The Role and Regulation of Thromboxane A2 Signaling in Cancer-Trojan Horses and Misdirection

Abstract

Over the last two decades, there has been an increasing awareness of the role of eicosanoids in the development and progression of several types of cancer, including breast, prostate, lung, and colorectal cancers. Several processes involved in cancer development, such as cell growth, migration, and angiogenesis, are regulated by the arachidonic acid derivative thromboxane A₂ (TXA₂). Higher levels of circulating TXA₂ are observed in patients with multiple cancers, and this is accompanied by overexpression of TXA₂ synthase (TBXAS1, TXA₂S) and/or TXA₂ receptors (TBXA2R, TP). Overexpression of TXA₂S or TP in tumor cells is generally associated with poor prognosis, reduced survival, and metastatic disease. However, the role of TXA₂ signaling in the stroma during oncogenesis has been underappreciated. TXA₂ signaling regulates the tumor microenvironment by modulating angiogenic potential, tumor ECM stiffness, and host immune response. Moreover, the by-products of TXA₂S are highly mutagenic and oncogenic, adding to the overall phenotype where TXA₂ synthesis promotes tumor formation at various levels. The stability of synthetic enzymes and receptors in this pathway in most cancers (with few mutations reported) suggests that TXA₂ signaling is a viable target for adjunct therapy in various tumors to reduce immune evasion, primary tumor growth, and metastasis.

Keywords: cancer; isoforms; stroma; thromboxane A2 receptor; thromboxane A2 synthase.

Figure 1

Schematic for TXA₂ biosynthesis and signaling. Eicosanoid Synthesis (1) begins with the release of arachidonic acid from membrane phospholipids by phospholipase A₂. Arachidonic acid is either metabolized to PGH₂ by the cyclo-oxygenase enzymes or is attacked by oxygen free radicals to produce F-series isoprostanes (2). TXA₂S then metabolizes PGH₂ into either TXA₂ (1) or 12-HHT and MDA (3). MDA forms DNA and Protein adducts to cause cellular damage and cell death (4) while BLT2 activation by 12-HHT enhances tumor formation (5). TP isoforms are derived from alternative splicing with TPα expression driven by promoter 1 and 2 (P1, P2) (6). TXA₂ and F-series isoprostanes activate TP (7) to produce propagate carcinogenesis.

Figure 2

TBXA2R Expression is Increased in The Majority of Tumors. (A) Expression of TBXA2R mRNA in 36 different cancer cell types. Data are mean ± SD showing 95% CI. Numbers above descriptors on the X-axis denote the number of individual cell lines from which the data were derived. (B) TCGA data from 59,132 patients’ samples showing the correlation between TBXA2R mRNA expression and methylation of the promoter (within 1 kb of transcriptional start site (TSS)). Red dotted line denotes linear correlation (R² = 0.442).

Figure 3

Elevated TBXA2R and TBXAS1 Expression Is Associated with Poor Prognosis. Data from the TCGA pan-cancer database for TBXA2R (A–C) and TBXAS1 (D–F) was correlated with overall (A,D), progression free (B,E) and disease free (C,F) survival in patient populations with low (♦) and high (♦, TBXA2R; ♦, TBXAS1) expression. n = number of patients in the cohort.

Figure 4

TBXAS1 Expression is Dysregulated in Multiple Tumors. (A) Expression of TBXAS1 mRNA in 36 different cancer cell types. Data are mean ±SD showing 95% CI. Numbers above descriptors on the X-axis denote the number of individual cell lines from which the data were derived. (B) TCGA data from 59,132 patient samples showing the correlation between TBXAS1 mRNA expression and methylation of the promoter (within 1 kb of transcriptional start site (TSS)). Red dotted line denotes linear correlation (R² = 0.029). (C) Correlation of TBXA2R and TBXAS1 expression for cancers in the TCGA pan cancer dataset.

Figure 5

TBXA2R and TBXAS1 are Genetically Stable Targets for Anti-Cancer Therapy. (A) Mutational analysis of TBXA2R and TBXAS1 mutants from the TCGA pan-cancer dataset. The different types of cancer in the analysis and the average total mutation count (MutCnt) for each patient are shown. Gain (■) and loss (■) of copy number are differentiated from missense (■) and frameshift (truncation, ■) mutations. (B) Schematic of TPα and TXA₂S showing the position of the most frequent mutations and how many times they occurred in (A). The 7 transmembrane regions of TP and active domains of TXA₂S (■) are shown for reference. (C,D) Correlation of change in copy number with mRNA expression for TBXA2R (C) and TBXAS1 (D) indicates whether changes in copy number are required for changes in transcription of both genes in cancer.

Figure 6

TXA₂S is Expressed in Tumor Stroma. Biopsies from colorectal (A) brain (B), lung (C) and prostate (D) cancer were immunostained with antibodies against TXA₂S. Tumor (T), leukocytes (L, ←) and fibroblasts (F, ←) all stain positively. Data are from the human protein atlas.

Figure 7

TXA₂S expression is elevated in common sites of metastasis. TXA₂S protein (blue) and RNA (Orange) expression was graphed against the frequency of metastasis from breast cancer (RNA expression from human protein atlas, metastasis data from [137]).