An ester derivative of tenacigenin B from Marsdenia tenacissima (Roxb.) Wight et Arn reversed paclitaxel-induced MDR in vitro and in vivo by inhibiting both P-gp and MRP2

Marsdenia tenacissima is a medicinal plant, used as a raw material for cancer treatment in China. In our previous studies, 11α-O-2-methylbutanoyl-12β-O-tigloyl-tenacigenin B (MT2), the main steroid aglycone isolated from M. tenacissima, was found to significantly enhance the antitumor activity of paclitaxel (PTX) in vivo. However, it is unclear whether MT2 reverses multidrug resistance (MDR) in tumors. To determine the role and mechanism of MT2 in reversing tumor MDR. MDR cell line HeLa/Tax was established from the human cervical carcinoma cell line HeLa by long-term exposure to subtoxic concentrations of PTX and was used to evaluate the ability of MT2 to restore chemosensitivity of cells both in vitro and in a nude mouse model. The expression of P-glycoprotein (P-gp) and multidrug resistance-associated protein 2 (MRP2) was determined using western blotting and immunohistochemistry. The substrate transport function was assessed using an MDR function assay kit. The binding modes of MT2 and P-gp were determined using the conformation-sensitive anti-P-gp antibodies. The permeability and transport properties of MT2 were analyzed in Caco-2 cell monolayers. Compared to parental cells, HeLa/Tax cells overexpress P-gp and MRP2 and are approximately 100–360 fold more resistant to the anticancer drugs PTX, docetaxel, and vinblastine. MT2 at 5 or 10 μmol/L significantly increased the sensitivity of HeLa/Tax to these three anticancer drugs (18–56-fold decrease in IC₅₀ value) and suppressed the expression of P-gp and MRP2. Knockdown of P-gp with small interfering RNA partially reversed MT2-induced sensitivity to PTX in HeLa/Tax cells. Moreover, MT2 directly inhibited P-gp-mediated substrate transport while interacting with membrane P-gp in non-substrate ways. MT2 was highly permeable and could not be transported in the Caco-2 cell monolayers. In nude mice bearing HeLa/Tax xenografts, the combination treatment with MT2 and PTX exerted a synergistic inhibitory effect on the growth of tumors and the expression of P-gp and MRP2 without increasing toxicity. MT2 is a potential agent for reversing MDR. It impedes membrane drug efflux pumps by suppressing P-gp and MRP2 expression, and directly inhibiting the transport function of P-gp.