Stevioside and its major metabolite, steviol, have been reported to affect ion transport in many types of tissues, such as the kidney, pancreas, and intestine. The effect of stevioside, steviol, and its analogs on intestinal Cl Ϫ secretion was investigated in a human T84 epithelial cell line. Short-circuit current measurements showed that steviol and analogs isosteviol, dihydroisosteviol, and isosteviol 16-oxime inhibited in a dosedependent manner forskolin-induced Cl Ϫ secretion with IC 50 values of 101, 100, 9.6, and 50 M, respectively, whereas the parent compound stevioside had no effect. Apical Cl Ϫ current measurement indicated that dihydroisosteviol targeted the cystic fibrosis transmembrane regulator (CFTR). The inhibitory action of dihydroisosteviol was reversible and was not associated with changes in the intracellular cAMP level. In addition, dihydroisosteviol did not affect calcium-activated chloride secretion and T84 cell viability. In vivo studies using a mouse closed-loop model of cholera toxin-induced intestinal fluid secretion showed that intraluminal injection of 50 M dihydroisosteviol reduced intestinal fluid secretion by 88.2% without altering fluid absorption. These results indicate that dihydroisosteviol and similar compounds could be a new class of CFTR inhibitors that may be useful for further development as antidiarrheal agents.

The application of brassinosteroids (BRs) has been reported to alleviate heat stress. This study investigated the effect of 7,8-dihydro-8a-20-hydroxyecdysone (DHECD)-a BR mimic-by comparison with 24-epibrassinolide (EBR) on the changes in photosynthetic performance, lipid peroxidation, and rice seed set. The results demonstrated that 10 28 M EBR and 10 27 M DHECD had the best actions to counteract the lethal heat temperature of 47°C for 2 h indicated by a reduction in the number of wilted leaves and an increase in the relative water content and leaf greenness. Moreover, plants treated with EBR or DHECD were exposed to high day/night temperatures of 40/30°C for 7 days. EBR-treated and DHECD-treated plants showed a high shoot fresh weight, leaf area, chlorophyll content, and carotenoid content. High temperature significantly decreased the leaf net CO 2 assimilation rate as well as increased lipid peroxidation. The application of EBR and DHECD maintained the high level of the net CO 2 assimilation rate by increasing the stomatal conductance and photochemical quenching. On the other hand, EBR and DHECD decreased the intracellular CO 2 content and nonphotochemical quenching leading to enhance photosynthesis under heat stress. EBR-treated and DHECD-treated plants significantly reduced their malondialdehyde and hydrogen peroxide contents as well as increasing their total soluble sugar contents. Moreover, BR treatments increased the filled seed of rice. This study confirmed that DHECDa BR mimic-has activities of heat stress alleviation similar to EBR.

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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.

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