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Abstract
The synthetic curcumin analog B5 is a potent inhibitor of thioredoxin reductase (TrxR) that has potential anticancer effects. The molecular mechanism underlying B5 as an anticancer agent is not yet fully understood. In this study, we report that B5 induces apoptosis in two human cervical cancer cell lines, CaSki and SiHa, as evidenced by the downregulation of XIAP, activation of caspases and cleavage of PARP. The involvement of the mitochondrial pathway in B5-induced apoptosis was suggested by the dissipation of mitochondrial membrane potential and increased expression of pro-apoptotic Bcl-2 family proteins. In B5-treated cells, TrxR activity was markedly inhibited with concomitant accumulation of oxidized thioredoxin, increased formation of reactive oxygen species (ROS), and activation of ASK1 and its downstream regulatory target p38/JNK. B5-induced apoptosis was significantly inhibited in the presence of N-acetyl-l-cysteine. Microscopic examination of B5-treated cells revealed increased presence of cytoplasmic vacuoles. The ability of B5 to activate autophagy in cells was subsequently confirmed by cell staining with acridine orange, accumulation of LC3-II, and measurement of autophagic flux. Unlike B5-induced apoptosis, autophagy induced by B5 is not ROS-mediated but a role for the AKT and AMPK signaling pathways is implied. In SiHa cells but not CaSki cells, B5-induced apoptosis was promoted by autophagy. These data suggest that the anticarcinogenic effects of B5 is mediated by complex interplay between cellular mechanisms governing redox homeostasis, apoptosis and autophagy.
Highlights
The thioredoxin (Trx) system, composed of thioredoxin reductase (TrxR), Trx, and NADPH, is a major cellular redox control mechanism that is often deregulated in malignancy [1]
We report that B5 induces apoptosis in two human cervical cancer cell lines, CaSki and SiHa, as evidenced by the downregulation of X-linked inhibitor of apoptosis (XIAP), activation of caspases and cleavage of PARP
We demonstrated that B5, a newly synthesized TrxR inhibitor and a curcumin analog, exhibited significant antitumor activity based on the modulation of the intracellular redox status and autophagy (Fig. 10)
Summary
The thioredoxin (Trx) system, composed of thioredoxin reductase (TrxR), Trx, and NADPH, is a major cellular redox control mechanism that is often deregulated in malignancy [1]. The Trx system may be involved in carcinogenesis in several ways. It can promote cancer cell proliferation through its modulatory effect on redox-regulated transcription factors and/or protein kinase signaling cascades [5]. Tumor cells usually have higher resistance against oxidative stress-induced apoptosis [10]. High levels of Trx expression are associated with tumor invasion and metastasis. Trx promotes matrix metalloproteinase activity and stimulates cancer cell invasion [14]. Trx over expression is linked to tumor metastasis [15] and is implicated in malignant potential of tumor cells [1]. The Trx system may represent an important therapeutic target in cancer treatment
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