The Equilibrative Nucleoside Transporters (ENTs) are a Na+-independent class of nucleoside transporters (SLC29) responsible for the uptake of a large number of nucleosides and nucleoside analogs [1
]. Among the four members identified (ENT1-4) [1
], ENT1 and ENT2 are the best-characterized.
ENT1 is the most abundant and widely distributed plasma membrane nucleoside transporter in human cells that mediates cellular entry of nucleoside-derived antineoplastic drugs such as gemcitabine, cytarabine, and fludarabine [2
]. Deficiency in ENT1 confers development of drug resistance in many in vitro models of hematological [5
] and solid tumors [6
]. On the other hand, high levels of hENT1 represent a positive predictive factor of response to gemcitabine in patients with advanced pancreatic cancer [9
], resected disease [10
] and metastatic lung disease [7
]. These observations make ENTs an important target for study to improve anticancer therapy and drug effectiveness, however little is known about the regulation mechanism of ENT1 expression or activity.
It was previously shown that hypoxia-activated transcription factor, HIF1α can negatively regulate hENT1 expression [11
]. In fact, heterogeneously distributed hypoxic areas are a characteristic property of locally advanced breast cancer and may become resistant to some forms of chemotherapy [12
]. On the other hand, it has been reported that inflammatory cytokines can downregulate several transporters (including ENT1) as a result of expression changes [13
], however the mechanism is not yet understood.
Importantly, the MAP kinases JNKs and p38 are preferentially activated by a variety of cell stresses including proinflammatory cytokines [15
] and chemotherapeutic drugs such as gemcitabine, taxol, cisplatin or transplatin [16
]. This suggests that JNK activation may be a common response to chemotherapy. Earlier gene array analysis experiments demonstrated a reduction in ENT1 expression in 3T3 cells expressing MKK7 [18
], a kinase involved in JNK activation [19
]. Collectively these observations strongly support the idea that stress-activated signaling pathways are involved in the regulation of nucleoside transport and may contribute to the development of drug resistance.
Using a murine model of chronic myelogenous leukemia we show that JNK-cJun signaling pathway activated by chemical stress negatively regulates mENT1 expression and activity. We also identified a novel and functional c-Jun binding site in the mouse promoter. Finally, our results suggest that the activation of JNK in response to chemotherapeutic nucleoside analogs could contribute to the development of resistance to these or other nucleoside-derived drugs.