Inhibition of mammalian RNA synthesis by the cytoplasmic Ca²⁺ buffer BAPTA. Analyses of [³H]uridine incorporation and stress-dependent transcription

To determine whether oscillations of cytoplasmic [Ca²⁺] might be involved in transcription regulated by the unfolded protein response (UPR), dermal fibroblasts were loaded with the widely used Ca²⁺ buffer BAPTA, which is expected to dampen cytoplasmic [Ca²⁺] changes without affecting resting [Ca²⁺]. BAPTA inhibited UPR-dependent transcription of the GRP78/BiP and EDEM genes. However, BAPTA also blocked cytoplasmic stress-dependent (UPR-independent) transcription of the HSP70 gene. These results led to the unexpected demonstration that BAPTA was a general inhibitor of cellular RNA synthesis in dermal fibroblasts. BAPTA is delivered to the cytoplasm as the acetoxymethyl (AM) ester BAPTA/AM, but released AM groups, as well as formaldehyde generated from AM breakdown, were ruled out as causes of RNA synthesis inhibition. BAPTA inhibited RNA synthesis in all mammalian cell types tested except CHO-K1. GRP78/BiP RNA induction in CHO-K1 cells was not blocked by BAPTA. Thus, there does not appear to be a critical requirement for cytoplasmic [Ca²⁺] changes in CHO-K1 UPR-dependent transcription. However, general inhibition of RNA synthesis by the [Ca²⁺] buffer BAPTA was unanticipated. This might possibly reflect a fortuitous interaction of BAPTA with the RNA synthesis machinery or a requirement for [Ca²⁺] changes.