Thyroid hormone. Aldosterone antagonism in cultured epithelial cells

Overlapping nongenomic and genomic actions of thyroid hormone and steroids

Nuclear receptors for thyroid hormone and steroids are members of a receptor superfamily with similar molecular organization, but discrete transcriptional functions that define genomic actions of these nonpeptide hormones. Nongenomic actions of thyroid hormone and estrogens and androgens are initiated outside the nucleus, at receptors in the plasma membrane or in cytoplasm; these actions are largely regarded to be unique to the respective hormones. However, there is an increasing number of descriptions of overlapping nongenomic and genomic effects of thyroid hormone and estrogens and testosterone. These effects are concentrated in tumor cells, where, for example, estrogens and thyroid hormone have similar mitogen-activate protein kinase (MAPK)-dependent proliferative actions on ERα-positive human breast cancer cells, and where dihydrotestosterone also can stimulate proliferation. Steroids and thyroid hormone have similar anti-apoptotic effects in certain tumors. But thyroid hormone and steroids also have overlapping or interacting nongenomic and genomic actions in heart and brain cells. These various effects of thyroid hormone and estrogens and androgens are reviewed here and their possible clinical consequences are enumerated.

The activity of Na+/K(+)-ATPase and abundance of its mRNA are regulated in rat myometrium during pregnancy

The Na+/K(+)-ATPase activity and expression of mRNAs encoding alpha and beta subunits of the enzyme were examined in rat myometrium at different stages of pregnancy. The enzyme activity appeared to increase during the pregnancy and reached the highest value at the 17th day. Northern blot analysis of total RNA isolated from the same myometrial samples shows the expressions of alpha 1, alpha 3 and beta mRNAs. A2 mRNA was not detectable. By the progression of pregnancy until the 17th day the expression of all the three kinds of mRNA increased. The change in the abundance of mRNA-beta was much more higher (12-fold) than that of mRNA-alpha 1 and -alpha 3 (4 and 2.5-fold, respectively). Furthermore, the expression of mRNA-beta sharply decreased after the 17th day, while the level of alpha subunits mRNAs barely changed. We conclude that transcriptional regulation of the Na+/K(+)-ATPase subunits could be different during this physiological process.

Cellular mechanisms of action of mineralocorticoid hormones

Mineralocorticoid hormones are a subset of steroid hormones that act primarily in epithelial tissues to regulate ion transport of Na+, K+ and H+. Cellular specificity is conferred by receptors which act in the nucleus to stimulate gene expression. Transcription and subsequent translation result in the production of new proteins which mediate the physiologic effects. The mechanisms involved in receptor specificity and localization, in regulation of gene activation, and in expression of transport effects are reviewed. The cellular actions of mineralocorticoids fit well with the general model of steroid hormone action but considerable questions remain at each step in the process.

Aldosterone increases the apical Na+ permeability of toad bladder by two different mechanisms

The aldosterone-induced augmentation of Na+ transport in toad bladder was analyzed by comparing the hormonal actions on the transepithelial short-circuit current and on the amiloride-sensitive 22Na+ uptake in isolated membrane vesicles. Incubating bladders with 0.5 microM aldosterone for 3 hr evoked more than a 2-fold increase of the short-circuit current (because of the activation or insertion of apical amiloride-blockable channels) but had no effect on the amiloride-sensitive Na+ transport in apical vesicles derived from the treated tissue. A longer incubation (e.g., 6 hr) produced an additional augmentation of the short-circuit current, which was accompanied by about a 3-fold increase of the channel activity in isolated membranes. The stimulatory effect of aldosterone sustained in vesicles was inhibited by the antagonist spironolactone (present at 1000-fold excess) and the protein synthesis inhibitor cycloheximide (1 microM). In addition, triiodothyronine and butyrate, previously reported to partly inhibit the aldosterone-induced increase in short-circuit current, blocked the hormonal effect in vesicles. It is suggested that aldosterone elevates the apical Na+ permeability of target epithelia by two different mechanisms: a relatively fast effect (less than or equal to 3 hr), which is insensitive to triiodothyronine or butyrate and is not sustained by the isolated membrane, and a slower or later (greater than 3 hr) response blocked by these reagents, which is preserved by the isolated membrane. The data also indicate that these processes are mediated by different nuclear receptors.