Accommodation of protein synthesis to chronic deprivation of intracellular sequestered calcium. A putative role for GRP78.

Mobilization of sequestered intracellular Ca2+ with EGTA or Ca2+ ionophores severely depresses rates of translational initiation in various mammalian cell types including C6 glial, GH3 pituitary and P3X63Ag8 myeloma cells. Within 2-3 h of continuous exposure to either chelator or ionophore, cells adapt or accommodate such that their rates of amino acid incorporation are restored to 40-70% of those of untreated controls. In GH3 and P3X63Ag8 cells, treatment with either a phorbol ester or a cAMP-elevating agent was required to obtain maximal degrees of accommodation of translational initiation. Following the development of accommodation, cells restored with optimal Ca2+ exhibited rates of amino acid incorporation identical with those of nontreated controls but remained resistance to inhibition on subsequent challenge with EGTA or ionophore. Development of translational tolerance to agents depleting Ca2+ stores did not involve alterations in cellular capacity or affinity for the cation. Invariably, the development of tolerance was preceded by transcriptionally dependent, preferential synthesis of the reticuloplasmin GRP78/BiP. In Ca2(+)-deprived GH3 cells, the synthesis of GRP78 was promoted by phorbol ester and cAMP with the extent of induction correlating directly with the degree of translational tolerance to ionophore. Cells pretreated with dithiothreitol, an alternate inducer of GRP78, also became tolerant to translational inhibition by Ca2+ ionophore or EGTA. Amino acid incorporation in nonsecreting NS-1-cloned myeloma cells, which constitutively express high levels of GRP78 and its mRNA, resisted inhibition by EGTA, ionophore, and dithiothreitol. Antisense oligodeoxynucleotides directed against GRP78 mRNA reduced amino acid incorporation in tolerant, but not in non-tolerant, preparations. These results predicate the existence of a mechanism whereby mammalian cells are capable of rapidly developing translational cross-tolerance to either depletion of sequestered Ca2+ or a reducing environment. A role for nascent GRP78 is strongly implicated in this accommodation mechanism.