Cyclooxygenase inhibitors modulate the p53/HDM2 pathway and enhance chemotherapy-induced apoptosis in neuroblastoma

Abstract

Cyclooxygenase-2 (COX-2) is upregulated in many tumors including neuroblastoma, and its overexpression has been implicated in resistance to p53-dependent apoptosis. Although p53 is rarely mutated in neuroblastoma, the p53 protein is rendered inactive via several mechanisms including sequestration in the cytoplasm. Here, we show that COX inhibitors inhibit the growth of neuroblastoma and when combined with low doses of chemotherapy, exert synergistic effects on neuroblastoma cells. Following COX inhibitor treatment, HDM2, which targets p53 for ubiquitin-mediated degradation, is downregulated, resulting in an attenuation of p53 ubiquitination and an increase in p53 half-life. The level of HDM2 phosphorylation at ser166, which influences both HDM2 and p53 subcellular distribution, is markedly diminished in response to COX inhibitors and is associated with increased p53 nuclear localization. Combining COX inhibitors with low-dose chemotherapy potentiates apoptosis and p53 stability, nuclear localization, and activity. p53 knockdown by siRNA resulted in the rescue of COX-inhibitor-treated cells, indicating that COX inhibitor-induced apoptosis is, at least in part, p53-dependent. Taken together, these results provide the first evidence that COX inhibitors enhance chemosensitivity in neuroblastoma via downregulating HDM2 and augmenting p53 stability and nuclear accumulation.

Cyclooxygenase inhibitors differentially modulate p73 isoforms in neuroblastoma

“…We have previously demonstrated that selective and non-selective COX inhibitors induce cell death in neuroblastoma, in part due to p53-dependent apoptosis (Lau et al, 2007). The selective COX-2 inhibitor celecoxib also suppressed the proliferation of p53-mutant neuroblastoma cells (SK-N-BE2, SK-N-FI and SK-N-AS) in vitro ( Figure 2a) and inhibited the growth of SK-N-BE2 xenografts (data not shown).…”

“…In addition, COX inhibitors have also been shown to target COX-2 independent signaling pathways (reviewed in Kashfi and Rigas (2005) and Grosch et al (2006)). We have shown that COX inhibitors modulate the p53/HDM2 pathway, resulting in increased p53 stability and nuclear accumulation (Lau et al, 2007). We observed that neuroblastoma cells harboring mutant-p53 were also sensitive to COX inhibitors, prompting us to ask whether p73 was involved in the potentiation of chemosensitivity by COX inhibitors.…”

“…We and others have reported that cyclooxygenase (COX) inhibitors induce cell death and enhance chemosensitivity in neuroblastoma (Johnsen et al, 2004;Lau et al, 2007;Ponthan et al, 2007). COX inhibitors are non-steroidal anti-inflammatory drugs commonly used as analgesics and anti-inflammatory agents, but have also been shown to have anticancer properties (Thun et al, 2002).…”

The dark side of a tumor suppressor: anti-apoptotic p53

“…Consistent with this, Cox-2 inhibitors enhanced chemosensitivity by downregulating HDM2 thereby augmenting p53 stability. 150 Secondly, Cox-2 was shown to physically interact with p53 both in vitro and in vivo suppressing not only basal, but also activation-induced transcriptional activity of p53, an effect that was mimicked by the addition of prostaglandin and abrogated by NS-398. 150 Such a negative feedback mechanism might be especially important during inflammatory responses that also result in DNA damage and that require the pro-apoptotic capabilities of p53 to be kept tightly in check.…”

“…150 Secondly, Cox-2 was shown to physically interact with p53 both in vitro and in vivo suppressing not only basal, but also activation-induced transcriptional activity of p53, an effect that was mimicked by the addition of prostaglandin and abrogated by NS-398. 150 Such a negative feedback mechanism might be especially important during inflammatory responses that also result in DNA damage and that require the pro-apoptotic capabilities of p53 to be kept tightly in check. 148 Although it remains to be elucidated whether p53 induces HB-EGF expression (and subsequently Akt and Cox-2 activation) in a direct or indirect manner, with the induction of this and the DDR1 pathway, p53 entails powerful antiapoptotic mechanisms that appear to be crucial determinants for the maintenance of the delicate balance between life and death in response to DNA damage.…”