Comparison of methyltrienolone and dihydrotestosterone binding and metabolism in human genital skin fibroblasts

We compared the characteristics of the specific binding to the androgen receptor and the metabolism of methyltrienolone (R1881) and 5α-dihydrotestosterone (DHT) in cultured human genital skin fibroblasts. Both R1881 and DHT bound to the intracellular androgen receptor with high affinity and low capacity, with maximum binding achieved by 20 min at 37°C. From 12 Scatchard analyses of whole cell binding in six different newborn foreskin fibroblast strains, the maximum binding capacity (B_max) expressed as mol × 10^-18/μg DNA for R1881 was 758 ± 50 (SEM) and for DHT was 627 ± 103 (SEM) while the binding affinity (K_d) expressed as 10^-9M for R1881 was 0.56 ± 0.06 (SEM) and for DHT was 0.72 ± 0.11 (SEM). Neither steroid was specificially bound within genital skin fibroblasts of patients with receptor (-), androgen insensitivity. From competition studies, the relative binding affinity of R1881 was 1.5-2.0 fold higher than for DHT. However, the dissociation rate of R1881 (t₁ = 68 h) from the androgen receptor was nearly twice as fast as for DHT (t₁) = 111 h). The macromolecular steroid complex with R1881 or DHT sedimented similarly at approximately 3S in linear sucrose density gradients. Nuclear bound steroid represented about half of the total cellular androgen binding with one-third of the nuclear steroid resistant to extraction by 0.5 M KCl and sonication. R1881 was not metabolized by foreskin fibroblasts nor did it alter the rate of testosterone and DHT transformation. R1881 was not appreciably bound (< 40%) by an anti-testosterone immunoglobulin which bound > 90% of the testosterone or DHT present over the same steroid concentration range. In summary, R1881 binds to the androgen receptor of genital skin fibroblasts with high affinity and specificity, but exhibits some molecular properties which differ from endogenous androgens.