Molecular Interactions of the Plant Steroid Hormone Epibrassinolide on Human Drug-Sensitive and Drug-Resistant...

Background: Small-cell lung cancer (SCLC) has a poor prognosis because it is often diagnosed after it has spread and develops multi-drug resistance. Epibrassinolide (EB) is a plant steroid hormone with widespread distribution and physiological effects. In plants, EB-activated gene expression occurs via a GSK-mediated signaling pathway, similar to Wnt-β-catenin signaling in animal cells that is elevated in cancer cells. Methods: This mechanistic parallel prompted investigations of the molecular interactions of EB on drug-sensitive (H69) and multi-drug-resistant (VPA) SCLC cells. Cellular and molecular investigations were performed. Results: Pharmacologic interactions between EB and the Wnt signaling inhibitors IGC-011 and PRI-724 were determined by the combination index method and showed antagonism, indicating that EB acts on the same pathway as these inhibitors. Following incubation of drug-sensitive and drug-resistant SCLC cells with EB, there was a reduction in β-catenin (e.g., 3.8 to 0.7 pg/µg protein), accompanied by a reduction in β-catenin promoter activity, measured by firefly luciferase-coupled promoter element transfection. Cellular β-catenin concentration is regulated by the active form of GSK3β. In Wnt signaling, active GSK3β is converted to inactive pGSK3β, thereby increasing the concentration of β-catenin. After incubation of SCLC cells with EB, there was a reduction in the inactive form (pGSK3β) and a relative increase in the active form (GSK3β). In vitro enzyme assays showed that EB did not inhibit purified GSK3β, but there was non-competitive inhibition when SCLC cell extracts were used as the source of enzyme. This indirect inhibition by EB indicates that it may act on the Wnt pathway by blocking the phosphorylation of GSK3β. The protein levels of three SCLC tumor markers, namely, NSE, CAV1, and MYCL1, were elevated in drug-resistant SCLC cells. EB incubation led to a significant reduction in the levels of the three markers. Two major effects of EB on SCLC cells are the promotion of apoptosis and the reversal of drug resistance. Transcriptional analyses showed that after exposure of SCLC cells to EB, there were increases in the expression of genes encoding apoptotic inducers (e.g., BAX and FAS) and effectors (e.g., CASP3) and reductions in the expression of genes encoding apoptosis inhibitors (e.g., survivin). PGP1 and MRP1, two membrane efflux pumps expressed in SCLC cells, were elevated in drug-resistant cells, but EB incubation did not affect these protein levels. Cellular assays of drug efflux by PGP1 showed an increase in drug-resistant cells, but EB did not alter efflux activity. Following exposure to human liver microsomes, EB was metabolized by NADPH-dependent oxidation and UDPG-dependent glucuronidation, as evidenced by the elimination of EB cytotoxicity against SCLC cells. Conclusions: Taken together, these data indicate that EB, a steroid hormone in plants consumed in the human diet, is pharmacologically active in drug-sensitive and drug-resistant SCLC cells in the Wnt signaling pathway, alters apoptotic gene expression, and is a substrate for microsomal modifications.