Activation of protein kinase C decreases equilibrative nucleobase transporter 1-mediated substrate uptake via phosphorylation of threonine 231

Protein kinase C (PKC) signalling has been shown to be dysregulated in various cancers including acute lymphoblastic leukemia (ALL). We have previously determined that changes in the expression levels of SLC43A3-encoded equilibrative nucleobase transporter 1 (ENBT1) can significantly alter 6-mercaptopurine (6-MP) toxicity in ALL cells. 6-MP is a common drug used in ALL chemotherapy. Furthermore, it has been reported that activation of PKC by phorbol 12-myristate 13-acetate (PMA) impacts nucleobase uptake via an ENBT1-like transporter in Lilly Laboratories Culture-Porcine Kidney 1 (LLC-PK1) cells. We hypothesized that activation of PKC would also alter ENBT1-mediated uptake of nucleobases in leukemia cell models. Using MOLT-4, SUP-B15, and K562 cells, we incubated the cells with PMA or its inactive isoform 4α-PMA for 30 min and determined changes to ENBT1-mediated substrate uptake. All of the cell lines tested showed decreased ENBT1-mediated substrate uptake when exposed PMA, relative to that observed using 4α-PMA. Pre-incubation with the broad-spectrum PKC inhibitor, Gö6983, reversed the decrease caused by PMA. Finally, to determine the residue responsible for this PKC-mediated effect, we transiently transfected HEK293 cells (which do not express endogenous ENBT1) with wild-type SLC43A3 transcript or constructs mutated to modify the predicted PKC sites in ENBT1. We found that the mutation of threonine 231 to alanine prevents the decrease in ENBT1-mediated uptake following incubation with PMA, suggesting its involvement. This study shows that activation of PKC decreases ENBT1-mediated uptake, suggesting that aberrant activation of PKC in ALL could decrease ENBT1-mediated 6-MP uptake potentially leading to decreased therapeutic efficacy.