Cyst enlargement in
polycystic kidney disease (PKD) involves cAMP-activated proliferation of
cyst-lining epithelial cells and transepithelial fluid secretion into the
cyst lumen via
cystic fibrosis transmembrane conductance regulator (CFTR)
chloride channel. This study aimed to investigate an inhibitory effect and detailed mechanisms of
steviol and its derivatives on
cyst growth using a
cyst model in Madin-Darby canine kidney (MDCK) cells. Among 4
steviol-related compounds tested,
steviol was found to be the most potent at inhibiting MDCK
cyst growth.
Steviol inhibition of
cyst growth was dose-dependent;
steviol (100 microM) reversibly inhibited
cyst formation and
cyst growth by 72.53.6% and 38.2±8.5%, respectively.
Steviol at doses up to 200 microM had no effect on MDCK cell viability, proliferation and apoptosis. However,
steviol acutely inhibited
forskolin-stimulated apical
chloride current in MDCK epithelia, measured with the Ussing chamber technique, in a dose-dependent manner. Prolonged treatment (24 h) with
steviol (100 microM) also strongly inhibited
forskolin-stimulated apical
chloride current, in part by reducing
CFTR protein expression in MDCK cells. Interestingly,
proteasome inhibitor,
MG-132, abolished the effect of
steviol on
CFTR protein expression. Immunofluorescence studies demonstrated that prolonged treatment (24 h) with
steviol (100 microM) markedly reduced CFTR expression at the plasma membrane. Taken together, the data suggest that
steviol retards MDCK
cyst progression in two ways: first by directly inhibiting CFTR
chloride channel activity and second by reducing CFTR expression, in part, by promoting proteasomal degradation of CFTR.
Steviol and related compounds therefore represent
drug candidates for treatment of
polycystic kidney disease.