Divergent roles of CXCR3 isoforms in promoting cancer stem-like cell survival and metastasis.

There is growing evidence that several chemokine receptors including CXCR3 contribute to metastasis of breast and other cancers, however, in order to target CXCR3 effectively, it is critical to understand the relative contribution of each CXCR3 isoform. Furthermore, the possible contribution of either major CXCR3 isoform (CXCR3-A, CXCR3-B) to cancer stem cell behavior has not been reported. We employed primary invasive ductal carcinomas, a panel of breast cell lines, and a xenograft model of metastatic breast cancer to examine the role of CXCR3 isoforms in the behavior of breast cancer stem-like cells and the contribution of each isoform to metastasis. In primary human breast cancer specimens as well as established breast cancer cell lines, CXCR3-A is more highly expressed than CXCR3-B. Conversely, immortalized normal MCF10A cells express more CXCR3-B relative to CXCR3-A. Overexpression of CXCR3-B in MDA-MB-231 basal-like cells inhibits CXCR3 ligand-stimulated proliferation, which is accompanied by reduced ligand-mediated activation of ERK1/2 and p38 kinases. Likewise, metastatic capacity is reduced in vivo by higher levels of CXCR3-B, and migratory and invasive properties are inhibited in vitro; conversely, silencing of CXCR3-B enhances lung colonization. In contrast to the anti-metastatic and anti-proliferative roles of CXCR3-B in the non-stem cell population, this isoform supports a cancer stem-like cell phenotype. CXCR3-B is markedly elevated in mammosphere-forming parental cells and overexpressing CXCR3-B further enhances mammosphere-forming potential as well as growth in soft agar; stem-like behavior is inhibited in MDA-MB-231shCXCR3-B cells. Targeting of both CXCR3 isoforms may be important to block the stem cell-promoting actions of CXCR3-B, while inhibiting the pro-proliferative and metastasis-promoting functions of CXCR3-A.

Multiple drug resistance-associated protein 4 (MRP4), prostaglandin transporter (PGT), and 15-hydroxyprostaglandin dehydrogenase (15-PGDH) as determinants of PGE2 levels in cancer.

The cyclooxygenase-2 (COX-2) enzyme and major lipid product, prostaglandin E2 (PGE2) are elevated in many solid tumors including those of the breast and are associated with a poor prognosis. Targeting this enzyme is somewhat effective in preventing tumor progression, but is associated with cardiotoxic secondary effects when used chronically. PGE2 functions by signaling through four EP receptors (EP1-4), resulting in several different cellular responses, many of which are pro-tumorigenic, and there is growing interest in the therapeutic potential of targeting EP4 and EP2. Other members in this signaling pathway are gaining more attention. PGE2 is transported out of and into cells by two unique transport proteins. Multiple Drug Resistance-Associated Protein 4 (MRP4) and Prostaglandin Transporter (PGT) modulate PGE2 signaling by increasing or decreasing the levels of PGE2 available to cells. 15-hydroxyprostaglandin dehydrogenase (15-PGDH) metabolizes PGE2 and silences the pathway in this manner. The purpose of this review is to summarize the extensive data supporting the importance of the COX-2 pathway in tumor biology with a focus on more recently described pathway members and their role in modulating PGE2 signaling. This review describes evidence supporting roles for MRP4, PGT and 15-PGDH in several tumor types with an emphasis on the roles of these proteins in breast cancer. Defining the importance of these latter pathway members will be key to developing new therapeutic approaches that exploit the tumor-promoting COX-2 pathway.

Prostaglandin E receptor EP4 is a therapeutic target in breast cancer cells with stem-like properties.

