Albumin-stimulated TGFbeta-1 in renal tubular cells is associated with activation of MAP kinase.
BACKGROUND
Proteinuric renal diseases are often associated with progressive tubulointerstitial fibrosis that usually defines the degree and rate of progression of renal failure. Glomerular filtration of excess albumin, the dominant protein in proteinuria, into proximal tubule could provide the stimulus to induce certain fibrogenic cytokines from proximal tubular cells (PTC), which may account for fibrosis in the interstitium. To explore this hypothesis we tested the effect of bovine albumin in PTC in culture on the expression and secretion of transforming growth factor (TGF) beta-1, a prominent fibrogenic cytokine.


METHODS
TGFbeta-1 expressed by cultured PTC was measured by enzyme-linked immunosorbent assay (ELISA) and indirectly by reverse-transcription polymerase chain reaction (RT-PCR). Relative messenger ribonucleic acid (mRNA) levels were measured in ethidium bromides stained gels, by comparison to transcripts for 18s ribosomal RNA. Activated mitogen-activated protein (MAP) kinase was estimated by Western blot with phosphotyrosine-specific antibody.


RESULTS
Following incubation of PTC with albumin determination of TGF beta-1 mRNA in PTC and TGF beta-protein in culture medium both indicated a time- and dose-dependent increase. MAP kinase (p44/42) was activated within 5 min of exposure to albumin. Inhibition of the MAP kinase cascade by PD98059 attenuated the effect of albumin on TGF beta-1 expression.


CONCLUSIONS
These observations suggest that overexpression of TGF beta-1 by PTC in response to albumin is regulated through a MAP kinase signaling pathway. This mechanism may play a role in the development of interstitial fibrosis in proteinuric states.
Department of Pediatrics, Section of Nephrology, School of Medicine, University of California, Davis, CA 95616, USA. jzhao@ucdavis.edu
Jun
J
Zhao
Alfonso
A
Tramontano
Sudesh Paul
SP
Makker
D000418:Albumins / Q000494:pharmacology*; D000818:Animals; D002417:Cattle; D002478:Cells, Cultured; D004797:Enzyme-Linked Immunosorbent Assay; D015151:Immunoblotting; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D020928:Mitogen-Activated Protein Kinases / Q000378:metabolism*; D051381:Rats; D020133:Reverse Transcriptase Polymerase Chain Reaction; D016212:Transforming Growth Factor beta / Q000378:metabolism*
D000418:Albumins; D016212:Transforming Growth Factor beta; D020928:Mitogen-Activated Protein Kinases