Studies on Renal Growth Regulation by Urea and Ammonia in Normal Rat Kidney Cells

Abstract

The hypothesis that protein metabolites regulate renal growth has been considered for decades, and the overall objective of this work was to test this hypothesis in vitro using an established line of renal tubular epithelial cells, NRK-52E. The addition of urea to the medium of confluent cultures stimulated DNA synthesis which was dependent on the concentration of urea, but independent of the presence of serum. Urea enhanced proliferation of subcon-fiuent cultures of NRK cells in the presence of dilute serum to a similar degree as epidermal growth factor (EGF). Either deletion of serum from the medium or allowing the cultures to achieve confluence prevented the proliferative response to urea, but not EGF. Ammonium chloride stimulated the uptake of ³H-thymidine but did not increase cell number. Ammonium chloride increased the uptake of ³H-leucine and total cellular protein/DNA, while urea had no effect on these markers of growth. The rate of hydrolysis of urea to ammonia in vitro was not altered by the presence of NRK cells. These results suggest that urea and ammonia may regulate renal mass, and that their actions are separate and different.

Recombinant Forms of the Neurotrophic Factor Pigment Epithelium-Derived Factor Activate Cellular Metabolism and Inhibit Proliferation of the RAW Macrophage Cell Line

“…Previously, urea has only been shown to affect kidney-derived cells, enhancing their proliferation [27,28]. The concentrations of urea used in this study (1-10 mM) are lower than those seen by kidney cells but comparable to concentrations normally present in blood, suggesting that the effects of urea on the RAW cells could be physiologically relevant.…”

Urea stress is more akin to EGF exposure than to hypertonic stress in renal medullary cells

“…Urea also activated other effector arms of a receptor tyrosine kinase including activation of Shc and recruitment of Grb2 (41); activation of phospholipase C-␥, the relatively receptor tyrosine kinase-specific phospholipase, and production of inositol trisphosphate (6); and activation of phosphatidylinositol-3-kinase and Akt (41). In addition, data from several groups support a role for urea in either complete or incomplete mitogenesis of renal epithelial cells (4,5,19). Nonetheless, the identity of the urea-sensing molecule has remained elusive, and the specificity of these urea-dependent signaling events to the activation of a putative upstream receptor tyrosine kinase remains to be established.…”

“…Urea also activated other receptor tyrosine kinase effectors including Shc, with recruitment of Grb2 (41); phospholipase C-␥, with production of inositol 1,4,5-trisphosphate (6); and phosphatidylinositol-3-kinase, with subsequent activation of Akt (41). Urea may also exert a proproliferative effect on renal epithelial cells (4,5,19) in marked contrast to the effect of hypertonic stress. Furthermore, urea pretreatment affords cytoprotection from the adverse consequences of hypertonic stress (33,39), much like a peptide mitogen (21,39).…”

Cellular Response to Hyperosmotic Stresses

“…Interestingly, mitogenesis in response to urea treatment was not observed in the mIMCD3 cells that were used for most of these studies (292). Nevertheless, indices of proliferation in response to urea treatment have been observed in other urea-responsive renal epithelial cell lines such as MDCK, LLC-PK 1 , and NRK-52E (55,56,174). Thus this urea-induced mitogenic signaling pathway has been thoroughly documented, but its physiological role remains unclear.…”