The cyclooxygenase pathway is strongly implicated in breast cancer progression but the role of this pathway in the biology of breast cancer stem/progenitor cells has not been defined. Recent attention has focused on targeting the cyclooxygenase 2 (COX-2) pathway downstream of the COX-2 enzyme by blocking the activities of individual prostaglandin E (EP) receptors. Prostaglandin E receptor 4 (EP4) is widely expressed in primary invasive ductal carcinomas of the breast and antagonizing this receptor with small molecule inhibitors or shRNA directed to EP4 inhibits metastatic potential in both syngeneic and xenograft models. Breast cancer stem/progenitor cells are defined as a subpopulation of cells that drive tumor growth, metastasis, treatment resistance, and relapse. Mammosphere-forming breast cancer cells of human (MDA-MB-231, SKBR3) or murine (66.1, 410.4) origin of basal-type, Her-2 phenotype and/or with heightened metastatic capacity upregulate expression of both EP4 and COX-2 and are more tumorigenic compared to the bulk population. In contrast, luminal-type or non-metastatic counterparts (MCF7, 410, 67) do not increase COX-2 and EP4 expression in mammosphere culture. Treatment of mammosphere-forming cells with EP4 inhibitors (RQ-15986, AH23848, Frondoside A) or EP4 gene silencing, but not with a COX inhibitor (Indomethacin) reduces both mammosphere-forming capacity and the expression of phenotypic markers (CD44(hi)/CD24(low), aldehyde dehydrogenase) of breast cancer stem cells. Finally, an orally delivered EP4 antagonist (RQ-08) reduces the tumor-initiating capacity and markedly inhibits both the size of tumors arising from transplantation of mammosphere-forming cells and phenotypic markers of stem cells in vivo. These studies support the continued investigation of EP4 as a potential therapeutic target and provide new insight regarding the role of EP4 in supporting a breast cancer stem cell/tumor-initiating phenotype.

Aberrant expression of a beta-catenin gain-of-function mutant induces hyperplastic transformation in the mouse cornea.

Beta-catenin signaling has been shown to play a fundamental role in embryonic development and tumorigenesis. In this study, we investigated the role of beta-catenin (Ctnnb1) in corneal homeostasis and tumorigenesis. Conditional expression of a murine Ctnnb1 gain-of-function mutation alone caused corneal neoplasia and neovascularization, resembling human ocular surface squamous neoplasia (OSSN). These corneas displayed an upregulation of cell proliferative markers (PCNA and p63), while presenting downregulation of both the Pax-6 transcription factor and the corneal differentiation marker cytokeratin 12. In addition, the expression of limbal-type keratin 15 ectopically extended to cornea, but the pattern of conjunctival keratin 4 and epidermal keratin 10 were unchanged. Moreover, epithelial E-cadherin and laminins decreased concomitantly with elevated levels of MMP-7. We also noticed a dramatic upregulation of pro-angiogenic factors (Vegf-A, Vegfr1) and angiopoietins in these corneas. Interestingly, all human OSSN specimens examined revealed nuclear beta-catenin immunoreactivity. Taken together, these results argue that beta-catenin activation is a crucial step during OSSN pathogenesis. Thus, inhibition of beta-catenin might be beneficial for treating this disease.

The Chemokine Receptor CXCR3 Isoform B Drives Breast Cancer Stem Cells.

