Ribosomal protein S6 kinase is activated after folic acid injury and epidermal growth factor administration but not after unilateral nephrectomy in the rat kidney

Role of Mammalian Target of Rapamycin Signaling in Compensatory Renal Hypertrophy

Loss of functioning nephrons stimulates the growth of residual kidney tissue to augment work capacity and maintain normal renal function. This growth largely occurs by hypertrophy rather than from hyperplasia of the remaining nephrons. The signaling mechanisms that increase RNA and protein synthesis during compensatory renal hypertrophy are unknown. This study found that the remaining kidney hypertrophied 42% by 16 d after unilateral nephrectomy (UNX) in DBA/2 mice. Immunoblotting analysis revealed increased phosphorylation of the 40S ribosomal protein S6 (rpS6) and the eukaryotic translation initiation factor (eIF) 4E-binding protein 1 (4E-BP1), the two downstream effectors of the mammalian target of rapamycin (mTOR). The highly specific mTOR inhibitor rapamycin blocked UNX-increased phosphorylation of both rpS6 and 4E-BP1. UNX increased the content of not only 40S and 60S ribosomal subunits but also 80S monosomes and polysomes in the remaining kidney. Administration of rapamycin decreased UNX-induced polysome formation and shifted the polysome profile in the direction of monosomes and ribosomal subunits. Pretreatment of the mice with rapamycin inhibited UNX-induced hypertrophy. These studies demonstrate that activation of the mTOR signaling pathway in the remaining kidney after UNX plays an essential role in modulating RNA and protein synthesis during development of compensatory renal hypertrophy.

Influence of the renin-angiotensin system on epidermal growth factor expression in normal and cyclosporine-treated rat kidney

BACKGROUND

Epidermal growth factor (EGF) plays an important role in renal tubular regeneration after ischemic injury in kidney. The present study reports the association between the renin-angiotensin system (RAS) and EGF, and the effect of angiotensin II blockade with losartan (LSRT) on EGF expression in an experimental model of chronic cyclosporine (CsA) nephrotoxicity in rats.

METHODS

Two separate experiments were performed. In the first experiment, rats on the normal-salt diet (NSD; 0.3%) or low-salt diet (LSD; 0.05%) were treated with or without LSRT for four weeks. In the second experiment, rats on the NSD or LSD were given vehicle (VH group, olive oil, 1 mg/kg per day) or CsA (15 mg/kg per day) or CsA (15 mg/kg per day) plus LSRT (100 mg/L per day). Renal function, histopathology, TUNEL staining, plasma renin activity (PRA), and the expression of renin and EGF were studied.

RESULTS

Normal rats on the LSD showed significantly increased EGF expression (cortex, 2.6-fold; medulla, 1.7-fold) and significantly decreased EGF expression with the LSRT treatment compared with the rats treated with the NSD (cortex, 74.8 vs. 10%; medulla, 22.5 vs. 5%). In contrast, the CsA-treated rats on the LSD had a significantly lower EGF expression (cortex, 98 vs. 53%; medulla, 94 vs. 14%); however, concomitant administration of LSRT increased the EGF expression (cortex, 91- vs. 3.8-fold; medulla, 19- vs. 2.4-fold) compared with the rats on the NSD. In the normal and CsA-treated LSD rats, EGF expression was well correlated with PRA. In addition, EGF expression was well correlated with the interstitial fibrosis score (r = 0.664, P < 0.01) or number of TUNEL-positive cells (r = 0.822, P < 0.01) in CsA-treated LSD rats.

CONCLUSIONS

These results suggest that angiotensin II blockade with LSRT decreases EGF expression in normal rats on the LSD, but it protects EGF expression in CsA-induced nephrotoxicity. This finding provides a new perspective on the renoprotection of angiotensin II blockade in chronic CsA nephrotoxicity.

Inhibitors of cyclic nucleotide phosphodiesterase isozymes block renal tubular cell proliferation induced by folic acid

In previous studies we observed that inhibition of cyclic 3',5'-nucleotide phosphodiesterase (PDE) isozymes, namely isozyme PDE3, suppresses proliferation of rat renal glomerular mesangial cells in vitro and in vivo. To determine whether activation of the cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA) signaling pathway coupled to specific PDE isozymes modulates accelerated proliferation of renal epithelial cells, we investigated the effect of selective PDE isozyme inhibition on renal epithelial cell proliferation induced in rats by injection of folic acid (FA). In extracts from suspensions of renal cortical tubules, cAMP was metabolized predominantly by isozyme PDE4; activity of PDE3 was about three times lower. The increase in proliferative activity of renal cortical tissue from FA-injected rats, evaluated by immunostaining with Mib-1 antibody, was limited to tubular epithelial cells. Administration of the PDE3 inhibitors cilostazol or cilostamide together with the PDE4 inhibitor rolipram blocked mitogenic synthesis of DNA, as determined by (3H)-thymidine incorporation into renal cortical DNA, in FA-treated rats. FA injection caused an increase of more than 10-fold in proliferating cell nuclear antigen (PCNA) in renal cortical tissue; administration of the potent PDE3 inhibitor lixazinone or, to a lesser degree, cilostazol suppressed these high PCNA levels, whereas rolipram alone had no effect. The results indicate that FA-stimulated in vivo proliferation of renal tubular epithelial cells is down-regulated by activation of a cAMP-PKA signaling pathway linked to PDE3 isozymes. These observations are consistent with the notion that negative crosstalk between cAMP signaling and mitogen-stimulated signaling pathways regulates mitogenesis of renal cells of different terminal differentiation, including tubular epithelial cells.

