Structure-activity relationships for agonistic and antagonistic mineralocorticoids

Spectroscopic evaluation of synthesized 5β-dihydrocortisol and 5β-dihydrocortisol acetate binding mechanism with human serum albumin and their role in anticancer activity

Our study focus on the biological importance of synthesized 5β-dihydrocortisol (Dhc) and 5β-dihydrocortisol acetate (DhcA) molecules, the cytotoxic study was performed on breast cancer cell line (MCF-7) normal human embryonic kidney cell line (HEK293), the IC₅₀ values for MCF-7 cells were 28 and 25 μM, respectively, whereas no toxicity in terms of cell viability was observed with HEK293 cell line. Further experiment proved that Dhc and DhcA induced 35.6 and 37.7% early apoptotic cells and 2.5, 2.9% late apoptotic cells, respectively, morphological observation of cell death through TUNEL assay revealed that Dhc and DhcA induced apoptosis in MCF-7 cells. The complexes of HSA-Dhc and HSA-DhcA were observed as static quenching, and the binding constants (K) was 4.7 ± .03 × 10⁴ M^-1 and 3.9 ± .05 × 10⁴ M^-1_, and their binding free energies were found to be -6.4 and -6.16 kcal/mol, respectively. The displacement studies confirmed that lidocaine 1.4 ± .05 × 10⁴ M^-1 replaced Dhc, and phenylbutazone 1.5 ± .05 × 10⁴ M^-1 replaced by DhcA, which explains domain I and domain II are the binding sites for Dhc and DhcA. Further, FT-IR, synchronous spectroscopy, and CD results revealed that the secondary structure of HSA was altered in the presence of Dhc and DhcA. Furthermore, the atomic force microscopy and transmission electron microscopy showed that the dimensions like height and molecular size of the HSA-Dhc and HSA-DhcA complex were larger compared to HSA alone. Detailed analysis through molecular dynamics simulations also supported greater stability of HSA-Dhc and HSA-DhcA complexes, and root-mean-square-fluctuation interpreted the binding site of Dhc as domain IB and domain IIA for DhcA. This information is valuable for further development of steroid derivative with improved pharmacological significance as novel anti-cancer drugs.

General overview of mineralocorticoid hormone action

The adrenal cortex elaborates two major groups of steroids that have been arbitrarily classified as glucocorticoids and mineralocorticoids, despite the fact that carbohydrate metabolism is intimately linked to mineral balance in mammals. In fact, glucocorticoids assured both of these functions in all living cells, animal and photosynthetic, prior to the appearance of aldosterone in teleosts at the dawn of terrestrial colonization. The evolutionary drive for a hormone specifically designed for hydromineral regulation led to zonation for the conversion of 18-hydroxycorticosterone into aldosterone through the catalytic action of a synthase in the secluded compartment of the adrenal zona glomerulosa. Corticoid hormones exert their physiological action by binding to receptors that belong to a transcription factor superfamily, which also includes some of the proteins regulating steroid synthesis. Steroids stimulate sodium absorption by the activation and/or de novo synthesis of the ion-gated, amiloride-sensitive sodium channel in the apical membrane and that of the Na+/K+-ATPase in the basolateral membrane. Receptors, channels, and pumps apparently are linked to the cytoskeleton and are further regulated variously by methylation, phosphorylation, ubiquination, and glycosylation, suggesting a complex system of control at multiple checkpoints. Mutations in genes for many of these different proteins have been described and are known to cause clinical disease.

Effect of various 6-dehydro-corticosteroids, 9, 11-dehydro-DOCA and 9 alpha-fluoro-DOCA on the fluxes of sodium and potassium

The introduction of a double bond at carbons 6 and 7 (6-dehydro-derivatives) of deoxycorticosterone acetate (DOCA), cortisol-21-acetate, 9 alpha-fluorocortisol-21-acetate (9 alpha-F-C-ac) and aldosterone-21-acetate substantially reduces affinity for Type II receptors but not for Type I receptors. Such a modification changes the effect of these steroids on urinary excretion of Na+ and K+. 6-Dehydro-derivatives will thus bind preferentially to receptor Type I inducing the retention of sodium and compete with mineralocorticoids for such receptors. The increase in both natriuresis and kaliuresis when corticosteroids and their 6-dehydro-derivatives are administered together may be interpreted as evidence for a Type II receptor mediation of those ion fluxes. The ionic changes are not mediated by the (Na+ + K+)-ATPase system. The fluoration at 9 and the dehydrogenation at C9C11 of DOCA result in a strong increase of binding to Type I receptor and of sodium retention.

