Different binding and degradation of proinsulin, insulin and insulin-like growth factor-1 (IGF-1) in cultured renal proximal tubular cells. Implications for the prolonged serum half-life of proinsulin

Renal proximal tubular cell growth and differentiation are differentially modulated by renotropic growth factors and tyrosine kinase inhibitors

BACKGROUND

The renotropic growth factors (GFs), hepatocyte GF (HGF), epidermal GF (EGF), and insulin-like GF-I (IGF-I) accelerate renal regeneration in animal models after toxic or ischemic injury. These GFs initiate their biological effects on renal tubular cells by interaction with specific transmembrane receptor tyrosine kinases.

MATERIALS AND METHODS

In the proximal tubular cell line PT-1, the biological effects of HGF, EGF, and IGF-I and the growth-inhibitory effects of different tyrosine kinase inhibitors (TKIs) were investigated. Receptor binding and tyrosine kinase phosphorylation were determined by ligand binding studies and Western blot analysis.

RESULTS

HGF, EGF, and IGF-I bound with nanomolar affinity to their specific cell membrane receptor tyrosine kinases. In contrast to EGF or IGF-I, HGF induced a variety of cell morphological changes, including cell scattering, formation of tubular structures, and expression of long microvilli on the apical cell membrane. HGF was a 10-fold more potent and more effective growth promoter than EGF or IGF-I. Among the TKIs tested, the mitogenic effect of HGF could be more specifically inhibited by emodin and tyrphostin, that of EGF by methyl-2,5-dihydroxycinnamate, lavendustin A, and genistein, and that of IGF-I by geldanamycin.

CONCLUSIONS

In contrast to EGF and IGF-I, HGF stimulated both growth and differentiation of renal proximal tubular cells, demonstrating the amazing biological potency of this renotropic growth factor. Selective TKIs may be a promising approach to modulate diseases with abnormalities in protein kinase signalling pathways such as renal cell carcinoma.

Endothelin release by rabbit proximal tubule cells: modulatory effects of cyclosporine A, tacrolimus, HGF and EGF

BACKGROUND

Previous studies have suggested that endothelins, a family of 21 amino acid peptides with potent vasoconstrictive and mitogenic properties, are involved in the pathogenesis of acute and chronic renal failure. In addition, endothelin seems to play an important role in mediating the nephrotoxic side effects of cyclosporine A (CsA) and tacrolimus. The present study investigated the production of endothelin-1 (ET-1) and endothelin-3 (ET-3) by bipolar differentiated rabbit proximal tubule cells (PT-1 cells), and the modulatory effect of CsA, tacrolimus, hepatocyte growth factor (HGF) and epidermal growth factor (EGF) on ET-1 and ET-3 release.

METHODS

ET-1 and ET-3 mRNA was detected by RT-PCR, immunoreactive endothelin was localized to PT-1 cells by immunofluorescence staining with antibodies against ET-1 and ET-3. ET-1 and ET-3 release into the culture medium was determined by specific radioimmunoassays after solid phase extraction.

RESULTS

PT-1 cells exhibited a time-dependent increase of ET-1 release up to an incubation period of 36 hours, whereas ET-3 release already reached a steady state level after four hours. PT-1 cells, cultured on filter membranes, released a significantly higher amount of immunoreactive ET-1 into the basolateral compartment than into the apical compartment. ET-3 release did not differ significantly between the basolateral and the apical compartment. Supplementation of the cell culture medium with 10% fetal calf serum induced a marked increase of the basolateral and apical ET-1 release, whereas ET-3 release was only slightly increased. CsA and tacrolimus (0.5 to 5000 microgram/liter) induced a significant, dose-dependent increase of ET-1 and ET-3 release by PT-1 cells with a maximum stimulation at a CsA concentration of 500 microgram/liter (P < 0.001) and a tacrolimus concentration of 50 microgram/liter (P < 0.001). HGF and EGF (10-10 to 10-8 mol/liter) exerted a significant (P < 0.001) dose-dependent inhibitory effect on ET-1 release, whereas ET-3 release was not significantly reduced. Coincubation of PT-1 cells with CsA or tacrolimus and HGF or EGF also resulted in a marked reduction of ET-1 release.

CONCLUSIONS

The present data suggest that ET-1 and ET-3 release by cultured rabbit proximal tubule cells are regulated differently, and that the stimulatory effect of CsA and tacrolimus on ET-1 release is antagonized by HGF and EGF.

Pleiotropic effects of hepatocyte growth factor in proximal tubule involve different signaling pathways

Hepatocyte growth factor (HGF) accelerates renal tubule cell regeneration and induces tubulogenic differentiation via the intracellular tyrosine kinase (TK) domain of its receptor, the proto-oncogene c-Met. We tested whether different signaling pathways may be involved by examining HGF binding and effects on cell proliferation, migration, scattering, and tubulogenic differentiation in the bipolar differentiating rabbit proximal tubule cell line PT-1 under serum-free conditions in the presence or absence of the protein TK inhibitors (PTKIs) herbimycin-A, genistein, methyl-2,5-dihydroxycinnamate, and geldanamycin. These PTKIs inhibit pp60(c-src), a nonreceptor TK involved in cell-growth control. HGF bound to a single high-affinity receptor class, increased microvilli numbers 1.5-fold, enhanced cell proliferation and migration 1.8-fold, and stimulated formation of tubule structures 2.2-fold. PTKI inhibited the mitogenic and motogenic effects of HGF with different potencies and comparable maximal effects but had no specific influence on HGF-induced tubulogenic cell differentiation. These data underline the importance of pp60(c-src) in mediating mitogenic and motogenic effects of HGF, whereas stimulation of tubulogenic cell differentiation may be transduced by a pp60(c-src)-independent pathway.