Functional Comparison of Human and Murine Equilibrative Nucleobase Transporter 1
Authors: Kim Chan S. , Ruel Nicholas M. , Hammond James R. 
Published: May 2022

BackgroundThiopurines are a key component for the immunosuppressive therapy of various leukemias, inflammatory bowels disease, and other autoimmune disorders. Thiopurine therapy discontinuation/interruption is associated with a higher risk of relapse, but is common due to the prevalence of severe and potentially fatal adverse events, such as hepatotoxicity.Drug transporter expression is a known factor for patient variability in drug response and toxicity. We have recently established that the SLC43A3‐encoded transporter, equilibrative nucleobase transporter 1 (ENBT1), is the primary mechanism by which the thiopurine, 6‐mercaptopurine (6‐MP) enters cells. ENBT1 is known to be highly expressed in human hepatocytes, however, the relationship between ENBT1 and thiopurine‐induced hepatotoxicity has not been explored in the literature.To investigate this paradigm, our lab has proposed developing a novel SLC43A3 knockout mouse model. However, the functional differences between human and murine ENBT1, in mediating 6‐MP transport, has equally not been explored. Evidence that suggests ENBT1 is functionally similar between species, would be an essential foundation to assist bridging the gap between our proposed animal model and clinical studies within the literature.HypothesisWe hypothesize that hSLC43A3‐encoded hENBT1 and mslc43a3‐encoded mENBT1 are functionally similar and will have non‐significantly different 6‐MP transport kinetics and resulting cytotoxicity.MethodsThe ENBT1‐deficient human embryonic kidney 293 (HEK293) cell line was stably transfected with either hSLC43A3 or mslc43a3. [3H]Adenine and [3H]6‐MP were used in an oil‐stop centrifugation assay to assess ENBT1‐mediated transport activity in wildtype and transfected HEK293 cell lines. To determine cytotoxicity, wildtype and transfected HEK293 cell lines were incubated for 48 hours with a range of 6‐MP concentrations (78 nM – 1.28 mM) before being assessed via MTT cell viability assay.ResultsOil stop centrifugation assay of ENBT1‐mediated [3H]adenine and [3H]6‐MP transport revealed that hENBT1 and mENBT1 have similar transport kinetics (Km and Vmax), where resulting Michaelis‐Menten curves were not significantly different (unpaired T‐test: Adenine ‐ t16=0.32, p=0.76, n=5 & 6‐MP ‐ t16=0.64, p=0.53, n=4). Adenine inhibition of ENBT1‐mediated [3H]6‐MP transport and 6‐MP inhibition of ENBT1‐mediated [3H]adenine transport also showed that hENBT1 and mENBT1 have concurring inhibition kinetics (Ki), inhibition curves were not significantly different (unpaired T‐test: Adenine: t17=0.29, p=0.77, n=5 & 6‐MP: t14=0.18, p=0.86, n=4). Subsequent MTT cell viability assay showed that wildtype HEK293 are relatively 6‐MP resistant, while the transfected‐HEK293s are 6‐MP sensitive and have non‐significantly different cell viability curves (unpaired T‐test: t28=0.13, p=0.89, n=5).ConclusionOur results show that hENBT1 and mENBT1 are functionally similar in regards to ENBT1‐mediated adenine and 6‐MP transport in a stably transfected cell line. Additional work is necessary to investigate whether this is the case in human and murine hepatic cell lines/primary hepatocytes that have endogenous SLC43A3/ENBT1 expression and a larger range of xenobiotic metabolizing enzymes that may alter the equilibrium of 6‐MP within cells.