Cyclooxygenase–2 and Tumor Biology

Shigeru Kanaoka; Tetsunari Takai; Ken‐ichi Yoshida    First Department of Medicine, Hamamatsu University School of Medicine, 1–20–1 Handayama, Hamamatsu 431–3192, Japan

2 Introduction

It is now accepted that the use of regular nonsteroidal anti‐inflammatory drugs (NSAIDs) reduces the risk of cancer, particularly in the colon [1]. NSAIDs prevent colorectal adenomas and cancer, and induce polyp regression in familial adenomatous polyposis (FAP) [2–6]. This protection against colorectal tumor development seems to be related particularly to cyclooxygenase‐2 (COX‐2) inhibition by NSAIDs, although this is probably not the only mechanism [7, 8]. Indeed selective COX‐2 inhibitors significantly reduced numbers and burden of colorectal polyps in patient populations with FAP [9, 10].

Inhibition of COX leads to reduced conversion of arachidonic acid to proinflammatory prostaglandins (PGs) and other bioactive lipids, including PGE2, PGF2α, PGD2, PGI2, and thromboxane (TX) A2. However, the precise mechanism for the anticancer effect remains unknown.

This chapter describes proposed mechanisms for the role of COX‐2 in carcinogenesis and discusses the use of COX‐2 as a biomarker in the evaluation of neoplasm. We also introduce a novel method to detect COX‐2 messenger RNA (mRNA) in feces from subjects with colorectal neoplasms.

3 Proposed Roles for COX‐2 in Carcinogenesis

To date, at least six mechanisms by which COX‐2 contributes to tumorigenesis and the malignant phenotype of tumor cells have been identified, including induction of proliferation, inhibition of apoptosis, increased angiogenesis, enhanced invasiveness, immune modulation, and increased mutagenesis.

3.1 Proliferation

Although COX‐2‐derived PGE2 in colorectal carcinoma cells stimulates cell proliferation [11], the downstream signaling pathways involved in PGE2‐induced proliferation are still unknown. Forced expression of COX‐2 in human colorectal cancer (CRC) cells can stimulate cellular proliferation through transactivation of the epidermal growth factor receptor (EGFR) [12]. PGE2 activates matrix metalloproteinase (MMP) activity resulting in the release of active EGFR ligand from the plasma membrane, leading to increased expression of amphiregulin, a ligand of EGFR [13]. Activation of EGFR signaling leads to increased MAPK activity resulting, in turn, in AP‐1‐mediated induction of COX‐2 transcription [14]. Evidence that a combination treatment using a nonselective NSAID and an EGFR tyrosine kinase inhibitor significantly decreased polyp formation in ApcMin mice supports the notion that combinations of different agents for cancer prevention and treatment may be more effective than a single agent therapy that is targeted to one molecule [13].

3.2 Apoptosis

Tumor growth is dependent on the disruption of the normal balance of cell proliferation and apoptosis [15]. Prolonged cell survival through inhibition of apoptosis can facilitate the accumulation of successive genetic mutations, which favors tumor progression. The regulation of apoptosis is cell‐type specific and dependent on the balance of pro‐ and antiapoptotic factors. Although a number of experimental studies have demonstrated a clear positive correlation between COX‐2 expression and inhibition of apoptosis [16, 17], the underlying molecular mechanisms are still not fully understood. A reduction in the rate of cells undergoing apoptosis has been found in sporadic adenomas and colonic carcinomas, as well as the colorectal mucosa of FAP patients [18, 19], and is possibly due to increased COX‐2 expression found in these cells. Furthermore, overexpression of COX‐2 in rat intestinal epithelial (RIE) cells increased their resistance to butyrate‐induced apoptosis and led to increased Bcl‐2 protein expression [20]. The COX‐2 selective inhibitor NS398 induced apoptosis in 15 CRC cell lines, while more significant effects were observed in cells expressing COX‐2 [21], suggesting that NSAIDs can induce apoptosis in tumor cells by inhibiting the COX‐2 pathway. Evidence that COX‐2 regulates the apoptotic rate of tumor cells further supports the concept that the COX‐2 pathway plays a key role in preventing apoptosis in CRCs.

The role of COX‐2 in preventing apoptosis is likely mediated by COX‐2‐derived prostaglandins. Previous reports have shown that COX‐2‐derived PGE2 is particularly high in human colon cancers [22] and that PGE2 inhibits apoptosis induced by either nonselective NSAIDs or the COX‐2 selective inhibitor SC‐58125 in human colon carcinoma cells [23]. These findings support the hypothesis that COX‐2‐derived PGE2 may increase the resistance of colon carcinoma cells to programmed cell death. However, the mechanisms by which PGE2 modulates apoptosis are still largely unknown. PGE2 may regulate programmed cell death and reduce the basal apoptotic rate by increasing levels of antiapoptotic proteins, such as Bcl‐2 [23], or other members of the bcl gene family such as Mcl‐1. Another possibility is the involvement of PGE2 in the induction of NF‐κB (p65/Rel A) transcriptional activity [24], a component of the antiapoptotic signaling pathway. However, COX‐independent effects of NSAID‐induced apoptosis have also been reported [25].

Results from other studies suggest that COX‐2‐mediated apoptosis is not only operative in colorectal cancer cells but also in other tumor cell lines. A clear correlation has been observed between PGE2 levels and decreased apoptosis in murine lung adenoma xenografts [26, 27], while an increased rate of apoptosis is seen in the human prostate carcinoma cell line LNCaP following exposure to the COX‐2 inhibitor NS398 [28]. In addition, similar results have been reported for cancers of gall bladder [29], head and neck [30], esophagus [31, 32], cervix [33], pancreas [34], and lung [35].

3.3 Angiogenesis

The ability to induce angiogenesis is essential for most solid tumors to enable them to grow beyond 2–3 mm in diameter. Angiogenesis may also provide an important path for metastasis. Tumor cells ensure their own growth by secreting vascular growth factors such as vascular endothelial growth factor (VEGF), basic fibroblast growth factor (bFGF), and platelet‐derived growth factor (PDGF) that stimulate angiogenesis. Among the growth factors identified so far, VEGF seems to be the most prominent and the most important factor for tumor angiogenesis [36]. Hurwitz et al. have shown that bevacizumab, an antibody against VEGF, extended progression‐free survival in patients with metastatic CRC by 10 months [37]. Thus, VEGF appears to be a useful target for the treatment of CRC.

Cancer cells as well as other cells attracted to sites of inflammation produce proangiogenic factors, leading to endothelial cell recruitment, proliferation, and assembly. Genetic studies demonstrate that colorectal tumor growth and vascular density are significantly attenuated in COX‐2−/− mice, supporting the concept that COX‐2 plays a crucial role in tumor‐associated angiogenesis [38, 39].

COX‐2 may modulate angiogenesis in several ways, since both nonselective and selective NSAIDs block the production of angiogenic factors and inhibit the proliferation, and migration of vascular endothelial cells as well as subsequent tube formation [40–43]. Inhibition of COX‐2 by both nonselective and selective NSAIDs decreases endothelial tube formation, the inhibitory effect being mediated either directly or indirectly via a MAP kinase‐dependent pathway [40]. These results further confirm that COX‐2 plays a crucial role in tumor‐associated angiogenesis and suggest that NSAIDs may have potential for use as antiangiogenic agents.

Overexpression of COX‐2 in CRC cells can stimulate the production of angiogenic factors such as VEGF and bFGF [40]. Increased production of PGE2 is thought to mediate the major role of COX‐2...