Txr1: a transcriptional regulator of thrombospondin-1 that modulates cellular sensitivity to taxanes

Abstract

Using transcripts initiated at a chromosomally integrated retrovirus-based promoter to perturb gene expression randomly in human prostate cancer cells, we isolated cell clones resistant to taxane lethality and discovered the role of a previously uncharacterized gene, txr1, in this phenotype. We show that txr1 impedes taxane-induced apoptosis in tumor cells by transcriptionally down-regulating the production of thrombospondin-1 (TSP-1)-known earlier for both its anti-angiogenic and proapoptotic actions. Decrease of Txr1 or treatment with TSP-1 or TSP-1 mimetic peptide sensitized cells to taxane cytotoxicity by activating signaling through the CD47 receptor (also known as the integrin-associated protein), whereas interference with CD47 function reduced taxane-induced cell death. Cellular abundance of Txr1 and TSP-1 varied inversely, and alteration of the level of both proteins correlated highly with taxol resistance in 13 of 19 NCI-60 cancer cell lines. Our results reveal a hitherto unsuspected mechanism of taxane resistance, elucidate the role of txr1 in this resistance, and identify txr1 as a regulator of TSP-1 production and an agent for its chemotherapeutic modulation.[Keywords: Thrombospondin-1; CD47; taxanes; microarray; angiogenesis; Txr1] Supplemental material is available at http://www.genesdev.org. Taxanes are a family of major chemotherapeutic agents that have anti-neoplastic effects against a wide spectrum of human cancers (Rowinsky 1997). The anti-proliferative actions of the two clinically employed taxanes, paclitaxel (taxol) and docetaxel, stem from their ability to interact with ␤-tubulin and promote tubulin polymerization and stability; this event blocks cell cycling at the G 2 -M transition and results in eventual cell death by apoptosis (Horwitz 1992). Despite a dramatic response of susceptible tumors to initial treatment with taxanes, the subsequent development of resistance has proved to be a major limitation to long-term use of these agents as anticancer drugs (Horwitz et al. 1993). A widely studied mechanism associated with such resistance is the exclusion of taxanes and other drugs from cells by up-regulation of ATP-dependent cell membrane P-glycoproteins (i.e., transporter-related multidrug resistance [MDR]) (Bradley and Ling 1994). Genetic mutations that interfere with the binding of taxanes to microtubules have also been extensively investigated as a cause of taxol resistance (Giannakakou et al. 1997;Kavallaris et al. 1997;Ranganathan et al. 1998;Monzo et al. 1999).In recent years, several approaches that use functionbased screens for the discovery of cellular genes whose inactivation or overexpression yields phenotypic properties of interest have been developed (Li and Cohen 1996;Kimchi 2003;Primiano et al. 2003;Berns et al. 2004;Lu et al. 2004). One of these (Li and Cohen 1996) uses an experimentally regulated chromosomally integrated virus-based promoter to modulate the expression of genes at the site of viral insertion. Using this approach, we identified a previously unknown gene, tx...

Tumoral Expression of TXR1 and TSP1 Predicts Overall Survival of Patients with Lung Adenocarcinoma Treated with First-line Docetaxel-Gemcitabine Regimen

“…A recently reported in vitro study proposed another mechanism of resistance to taxanes (14). Using function-based screens, the investigators identified a previously unknown gene, TXR1 (15), whose protein overexpression conferred resistance to paclitaxel.…”

Sensitization of breast cancer cells to taxol by inhibition of taxol resistance gene 1

“…Inactivations of TXR1 by CD47, or activation of TSP1, were also shown to enhance taxolinduced cytotoxicity in tumor cells. In combination, results by these authors suggested that TXR1, as a thrombin-mediated regulator, was a potential therapeutic target (12). However, the correlation between TXR1 and drug resistance in breast cancer remained to be investigated.…”

“…Cohen et al (12) identified the taxol resistance gene 1 (TXR1) and showed that it reduced the secretion of the apoptosis-inducing protein, thrombospondin-1 (TSP1). TSP1 has been shown to induce apoptosis and inhibit angiogenesis (13), roles that are consistent with a taxol-resistant phenotype.…”

CD47, a multi-facetted target for cancer immunotherapy

“…TSP-1/CD47 interaction can also differentially affect cancer cell sensitivity toward chemotherapeutics. For instance, TSP-1 sensitized taxol-resistant prostate cancer cells to taxol treatment (Lih et al, 2006), whereas a blocking anti-CD47 antibody decreases cytotoxicity of taxol treatment. Of note, taxol resistance is partly regulated by the txr1 gene, a gene known to downregulate TSP-1 expression.…”

“…Of note, taxol resistance is partly regulated by the txr1 gene, a gene known to downregulate TSP-1 expression. Correspondingly, treatment with a TSP-1 mimetic peptide sensitized cells to taxol by activating CD47 signaling (Lih et al, 2006). In contrast, treatment of thyroid carcinoma cells with a CD47-binding peptide reduced doxorubicin and camtothecin cytotoxicity (Rath et al, 2006).…”