Transient inhibition of protein synthesis induces the immediate early gene VL30: Alternative mechanism for thapsigargin-induced gene expression

Induction of gene expression in response to calcium ionophores or thapsigargin, which inhibits the calcium-ATPase responsible for sequestering intracellular calcium, has frequently been attributed to direct stimulatory events subsequent to the elevation of intracellular free calcium. VL30 is a murine gene that is transcriptionally induced in response to a large array of mitogenic and transforming stimuli. We have shown previously that an enhancer element within the VL30 promoter region is dependent upon cotreatment with thapsigargin or calcium ionophore for a full-scale induction of gene expression. In this report, we demonstrate that both thapsigargin and calcium ionophores induce a transient inhibition of protein synthesis in Rat-1 cells transfected with a VL30 enhancer-driven reporter construct. Recovery of protein synthesis is facilitated by cotreatment with epidermal growth factor or phorbol esters. Furthermore, treatment with cycloheximide or DTT, which inhibit protein synthesis without altering intracellular calcium levels, can substitute for thapsigargin or ionophores in stimulating VL30 gene expression. These results suggest that the stimulatory effects of thapsigargin and calcium ionophores on VL30 expression may be mediated, at least in part, by the ability of these agents to initiate stress responses associated with the inhibition of protein synthesis.