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A more recent version of this article appeared on September 1, 2002
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Submitted on May 6, 2002
Accepted on June 13, 2002
1 University of Texas Health Science Center San Antonio, Department of Physiology, MC 7756, 7703 Floyd Curl Drive, San Antonio, TX 78229-3900
* Corresponding author. E-mail address: stockand{at}uthscsa.edu.
Aldosterone in some tissues increases expression of the mRNA encoding the small monomeric G protein Ki-RasA (Spindler et al., 1997). Renal A6 epithelial cells were used to determine whether induction of Ki-ras leads to concomitant increases in the total as well as active levels of Ki-RasA and whether this then leads to subsequent activation of its effector mitogen-activated protein kinase (MAPK/ERK) cascade. The molecular basis and cellular consequences of this action were specifically investigated. We identified the intron 1 - exon 1 region (rasI/E1) of the mouse Ki-ras gene as sufficient to reconstitute aldosterone responsiveness to a heterologous promotor. Aldosterone increased reporter gene activity containing rasI/E1 3-fold. Aldosterone increased the absolute and GTP-bound levels of Ki-RasA by a similar extent suggesting that activation resulted from mass action and not effects on GTP binding/hydrolysis rates. Aldosterone significantly increased Ki-RasA and MAPK activity as early as 15 min. with activation peaking by 2 hrs. and waning after 4 hrs. Inhibitors of transcription, translation and a glucocorticoid receptor antagonist attenuated MAPK signaling. Similarly, rasI/E1 driven luciferase expression was sensitive to glucocorticoid receptor blockade. Overexpression of dominant-negative RasN17, addition of antisense Ki-rasA and inhibition of MEK also attenuated steroid-dependent increases in MAPK signaling. Thus, activation of MAPK by aldosterone is dependent, in part, on a genomic mechanism involving induction of Ki-ras transcription and subsequent activation of its downstream effectors. This genomic mechanism has a distinct time course from activation by traditional mitogens, such as serum, which affect the GTP binding state and not absolute levels of Ras. The result of such a genomic mechanism is that peak activation of the MAPK cascade by adrenal corticosteroids is delayed but prolonged.
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