Mechanisms of Testosterone Effects on the Transport Protein P-Glycoprotein

P-glycoprotein (Pgp) is an efflux membrane transport protein that plays an important role in the protection of tumor cells from cytostatics and drug pharmacokinetics. Testosterone is known to be able to reduce Pgp activity and expression, but the mechanisms of its action remain unexplored. The aim of this study was to evaluate the mechanism underlying the effect of testosterone on Pgp functioning, specifically, the role of androgen (AR), pregnane X (PXR), and constitutive androstane (CAR) receptors in this mechanism both in vitro and in vivo. In in vitro experiments on Caco-2 cell line, testosterone (1 and 10 µM, 24 h) reduced the Pgp level (Western blotting) compared to the control. The PXR inhibitor ketoconazole (1 and 10 µM, 24 h) did not affect the Pgp level, while the CAR inhibitor CINPA (10 µM) reduced it. The combination of CINPA (10 µM) and testosterone (10 µM) also reduced the Pgp level, however, the latter was not significantly different from the values obtained with CINPA and testosterone applied individually. In in vivo experiments on male Chinchilla rabbits, single intramuscular injection of testosterone undecanoate (24 mg/kg b.w.) increased the serum testosterone level on day 21 compared to the control, whereas after orchiectomy, its content decreased on postoperative day 21. Relative Pgp and CAR levels in the rabbit jejunum decreased after testosterone injection and increased with orchiectomy. PXR and АR levels in the rabbit jejunum did not change significantly. Thus, both in vitro and in vivo experiments demonstrated that testosterone inhibits the CAR and thus reduces the level of the transport protein Pgp.