TAUTOMYCIN INHIBITS PHOSPHATASE-DEPENDENT TRANSFORMATION OF THE RAT-KIDNEY MINERALOCORTICOID RECEPTOR

The binding of aldosterone (ALDO) to the mineralocorticoid receptor (M R) induces a conformational change of the protein referred to as 'tran sformation'. This feature can be evidenced in vivo by the capacity of the MR to interact with chromatin, and in vitro by the ability of the MR to bind to DNA strands or to shift the sedimentation coefficient (S ) to lower values. The transformation process allows MR to work as a t ranscription factor after interacting with specific sequences of DNA. The signal transduction pathway for the MR transformation remains unkn own. As a first step towards elucidating the mechanism of steroid-depe ndent MR transformation, we asked if the MR-signaling pathway is affec ted by the phosphorylation status of the MR-heterocomplex, and how tha t pathway may be regulated. Incubation of preformed [H-3]ALDO-MR compl ex with bovine intestinal alkaline phosphatase led to an increase in t he rate of MR-transformation (measured as 9.4-5.4S shift). This alkali ne phosphatase-dependent MR transformation was inhibited by the specif ic alkaline phosphatase-type inhibitor levamisole, and was not evident in incubations performed with acid phosphatases. A direct correlation between the DNA-cellulose binding capacity of the [H-3]ALDO-MR comple x and the percentage of transformed 5.4S MR form was also observed. Wh en rat kidney cytosol was incubated in the absence of both exogenous p hosphatase and stabilizing agents (such as molybdate or vanadate), MR transformation also took place, in a time- and temperature-dependent p rocess. In contrast with the inhibitory effect observed upon alkaline phosphatase-promoted transformation, levamisole was unable to inhibit the endogenous transforming activity of MR, suggesting that an endogen ous phosphatase other than those which belong to the alkaline-type may be responsible for that transformation. Tautomycin, a polyketide prod uced by the soil bacteria Streptomyces which inhibits serine/threonine phosphatases of the PP1/PP2A subgroup, was able to inhibit the endoge nous phosphatase activity in a concentration-dependent form (K-iapp = 7.35 nM). These results support the idea that the endogenous renal act ivity involved in the regulation of rat kidney MR transformation may b e a protein phosphatase which belongs to the PP1/PP2A subgroup. (C) 19 98 Elsevier Science Ireland Ltd. All rights reserved.