Monday, August 23, 2010

On the function of anabolic steroids


Anabolic steroids are synthetic derivatives of testosterone that display androgenic and anabolic effects in a tissue dependent manner. The primary anabolic effect stems from increased protein synthesis and decreased protein catabolism within muscle tissue accelerating muscle growth and strength. The major androgenic properties also come from protein synthesis regulation yet primarily within the gonads and brain. This regulation leads to the development of characteristics such as facial hair and increased aggressiveness. Furthermore, anabolic steroids can also mimic the function of the female hormone estrogen promoting some female secondary characteristics such as breast development.
The hydrophobic anabolic steroid molecule (Fig. 1 depicts the hydrophobic nature of the anabolic steroid oxymetholone) is permeable to cell membranes. This permeability facilitates anabolic steroid penetration across the cell membrane where it binds to the androgen receptor located in the cytoplasm of that cell. Anabolic steroid binding (Fig. 2 depicts the androgen receptor ligand binding domain in ribbon diagram with red alpha helices, blue beta strands, and green loops bound to testosterone colored white emphasized by yellow arrow) changes the androgen receptor conformation causing the dissociation of bound heat shock proteins, homodimerization, and exposure of its nuclear localization signal. All of this causes the anabolic steroid-receptor complex to diffuse into the nucleus. In the nucleus, the androgen receptor regulates gene expression by binding to specific DNA sequences known as promoters and interacting with other proteins in the nucleus that either activate or inactivate gene transcription (Fig. 3 demonstrates the functional scheme for anabolic steroids on the androgen receptor). Activation of transcription results in increased synthesis of messenger RNA which is translated by ribosomes into certain proteins. Two target genes activated by the androgen receptor are the muscle protein actin which is essential for muscle growth, and the insulin-like growth factor I (IGF-1) which stimulates skeletal muscle growth.
Because anabolic steroids possess desirable anabolic properties along with unwanted androgenic and estrogen-like qualities, designing anabolic steroids with greater anabolism and fewer androgenic and estrogenic properties is of great interest to the medical community. In order to augment these properties, rational drug design (see post 7-26-10) has been used to modify the anabolic steroid’s chemical structure enhancing the anabolic-steroid androgen-receptor interaction and minimizing anabolic steroid binding to the estrogen receptor. The design strategy for the anabolic steroid tetrahydrogestrinone (THG) lucidly demonstrates the structure-activity relationship where even minor modifications in the structure have a great impact on the receptor interaction. THG was chemically modified with two diethyl groups added to the 13th and 17th carbon positions within the steroid molecule (Fig. 4 is the chemical structure of THG highlightling the two key diethyl groups with asterisks). These modifications establish greater van der Waals interactions with a distance of ~4 angstroms between THG and the androgen receptor (Fig. 5 shows the androgen receptor bound to testosterone in cyan with a superpositioned THG in magenta highlighting with red asterisks the additional van der Waal contacts between THG and the androgen receptor) and generate steric clashes between THG and the estrogen receptor with a distance of less than 1 angstrom abrogating THG binding (Fig 6 shows the estrogen receptor bound to estrone in cyan with a superpositioned THG molecule in magenta highlighting with red asterisks the steric clashes between THG and the estrogen receptor). Up to this point a diverse group of anabolic steroids have been synthesized with similarly optimized characteristics. However, no anabolic steroid has eliminated all androgenic and estrogen-like effects because of the inseparable nature of anabolic and androgenic function combined with the inability to completely eliminate estrogen receptor interaction and activation.

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