Texas.
Insights
Epidermal growth factor (EGF) inhibits sodium-phosphate cotransport in rat kidneys. This inhibition occurs by reducing NaPi-2 protein levels, not mRNA, in both young and weaned rats.
Area of Science:
- Nephrology
- Molecular Biology
- Developmental Biology
Background:
- Phosphate reabsorption in the proximal tubule is crucial for mineral homeostasis.
- Sodium-gradient dependent phosphate cotransport (Na-Pi cotransport) plays a key role in this process.
- Epidermal growth factor (EGF) is a known regulator of cellular processes, but its effect on renal phosphate transport is not fully understood.
Purpose of the Study:
- To investigate the effect of epidermal growth factor (EGF) on sodium gradient-dependent phosphate transport (Na-Pi cotransport) in suckling and weaned rats.
- To determine the specific mechanisms by which EGF influences Na-Pi cotransport, focusing on the NaPi-2 cotransporter.
- To compare the regulation of Na-Pi cotransport between different developmental stages (suckling vs. weaned rats).
Main Methods:
- Isolated brush border membrane vesicles (BBMVs) from proximal tubules of suckling and weaned rats were used.
- Na-Pi cotransport activity was measured using radiolabeled phosphate uptake assays.
- NaPi-2 protein abundance was assessed using Western blotting.
- NaPi-2 mRNA levels were quantified using RT-PCR.
- The effect of chronic EGF treatment on these parameters was evaluated.
Main Results:
- Weaned rats exhibited significantly higher basal Na-Pi cotransport activity and NaPi-2 protein and mRNA abundance compared to suckling rats.
- Chronic EGF treatment inhibited Na-Pi cotransport activity in BBMVs from both suckling and weaned rats.
- EGF-induced inhibition of Na-Pi cotransport was associated with a significant decrease in NaPi-2 protein abundance, but not NaPi-2 mRNA levels.
- The inhibitory effect of EGF was specific to Na-Pi cotransport, as Na-glucose cotransport remained unaffected.
Conclusions:
- Proximal tubule Na-Pi cotransport activity, NaPi-2 protein, and NaPi-2 mRNA are developmentally regulated, being higher in weaned than in suckling rats.
- EGF exerts an inhibitory effect on renal Na-Pi cotransport in both suckling and weaned rats.
- The mechanism of EGF-induced inhibition involves a post-transcriptional downregulation of NaPi-2 protein abundance, without altering NaPi-2 mRNA levels.
Abstract:
In the present study, we determined the effect of epidermal growth factor (EGF; 10 microgram/100 g body wt) on sodium gradient-dependent phosphate transport (Na-Pi cotransport) regulation in suckling (12-day-old) and weaned (24-day-old) rats. Weaned rats had higher proximal tubular brush border membrane vesicle (BBMV) Na-Pi cotransport activity (232 +/- 16 in weaned vs. 130 +/- 9 pmol. 10 s-1. mg protein-1 in suckling rats, P < 0.05). Chronic treatment with EGF induced inhibition of BBMV Na-Pi cotransport in both suckling (130 +/- 9 vs. 104 +/- 7 pmol. 10 s-1. mg protein-1, P < 0. 05) and weaned rats (232 +/- 16 vs. 145 +/- 9 pmol. 10 s-1. mg protein-1, P < 0.005). The inhibitory effect was selective for Na-Pi cotransport as there was no inhibition of Na-glucose cotransport. Weaned rats had a higher abundance of BBMV NaPi-2 protein than suckling rats (increase of 54%, P < 0.001) and a twofold increase in NaPi-2 mRNA. The EGF-induced inhibition of Na-Pi transport was paralleled by decreases in NaPi-2 protein abundance in both weaned (decrease of 26%, P < 0.01) and suckling (decrease of 27%, P < 0.01) animals. In contrast, there were no changes in NaPi-2 mRNA abundance. We conclude that proximal tubule BBMV Na-Pi cotransport activity, NaPi-2 protein abundance, and NaPi-2 mRNA abundance are higher in weaned than in suckling rats. EGF inhibits Na-Pi cotransport activity in BBMV isolated from suckling and weaned rats, and this inhibition is mediated via a decrease in NaPi-2 protein abundance, in the absence of a change in NaPi-2 mRNA.