Inflammation and breast cancer. Cyclooxygenase/prostaglandin signaling and breast cancer

Abstract

Many human cancers exhibit elevated prostaglandin (PG) levels due to upregulation of cyclooxygenase-2 (COX-2), a key enzyme in eicosanoid biosynthesis. COX-2 over-expression has been observed in about 40% of cases of invasive breast carcinoma and at a higher frequency in preinvasive ductal carcinoma in situ tumors, Extensive pharmacologic and genetic evidence implicates COX enzymes in neoplasia. Epidemiologic analyses demonstrate a protective effect of COX-inhibiting nonsteroidal anti-inflammatory drugs with respect to human cancer. Complementary experimental studies have established that both conventional nonsteroidal anti-inflammatory drugs and selective COX-2 inhibitors suppress mammary tumor formation in rodent breast cancer models. Furthermore, knocking out Cox-2 reduces mammary tumorigenesis and angiogenesis, and, conversely, transgenic COX-2 over-expression induces tumor formation. The utility of COX/PG signaling as a target for chemoprevention has been established by randomized controlled clinical trials. However, these studies also identified increased cardiovascular risk associated with use of selective COX-2 inhibitors. Thus, current efforts are directed toward identifying safer approaches to antagonizing COX/PG signaling for cancer prevention and treatment, with a particular focus on PGE2 regulation and signaling, because PGE2 is a key pro-tumorigenic prostanoid.

Figure 1

Eicosanoid metabolism and signaling. Cyclooxygenase (COX) enzymes convert arachidonic acid to the intermediate prostaglandin PGG₂, and thence to PGH₂. Subsequent enzymatic steps, catalyzed by specific isomerases, generate a variety of eicosanoid products. Thromboxane (TX) A₂ and prostacyclin (PGI₂), products of platelet COX-1 and endothelial COX-2, respectively, are thought to play opposing roles in cardiovascular biology. Most important in the context of epithelial tumorigenesis, PGE₂ is generated from PGH₂ through the action of PGE synthases. Signaling downstream of PGE₂ is initiated via interaction of PGE₂ with cognate PGE₂ receptors EP₁ to EP₄. PGE₂ signaling can be terminated via catabolism mediated by 15-hydroxyprostaglandin dehydrogenase (15-PGDH). Elevated PGE₂ levels in neoplastic tissues can thus be a consequence of COX-2 over-expression, PGE synthase modulation, and/or loss of 15-PGDH expression.

Figure 2

COX-2 expression in human breast tumors. Cyclooxygenase (COX)-2 protein has been detected in human breast biopsies in both (a) ductal carcinoma in situ and (b) infiltrating mammary carcinoma using immunohistochemistry on formalin-fixed tissue sections. Representative data are reproduced from [21] by permission of Wiley-Liss Inc., a subsidiary of John Wiley and Sons Inc. (Copyright (2000) American Cancer Society.)

Figure 3

COX-2 expression in human breast cancer correlates with decreased disease-free survival. Distant disease-free survival of breast cancer patients was plotted as a function of cyclooxygenase (COX)-2 expression: score 0 = no COX-2 expression (n = 133); score 1 = weak COX-2 expression (n = 854); score 2 = moderate COX-2 expression (n = 511); and score 3 = strong COX-2 expression (n = 78). Elevated expression of COX-2 protein correlated with reduced survival (P < 0.0001; log rank test). Reproduced from [26] with permission from the American Association of Cancer Research.

Figure 4

Knocking out Cox-2 reduces mammary tumorigenesis. Mouse mammary tumor virus (MMTV)/neu deletion mutant (NDL) mice, which express a mammary-targeted HER2/neu transgene, were crossed with Cox-2-deficient mice, and mammary tumor formation was evaluated in age-matched virgin MMTV/NDL females that were Cox-2 wild type (WT; n = 72), heterozygous (HET; n = 42), and null (NULL; n = 18). (a) Tumor multiplicity was significantly reduced in Cox-2 deficient mice (data shown are mean ± SEM. *P < 0.001, by likelihood ratio test. (b) The percentage of tumors in each of the indicated size categories was calculated for each genotype. The proportion of large tumors was significantly reduced in Cox-2 deficient MMTV/NDL animals relative to Cox-2 wild-type controls (P = 0.02). Reproduced with permission from [40].

Figure 5

Mammary gland vascularization is reduced in Cox-2 knockout mice. (a) Mammary gland tissue sections from age-matched virgin mouse mammary tumor virus (MMTV)/neu deletion mutant (NDL) females that were Cox-2 wild type (subpanels a to f) and Cox-2 null (subpanels g to l) were subjected to anti-CD31 immunohistochemistry, and counterstained with methyl green. Both the number and size of blood vessels were strikingly reduced in Cox-2 null samples. (b) Expression levels of angiogenesis-related genes were compared by quantitative reverse transcription polymerase chain reaction in MMTV/NDL mammary glands from Cox-2 wild-type (blue columns) and Cox-2 null females (yellow columns). The height of the columns indicate means normalized to the mean expression level of that gene in MMTV/NDL, Cox-2 wild-type samples; the bars indicate the standard error. Expression of VEGF, Ang1, and Flt1 was significantly reduced (P = 0.016, 0.049 and 0.010, respectively). The average of log values across all six genes for each mouse, representing a global effect, was significantly higher in wild-type tissues than in null tissues at P = 0.025. Reproduced with permission from [40].