Accumulation of unfolded proteins in the endoplasmic reticulum (ER) activates the unfolded protein response (UPR). In mammalian cells, UPR signals generated by several ER membrane resident proteins, including the bifunctional protein kinase endoribonuclease IRE1α, control cell survival and the decision to execute apoptosis. Processing of XBP1 mRNA by the RNase domain of IRE1α promotes survival of ER stress, while activation of the mitogen-activated protein kinase JNK by IRE1α late in the ER stress response promotes apoptosis. Here we show that activation of JNK in the ER stress response precedes activation of XBP1. This activation of JNK is dependent on IRE1α and TRAF2 and coincides with JNK-dependent induction of expression of several antiapoptotic genes, including cIAP1, cIAP2, XIAP, and BIRC6. ER-stressed jnk1-/- jnk2-/- mouse embryonic fibroblasts (MEFs) display more pronounced mitochondrial permeability transition and increased caspase 3/7 activity compared to wild type MEFs. Caspase 3/7 activity is also elevated in ER-stressed ciap1-/- ciap2-/-, and xiap-/- MEFs. These observations suggest that JNK-dependent transcriptional induction of several inhibitors of apoptosis contributes to inhibiting apoptosis early in the ER stress response.

Pseudohyphal growth and meiosis are two differentiation responses to nitrogen starvation of diploid Saccharomyces cerevisiae. Nitrogen starvation in the presence of fermentable carbon sources is thought to induce pseudohyphal growth, whereas nitrogen and sugar starvation induces meiosis. In contrast to the genetic background routinely used to study pseudohyphal growth (⌺1278b), nonfermentable carbon sources stimulate pseudohyphal growth in the efficiently sporulating strain SK1. Pseudohyphal SK1 cells can exit pseudohyphal growth to complete meiosis. Two stimulators of meiosis, Ime1 and Ime2, are required for pseudohyphal growth of SK1 cells in the presence of nonfermentable carbon sources. Epistasis analysis suggests that Ime1 and Ime2 act in the same order in pseudohyphal growth as in meiosis. The different behaviors of strains SK1 and ⌺1278b are in part attributable to differences in cyclic AMP (cAMP) signaling. In contrast to ⌺1278b cells, hyperactivation of cAMP signaling using constitutively active Ras2 G19V inhibited pseudohyphal growth in SK1 cells. Our data identify the SK1 genetic background as an alternative genetic background for the study of pseudohyphal growth and suggest an overlap between signaling pathways controlling pseudohyphal growth and meiosis. Based on these findings, we propose to include exit from pseudohyphal growth and entry into meiosis in the life cycle of S. cerevisiae.

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) causes ER stress and activates a signalling network known as the unfolded protein response (UPR). Here we characterise how ER stress and the UPR inhibit insulin signalling. We find that ER stress inhibits insulin signalling by depleting the cell surface population of the insulin receptor. ER stress inhibits proteolytic maturation of insulin proreceptors by interfering with transport of newly synthesised insulin proreceptors from the ER to the plasma membrane. Activation of AKT, a major target of the insulin signalling pathway, by a cytosolic, membrane-bound chimera between the AP20187-inducible FV2E dimerisation domain and the cytosolic protein tyrosine kinase domain of the insulin receptor was not affected by ER stress. Hence, signalling events in the UPR, such as activation of the JNK MAP kinases or the pseudokinase TRB3 by the ER stress sensors IRE1α and PERK, do not contribute to inhibition of signal transduction in the insulin signalling pathway. Indeed, pharmacologic inhibition and genetic ablation of JNKs, as well as silencing of expression of TRB3, did not restore insulin sensitivity or rescue processing of newly synthesised insulin receptors in ER-stressed cells.

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