Baicalein induces caspase-dependent apoptosis in    human melanoma A375 cells associated with elicitation of extrinsic apoptotic pathway and upregulation of cyclooxygenase-2 Free

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Baicalein is a flavone isolated from the dried root of a traditional Chinese medicinal herb, Scutelleria baicalensis Gorgi, which is used in China for treating cancers and inflammatory diseases. However, its mechanism of action was not known. The growth inhibitory effect of baicalein was first tested on a panel of cancer cell lines of different histotypes in vitro. Human melanoma A375 was then found to be the most responsive cell line, substantiated by the lowest IC₅₀ of 37.5 μM.

Baicalein exhibited a dose-dependent growth inhibition by inducing DNA fragmentation in A375 cells after S-phase cell cycle arrest, accompanied by the cleavage of poly-(ADP-ribose) polymerase (PARP). Caspase-8 and caspase- 3 activation were observed after 24 and 48 h of treatment, respectively. This was subsequently counter-proved by the inhibition of PARP cleavage after the administration of caspase-8 inhibitor (Z-IE(OMe)TD(OMe)-FMK) with the baicalein treatment. Sequential activation of caspase-8 and caspase-3 was evidenced by the inhibition of caspase-3 activation after the co-administration of caspase-8 inhibitor and baicalein. Activation of caspase-8 was observed as early as 24 h of treatment, revealing that caspase-8 would probably be the initiator of apoptosis, via the extrinsic death receptor signaling pathway. One of the apoptosis-promoting factors, TRAIL and its receptor, TRAIL-R1 (DR4) were both found to be upregulated after baicalein treatment, which was coherent with the caspase-8 activation.

COX-2, an enzyme responsible for arachidonic acid metabolism and prostaglandin synthesis, has been reported to be overexpressed in a number of cancers. As COX-2 expression is often induced by inflammatory mediators, we are therefore interested to know whether baicalein, a flavone from an anti-inflammatory herb, would cause cell death via modulation of COX-2 related pathways. Instead of downregulating COX-2, baicalein induced a prominent upregulation of COX-2 protein expression and prostaglandin E₂ production in the melanoma cells. Recent publication has also ascribed that COX-2 upregulation may serve a proapoptotic role via p53-dependent apoptosis. Our findings also support the view as phosphorylated p53 (Ser¹⁵) was elevated after baicalein treatment. Macroscopically, baicalein-induced cell death via upregulation of COX-2 was further evidenced by the enzyme upregulation in another responsive cell line human colorectal carcinoma Caco-2 (IC₅₀ = 84 μM), but not in those non-responsive cell lines such as prostate carcinoma PC-3 and lung carcinoma A549.

Findings from this study will help us understand the cell death mechanisms induced by baicalein more clearly and generate new knowledge about the functions of COX-2 in cancer biology, which is essential in developing novel anticancer agents targeting arachidonic acid metabolism.