TRAIL is as a promising anticancer therapy that lacks serious side effects due to its ability to selectively kill cancer cells without harming most normal cells. However, some highly malignant tumors are resistant to TRAIL-induced programmed cell death [
30]. The mechanisms underlying TRAIL resistance include decreased expression or dysfunction of DR4 and DR5, defects in the DISC that comprises FADD and caspase-8, overexpression of Bcl-2, Bcl-X
L, FADD-like interleukin-1β-converting enzyme-inhibitory protein or inhibitor of apoptosis protein, dysfunction of Bax and Bak, decreased release of second mitochondria-derived activator of caspases (Smac/Diablo) into the cytosol, and constitutive activation of survival signaling molecules, such as mitogen-activated protein kinase, Akt, and nuclear factor-kappa-B [
20,
30]. Therefore, we hypothesized that TRAIL resistance in cancer cells can be overcome by reversing the mechanisms by which TRAIL resistance is established, such as by upregulating DR4 and DR5, overexpressing pro-apoptotic proteins, downregulating anti-apoptotic proteins, and inhibiting key factors that regulate cell survival.
As a complex disease, cancer etiology involves multiple risk factors; therefore, the treatment of cancer may require a multi-target approach. Complementary and alternative medicine, which has traditionally used crude extracts or fractions of medicinal herbs, may offer new opportunities for cancer treatments involving a multi-component and multi-target strategy [
3]. In this study, we were the first to demonstrate that the crude extract of
D. sophia seeds effectively sensitized TRAIL-refractory A549 lung cancer cells to TRAIL-induced apoptosis by upregulating DR5 at the transcriptional level (Figs.
2 and
3).
D. sophia (L.) Webb ex Prantl (Flixweed) is an annual weed that belongs to the Brassicaceae (Cruciferae) family. It is widely distributed throughout Europe and temperate to tropical Asian countries and produces large numbers of tiny red to brown seeds (0.7–1.5 mm long) from early to late summer [
31]. In traditional folk medicine, different parts of
D. sophia have been used to treat jaundice, febrifuge, laxative, and furuncle in Middle Asia [
32] and for cough, edema, asthma, heart disease, and cancer in China [
33,
34]. A variety of phytochemicals have been identified from the extracts of aerial parts and seeds of
D. sophia. They are small molecules (amino acids, alcohols, aldehydes, ketones), cardiac glycosides (erysimoside, evobioside, helveticoside, strophanthidin), coumarins (bergapten, isoscopoletin, psoralene, scopoletin, xanthotoxin, xanthotoxol), fatty acids (arachic acid, capric acid, eicosenoic acid, erucic acid, lauric acid, linoleic acid, linolenic acid, myristic acid, oleic acid, palmitic acid, stearic acid), flavonoids (drabanemoroside, isoquercitrin, isorhamnetin, isorhamnetin–3-O-β-D-glucopyranoside, kaempferol, quercetin, quercetin 3-O-α-L-rhamnopyranosyl-(1 → 2)-α-L-arabinopyranose, quercetin-3-O-β-D-glucopyranoside), flavonol glycoside (artabotryside A), glucosinolates (gluconapin, sinigrin), lactones (descurainolide A and B), lignan (syringaresinol), nor-lignan (descuraic acid), lipids (epoxyacylglyceride, triacylglyceride), phenolic compounds (3,4,5-tritrimethoxy cinnamic acid, isovanillic acid,
p-benzoic acid,
p-hydroxybenzaldehyde, sinapic acid, syringic acid), phytosterol (daucosterol), sinapoyl glycosides (1,2-di-O-sinapoyl-β-D-glucopyranose, 1,2-disinapoylgentiobiose, 1,3-di-O-sinapoyl- β-D-glucopyranose), and a unique group of compounds (descurainin, descurainin A, descurainoside, descurainoside A and B) [
34‐
40]. The biological activities of the extract of
D. sophia, such as analgesic, antipyretic, anti-inflammatory, and cytotoxic effects may be attributed to some of aforementioned phytochemicals or unknown ones, and/or their combination. In the present study, however, we did not identify the active component(s) of EEDS responsible for the synergistic anticancer effect with TRAIL; this discovery remains for future study. The active components of EEDS can be identified using conventional activity-guided fractionation of crude extracts and spectroscopic analyses of isolated single phytochemicals.
We also demonstrated that the EEDS-induced upregulation of DR5 was mediated by CHOP, as revealed by the efficient inhibition of EEDS-induced DR5 expression by CHOP siRNA. CHOP is a transcription factor that is induced by various stresses, including ER stress (the unfolded protein response) [
29] and reactive oxygen species (ROS) [
41]. Since the relationship between CHOP and DR5 was discovered in 2004 [
42], numerous studies have shown that the induction of CHOP by stressful stimuli mediates DR5 upregulation during TRAIL sensitization via a consensus CHOP-binding element (GAGGATTGCGATC) in the DR5 promoter [
42,
43]. Biologically active phytochemicals isolated from medicinal plants or their crude extracts have been reported to enhance TRAIL-induced apoptosis via CHOP-dependent DR5 upregulation in cancer cells [
21,
29]. In addition to the CHOP-dependent pathway, CHOP-independent DR5 upregulation has been observed in certain cancer cells treated with TRAIL and other chemicals, such as Orlistat (Xenical
TM) [
44] and curcumin [
45], although the key mediators of DR5 upregulation have not been identified.