Molecular Characterization of Equilibrative Nucleoside Transporters in the Rat Carotid Body and Their Regulation by Chronic Hypoxia

G-Protein-Coupled Receptor (GPCR) Signaling in the Carotid Body: Roles in Hypoxia and Cardiovascular and Respiratory Disease

The carotid body (CB) is an important organ located at the carotid bifurcation that constantly monitors the blood supplying the brain. During hypoxia, the CB immediately triggers an alarm in the form of nerve impulses sent to the brain. This activates protective reflexes including hyperventilation, tachycardia and vasoconstriction, to ensure blood and oxygen delivery to the brain and vital organs. However, in certain conditions, including obstructive sleep apnea, heart failure and essential/spontaneous hypertension, the CB becomes hyperactive, promoting neurogenic hypertension and arrhythmia. G-protein-coupled receptors (GPCRs) are very highly expressed in the CB and have key roles in mediating baseline CB activity and hypoxic sensitivity. Here, we provide a brief overview of the numerous GPCRs that are expressed in the CB, their mechanism of action and downstream effects. Furthermore, we will address how these GPCRs and signaling pathways may contribute to CB hyperactivity and cardiovascular and respiratory disease. GPCRs are a major target for drug discovery development. This information highlights specific GPCRs that could be targeted by novel or existing drugs to enable more personalized treatment of CB-mediated cardiovascular and respiratory disease.

Effect of hypoxia on human equilibrative nucleoside transporters hENT1 and hENT2 in breast cancer

Elevated proliferation rates in cancer can be visualized with positron emission tomography (PET) using 3'-deoxy-3'-l-[¹⁸F]fluorothymidine ([¹⁸F]FLT). This study investigates whether [¹⁸F]FLT transport proteins are regulated through hypoxia. Expression and function of human equilibrative nucleoside transporter (hENT)-1, hENT2, and thymidine kinase 1 (TK1) were studied under normoxic and hypoxic conditions, and assessed with [¹⁸F]FLT-PET in estrogen receptor positive (ER⁺)-MCF7, triple-negative MDA-MB231 breast cancer (BC) cells, and MCF10A cells (human mammary epithelial cells). Functional involvement of hENT2 [¹⁸F]FLT transport was demonstrated in all cell lines. In vitro [¹⁸F]FLT uptake was higher in MDA-MB231 than in MCF7: 242 ± 9 vs. 147 ± 18% radioactivity/mg protein after 60 min under normoxia. Hypoxia showed no significant change in radiotracer uptake. Protein analysis revealed increased hENT1 (P < 0.0963) in MDA-MB231. Hypoxia did not change expression of either hENT1, hENT2, or TK1. In vitro inhibition experiments suggested involvement of hENT1, hENT2, and human concentrative nucleoside transporters during [¹⁸F]FLT uptake into all cell lines. In vivo PET imaging revealed comparable tumor uptake in MCF7 and MDA-MB231 tumors over 60 min, reaching standardized uptake values of 0.96 ± 0.05 vs. 0.89 ± 0.08 (n = 3). Higher hENT1 expression in MDA-MB231 seems to drive nucleoside transport, whereas TK1 expression in MCF7 seems responsible for comparable [¹⁸F]FLT retention in ER⁺ tumors. Our study demonstrates that hypoxia does not significantly affect nucleoside transport as tested with [¹⁸F]FLT in BC.-Krys, D., Hamann, I., Wuest, M., Wuest, F. Effect of hypoxia on human equilibrative nucleoside transporters hENT1 and hENT2 in breast cancer.

Bidirectional transport of 2-chloroadenosine by equilibrative nucleoside transporter 4 (hENT4): Evidence for allosteric kinetics at acidic pH

Adenosine has been reported to be transported by equilibrative nucleoside transporter 4 (ENT4), encoded by the SLC29A4 gene, in an acidic pH-dependent manner. This makes hENT4 of interest as a therapeutic target in acidic pathologies where adenosine is protective (e.g. vascular ischaemia). We examined the pH-sensitivity of nucleoside influx and efflux by hENT4 using a recombinant transfection model that lacks the confounding influences of other nucleoside transporters (PK15-NTD). We established that [³H]2-chloroadenosine, which is resistant to metabolism by adenosine deaminase, is a substrate for hENT4. Transport of [³H]2-chloroadenosine at a pH of 6.0 in PK15-NTD cells stably transfected with SLC29A4 was biphasic, with a low capacity (V_max ~ 30 pmol/mg/min) high-affinity component (K_m ~ 50 µM) apparent at low substrate concentrations, which shifted to a high capacity (V_max ~ 500 pmol/mg/min) low affinity system (K_m > 600 µM) displaying positive cooperativity at concentrations above 200 µM. Only the low affinity component was observed at a neutral pH of 7.5 (K_m ~ 2 mM). Efflux of [³H]2-chloroadenosine from these cells was also enhanced by more than 4-fold at an acidic pH. Enhanced influx and efflux of nucleosides by hENT4 under acidic conditions supports its potential as a therapeutic target in pathologies such as ischaemia-reperfusion injury.