Glucocorticoids: binding affinity and lipophilicity

The relative binding affinity of 35 steroids for the glucocorticoid receptor was determined in experiments in which the competition of various unlabeled steroids with either [6,7-3H]dexamethasone or [1,2-3H]hydrocortisone for the cytosolic glucocorticoid receptor of cultured human keratinocytes was measured. The data obtained were correlated with steroid lipophilicity, measured as the partition coefficient of the steroid between 1-octanol and pH 7.4 aqueous buffer. The introduction of various substituents on the steroid molecule induced changes in the binding affinity and was associated in some cases with concomitant changes in steroid lipophilicity. The substitution by a 17 alpha-OH or 21-OH group leads in all cases to a decrease in steroid lipophilicity and to an increase in affinity. In contrast, 17 alpha-OAc and especially 21-OAc substitution on hydrocortisone and betamethasone causes a decrease in the steroid affinity for the receptor and an increase in steroid lipophilicity. The elongation of the ester chain from acetate to valerate in both position C-17 and C-21 leads to the increase in both the binding affinity for the receptor and the lipophilicity of steroids. However, all 21-esters showed lower binding affinity than the parent alcohol. The binding affinity of the highly lipophilic 17 alpha, 21-diester was found to be lower than that of the 17 alpha-ester but higher than that of the 21-ester or of the parent alcohol. Only in the series of 17 alpha- and 21-esters is there a correlation between the binding affinity of steroids for the glucocorticoid receptor and their lipophilicity.

Effect of 6-dehydro-DOCA and 6-dehydro-9 alpha-fluorocortisol acetate on the excretion of sodium and potassium in the rat

Deoxycorticosterone acetate (DOCA) and 9 alpha-fluorocortisol acetate (9 alpha-F-Cac) can be modified by the introduction of a double bond at carbons 6 and 7 (6-dehydro-derivatives). Such a modification markedly changes the effect of the steroids on urinary excretion of Na+ and K+. Since 6-7 reduction of DOCA and 9 alpha-F-Cac substantially reduces affinity for Type II receptors but not Type I receptors, 6-dehydro-derivatives will thus bind preferentially to receptors influencing the retention of sodium (the "mineralocorticoid" or Type I receptor), and compete with mineralocorticoids for such receptors. We interpret the increase in both natriuresis and kaliuresis when mineralocorticoids and their dehydro-derivatives are administered together as evidence for a Type II receptor mediation of these ion fluxes.

Role of hydrophobic effects and polar groups in steroid-mineralocorticoid receptor interactions

To test whether hydrophobic interactions are the main driving forces in the association of steroids with mineralocorticoid receptors, the binding free energy calculated (delta Gc) using the surface area of the steroids accessible to water was compared with the observed free energy (delta GM) obtained from the Kd at the equilibrium estimated in the rat cytosol. The results are consistent with a binding process involving principally hydrophobic effects implying association of both faces of the steroid with the receptor. The discrepancies between calculated and observed values probably depend upon the mechanism of hydrogen bonding of the polar groups of the steroids to those on the receptor. Hydrogen bonds in an aqueous environment are likely between polar groups of the receptor and the 21-OH, 18-OH and 11-18 hemiacetal oxygen of the steroids. Strong hydrogen bonding in a hydrophobic environment from the 3-CO and 20-CO groups is a possibility (the latter with a dihedral angle C16-C17-C20-O20 of approximately equal to 7 degrees). Such a strong hydrogen bond from the 9 alpha-fluorine atom could account for the observed high affinity receptor binding of 9 alpha-fluorocortisol and 9 alpha-fluoroprednisolone. The loss of affinity in 11-deoxycortisol and cortisol may be explained by modification of the C17 side chain and repulsion forces from the 11 beta-OH.

The metabolism and hormonal activity of some ring-C aromatic steroids

The metabolism and steroid hormonal activity of the ring-C aromatic steroid 20 alpha,21-dihydroxy-17 beta-methyl-18-norpregna-4,8,11,13-tetraen-3-one (20 alpha,21-diol) was studied in rats and mice. The excretion of metabolites following intraperitoneal administration was monitored using tritium-labelled steroid. Of the total radioactivity, 15% was excreted in the urine after 2 days. Biliary excretion was rapid, with 80% of radioactivity collected over 12 h, while 67% was excreted in the faeces after 3 days. The metabolites isolated from bile and urine are conjugates, predominantly glucuronides, while only 5% of the dose was excreted as free unchanged steroid. The aglycones of the main metabolites from bile were identified as unchanged 20 alpha,21-diol and the two steroids 3 alpha,20 alpha,21-trihydroxy-17 beta-methyl-18-nor-5 alpha-pregna-8,11,13-triene and 3 beta, 20 alpha,21-trihydroxy-17 beta-methyl-18-nor-5 alpha-pregna-8,11,13-triene which resulted from reduction of the 4-ene-3-ketone system. Liver glycogen deposition, used as a measure of glucocorticoid activity, was induced in mice by 20 alpha,21-diol but not by its epimer the corresponding 20 beta,21-diol. Only the 20 alpha,21-diol was found to antagonise glycogen deposition when given simultaneously with cortisol. In adrenalectomised rats, the 20 alpha,21-diol produced a significant decrease in urinary Na+ excretion and increased K+ excretion. An equimolar dose of the 20 alpha,21-diol and deoxycorticosterone acetate gave a urinary excretion Na+/K+ ratio which reflected the combined mineralocorticoid effect of each steroid. The 20 beta, 21-diol produced no change in the urinary Na+/K+ ratio.