Epidermal growth factor in acute renal failure

Epidermal growth factor (EGF) is produced in large amounts in the kidney in the form of a membrane-bound high molecular weight precursor. This precursor is inserted in the apical plasma membrane of the EGF-producing cells, which are localized in the thick ascending limb and distal convoluted tubule in mouse and rat kidney, and probably also in human kidney. High levels of EGF are excreted in urine, although renal tissue contains little mature EGF. It modulates renal cell proliferation and differentiation in vitro, but the role of the distal tubular EGF and/or its precursor in vivo is unknown. The expression of EGF in the kidney and its liberation into the urine are quickly abolished during several types of drug- or ischemia-induced acute renal failure and also in ureteral obstruction. Moreover, its expression is restored only after morphological and functional recovery of the kidney. This absence of EGF in conditions in which its mitogenic properties would be most appropriate suggests that the EGF of renal origin is not acting as a mitogen during kidney regeneration. Nevertheless, since the number of EGF receptors, which are localized at the basolateral cell surface in most nephron segments, is increased in regenerating renal epithelia, EGF of systemic origin or other members of the EGF family of growth factors, released from infiltrated inflammatory cells at the sites of injury, could enhance cellular proliferation by interacting with the EGF receptor. Administration of EGF indeed has a mildly beneficial effect on recovery from acute renal injury.

Modification of immunoreactive EGF and EGF receptor after acute tubular necrosis induced by tobramycin or cisplatin

Acute tubular necrosis induced by aminoglycoside antibiotics and various other nephrotoxins is followed by a regenerative process which leads to the restoration of damaged tubules. Several lines of evidence indicate that tubular regeneration is mediated by polypeptide growth factors such as epidermal growth factor (EGF). Previous studies devoted to cisplatin nephrotoxicity have shown that this agent causes tubular cystic degeneration possibly related to an impairment of renal tissue repair. Thus, we examined on a comparative basis the time course of the regenerative response subsequent to tubular damage induced by tobramycin or cisplatin, particular attention being paid to renal EGF and its receptor. Female Sprague-Dawley rats (160-180 g body weight) were treated during 4 consecutive days with daily doses of 200 mg/kg tobramycin i.p. (BID) or 2 mg/kg cisplatin (once a day). Sham-treated rats were given 0.9% NaCl i.p. following the same protocol. Groups of experimental animals (n = 5-10) were terminated at increasing time intervals (1, 4, 7, 14, 21, 60 days) after cessation of treatment. One hour prior to sacrifice, each individual received i.p. 200 mg/kg 5-bromo-2'-deoxyuridine (BrdU) for the immunohistochemical demonstration of cell proliferation. Blood was collected at the time of sacrifice in order to assess glomerular filtration rate by measuring serum creatinine and BUN levels. Kidneys were analyzed with respect to total EGF determined by RIA in renal tissue homogenates, and soluble EGF was assayed in extracts prepared by centrifugation. Renal tissue was processed for the immunohistochemical detection of S-phase cells, of EGF, of EGF receptors, and of the intermediate filament vimentin, the latter being used as a marker of epithelium dedifferentiation. In absence of nephrotoxic alterations, EGF was immunolocalized in distal tubules, whereas EGF receptor immunostaining was seen in proximal tubules cells. Vimentin immunostaining was confined to glomeruli and blood vessels. Tobramycin and cisplatin caused acute tubular necrosis in proximal convoluted tubules and proximal straight tubules, respectively. Tissue damage was accompanied by renal dysfunction reflected by an elevation of serum creatinine and BUN levels. Tubular necrosis was followed by a proliferative response indicative of tubular regeneration. Regenerative hyperplasia was associated with a reduction of total immunoreactive EGF due to a decrease of tissue-bound proEGF. Tubules undergoing regenerative repair were characterized by a disappearance of EGF receptors and the presence of immunoreactive vimentin. In tobramycin-treated rats, renal dysfunction lasted for 4-7 days and was fully reversible, as indicated by the return of serum markers to normal values.

Regulation of epidermal growth factor receptor expression and growth by protein kinase C and retinoic acid in LLC-PK1 cells

The importance of receptor expression and protein kinase C to epidermal growth factor (EGF)-induced cell proliferation was studied in LLC-PK1 kidney cells. These cells have both high- and low-affinity binding sites for EGF. Neither transforming growth factor-beta nor tumor necrosis factor altered EGF receptor expression. On the other hand, retinoic acid induced a concentration-dependent increase in EGF binding that was maximal at 1 mumol/L. One micromolar of retinoic acid increased EGF binding from 0.38 +/- 0.01 fmol/10(6) cells in controls to 1.10 +/- 0.03 fmol/10(6) in treated cells at 18 hours (n = 8, P < 0.001). The increase in binding was the result of an increase in the Bmax of the high-affinity receptor. The upregulation of the EGF receptor induced by retinoic acid was associated with enhanced EGF-induced growth promotion. A 45-minute incubation of cells with phorbol 12-myristate 13-acetate caused a concentration-dependent decrease in EGF binding that was prevented by a 40-hour, 2 mumol/L pre-exposure to phorbol 12-myristate 13-acetate; 10(-8) mol/L EGF also caused a downregulation of the EGF receptor that was not prevented by phorbol 12-myristate 13-acetate or retinoic acid. Downregulation of protein kinase C did not interfere in the capacity of EGF to induce growth in these cells. These studies demonstrate that EGF receptor upregulation plays an important role in the control of EGF-induced cell growth. Protein kinase C regulates EGF binding in these cells; however, it is not necessary for EGF-induced growth promotion or receptor downregulation.