Intracellular BAPTA directly inhibits PFKFB3, thereby impeding mTORC1-driven Mcl-1 translation and killing Mcl-1-addicted cancer cells
Intracellular Ca2+ signals control several physiological and pathophysiological processes. The main tool to chelate intracellular Ca2+ is intracellular BAPTA (BAPTAi), usually introduced into cells as a membrane-permeant acetoxymethyl ester (BAPTA-AM). We previously demonstrated that BAPTAi enhanced apoptosis induced by venetoclax, a Bcl-2 antagonist, in diffuse large B-cell lymphoma (DLBCL). These findings implied a novel interplay between intracellular Ca2+ signaling and anti-apoptotic Bcl-2 function. Hence, we set out to identify the underlying mechanisms by which BAPTAi enhances cell death in B-cell cancers. In this study, we observed that BAPTAi alone induced apoptosis in lymphoma cell models that were highly sensitive to S63845, an Mcl-1 antagonist. BAPTAi provoked a rapid decline in Mcl-1 protein levels by inhibiting mTORC1-driven MCL-1 translation. Overexpression of nondegradable Mcl-1 rescued BAPTAi-induced cell death. We further examined how BAPTAi diminished mTORC1 activity and found that BAPTAi impaired glycolysis by directly inhibiting 6-phosphofructo-2-kinase/fructose-2,6-bisphosphatase 3 (PFKFB3) activity, an up to now unkown effect of BAPTAi. All aforementioned effects of BAPTAi were also elicited by a BAPTAi analog with low affinity for Ca2+. Thus, our work reveals PFKFB3 inhibition as an unappreciated Ca2+-independent mechanism by which BAPTAi impairs cellular metabolism and ultimately the survival of Mcl-1-dependent cancer cells. Our work has two important implications. First, direct inhibition of PFKFB3 emerged as a key regulator of mTORC1 activity and a promising target in the treatment of Mcl-1-dependent cancers. Second, cellular effects caused by BAPTAi are not necessarily related to Ca2+ signaling. Our data support the need for a reassessment of the role of Ca2+ in cellular processes when findings were based on the use of BAPTAi.