We are seeking to identify molecular targets that are relevant to breast cancer cells with stem-like properties. There is growing evidence that cancer stem cells (CSCs) are supported by inflammatory mediators expressed in the tumor microenvironment. The chemokine receptor CXCR3 binds the interferon-γ-inducible, ELR-negative CXC chemokines CXCL9, CXCL10, and CXCL11 and malignant cells have co-opted this receptor to promote tumor cell migration and invasion. There are 2 major isoforms of CXCR3: CXCR3A and CXCR3B. The latter is generated from alternative splicing and results in a protein with a longer N-terminal domain. CXCR3 isoform A is generally considered to play a major role in tumor metastasis. When the entire tumor cell population is examined, CXCR3 isoform B is usually detected at much lower levels than CXCR3A and for this, and other reasons, was not considered to drive tumor progression. We have shown that CXCR3B is significantly upregulated in the subpopulation of breast CSCs in comparison with the bulk tumor cell population in 3 independent breast cancer cell lines (MDA-MB-231, SUM159, and T47D). Modulation of CXCR3B levels by knock in strategies increases CSC populations identified by aldehyde dehydrogenase activity or CD44+CD24- phenotype as well as tumorsphere-forming capacity. The reverse is seen when CXCR3B is gene-silenced. CXCL11 and CXCL10 directly induce CSC. We also report that novel CXCR3 allosteric modulators BD064 and BD103 prevent the induction of CSCs. BD103 inhibited experimental metastasis. This protective effect is associated with the reversal of CXCR3 ligand-mediated activation of STAT3, ERK1/2, CREB, and NOTCH1 pathways. We propose that CXCR3B, expressed on CSC, should be explored further as a novel therapeutic target.

Multiple drug resistance-associated protein (MRP4) exports prostaglandin E2 (PGE2) and contributes to metastasis in basal/triple negative breast cancer.

Cyclooxygenase-2 (COX-2) and its primary enzymatic product, prostaglandin E2 (PGE2), are associated with a poor prognosis in breast cancer. In order to elucidate the factors contributing to intratumoral PGE2 levels, we evaluated the expression of COX-2/PGE2 pathway members MRP4, the prostaglandin transporter PGT, 15-PGDH (PGE2 metabolism), the prostaglandin E receptor EP4, COX-1, and COX-2 in normal, luminal, and basal breast cancer cell lines. The pattern of protein expression varied by cell line reflecting breast cancer heterogeneity. Overall, basal cell lines expressed higher COX-2, higher MRP4, lower PGT, and lower 15-PGDH than luminal cell lines resulting in higher PGE2 in the extracellular environment. Genetic or pharmacologic suppression of MRP4 expression or activity in basal cell lines led to less extracellular PGE2. The key finding is that xenografts derived from a basal breast cancer cell line with stably suppressed MRP4 expression showed a marked decrease in spontaneous metastasis compared to cells with unaltered MRP4 expression. Growth properties of primary tumors were not altered by MRP4 manipulation. In addition to the well-established role of high COX-2 in promoting metastasis, these data identify an additional mechanism to achieve high PGE2 in the tumor microenvironment; high MRP4, low PGT, and low 15-PGDH. MRP4 should be examined further as a potential therapeutic target in basal breast cancer.

Upregulation of Cyclooxygenase-2/Prostaglandin E2 (COX-2/PGE2) Pathway Member Multiple Drug Resistance-Associated Protein 4 (MRP4) and Downregulation of Prostaglandin Transporter (PGT) and 15-Prostaglandin Dehydrogenase (15-PGDH) in Triple-Negative Breast Cancer.

Elevated levels of cyclooxygenase-2 (COX-2) and prostaglandin E2 (PGE2) are indicators of a poor prognosis in breast cancer. Using several independent publicly available breast cancer gene expression databases, we investigated other members of the PGE2 pathway. PGE2 is produced by COX-2 and actively exported by multiple drug resistance-associated protein 4 (MRP4) into the extracellular microenvironment, where PGE2 can bind four cognate EP receptors (EP1-EP4) and initiate diverse biological signaling pathways. Alternatively, PGE2 is imported via the prostaglandin transporter (PGT) and metabolized by 15-prostaglandin dehydrogenase (15-PGDH/HPGD). We made the novel observation that MRP4, PGT, and 15-PGDH are differentially expressed among distinct breast cancer molecular subtypes; this finding was confirmed in independent datasets. In triple-negative breast cancer, the observed gene expression pattern (high COX-2, high MRP4, low PGT, and low 15-PGDH) would favor high levels of tumor-promoting PGE2 in the tumor microenvironment that may contribute to the overall poor prognosis of triple-negative breast cancer.