Ion Channels – Membrane Transport – Integrative Physiology
Kidney International (1999) 55, 956–962; doi:10.1046/j.1523-1755.1999.055003956.x
Tyrosine kinase inhibitors and immunosuppressants perturb the myo-inositol but not the betaine cotransporter in isotonic and hypertonic MDCK cells
MOHAMED G ATTA, STEPHEN C DAHL, H MOO KWON and JOSEPH S HANDLER
Division of Nephrology, Department of Medicine, Johns Hopkins University School of Medicine, Baltimore, Maryland, USA
Correspondence: Mohamed G. Atta, M.D., Johns Hopkins University, Division of Nephrology, Suite 416, 1830 East Monument Street, Baltimore, Maryland 21205, USA. E-mail: atta@welchlink.welch.jhu.edu
Received 26 June 1998; Revised 30 September 1998; Accepted 1 October 1998.
Abstract
Tyrosine kinase inhibitors and immunosuppressants perturb the myo-inositol but not the betaine cotransporter in isotonic and hypertonic MDCK cells.
Background.
The sodium/myo-inositol cotransporter (SMIT) and the betaine cotransporter (BGT1) are essential for the accumulation of myo-inositol and betaine, and hence cell survival in a hypertonic environment. The underlying molecular mechanism involves an increase in transcription of the SMIT and BGT1 genes through binding of a trans-acting factor to enhancer elements in the 5' flanking region of both genes, resulting in increased mRNA abundance and increased activity of the cotransporters. Current evidence regarding transcriptional and post-transcriptional regulation indicates that both cotransporters are regulated in parallel.
Methods.
To investigate the signal transduction of hypertonic stress, we examined the effect of tyrosine kinase inhibitors and immunosuppressants on the hypertonicity-induced activity of the two cotransporters in Madin-Darby canine kidney (MDCK) cells.
Results.
None of the agents studied affected BGT1 activity in isotonic or hypertonic conditions. Treatment of MDCK cells with genistein, a tyrosine kinase inhibitor, increased SMIT activity in hypertonic but not isotonic conditions. The stimulation of SMIT by genistein was accompanied by a parallel increase in mRNA abundance. In contrast, treating cells with tyrphostin A23, another tyrosine kinase inhibitor, or cyclosporine A, an immunosuppressant, inhibited SMIT activity in hypertonic cells. FK506, another immunosuppressant, increased SMIT activity, but only in isotonic conditions.
Conclusions
These results provide the first evidence of divergent regulatory pathways modulating SMIT and BGT activity.
Keywords:
hypertonicity, cell volume, osmolytes, cotransport, protein phosphorylation, transcription
