Release of Ca2+ from intracellular organelles by peptide analogues: evidence against involvement of metalloendoproteases in Ca2+ sequestration by the endoplasmic reticulum

Novel mastoparan analogs induce differential secretion from mast cells

Cationic amphiphilic peptides stimulate secretion via a receptor-independent action upon G proteins. We have previously utilized chimeric analogs of mastoparan (MP), including galparan (galanin(1-13)-MP ), as molecular probes of secretion. Here, we further resolve the structure-activity relationship of peptidyl secretagogs, including rationally designed chimeric MP analogs. The secretory efficacies of 10 MP analogs were significantly higher than 45 unrelated basic peptides. Comparative studies identified MP analogs that are differential secretagogs for 5-hydroxytryptamine (5-HT) and beta-hexosaminidase. Peptide-induced activation of phospholipase D (PLD), an enzyme intimately involved in regulated exocytosis [5], correlated with the secretion of beta-hexosaminidase but not 5-HT. Thus, these data indicate that different mechanisms are responsible for the exocytosis of 5-HT and beta-hexosaminidase, respectively. Moreover, mastoparan analogs are novel tools for probing the molecular details of exocytosis and other biological phenomena.

A dipeptide metalloendoprotease substrate completely blocks the response of cells in culture to cholera toxin

Prior exposure (15 min at 37 degrees C) of several cell types (Vero, SH-SY5Y neuroblastoma, human intestinal epithelial T84) to 3 mm N-benzoyloxycarbonyl-Gly-Phe-amide (Cbz-Gly-Phe-NH(2)), a competitive substrate for metalloendoproteases, completely suppressed cholera toxin (CT)-induced intracellular cAMP accumulation. The specificity of the inhibitory effect was demonstrated by the complete lack of effect of the dipeptide Cbz-Gly-Gly-NH(2), an inactive analogue of Cbz-Gly-Phe-NH(2). The effect was reversible and dose- (IC(50) as low as 0.2 mm depending on the cell type) and time-dependent. Adding Cbz-Gly-Phe-NH(2) during the lag phase caused a diminution of its inhibitory effect similar to that observed with brefeldin A (BFA). Whereas the dipeptide completely suppressed the CT-induced adenylate cyclase (AC) activity, a direct effect on AC is unlikely since the elevation of intracellular cAMP by forskolin was only slightly reduced. The A(1) peptide of CT and NAD(+) activated the AC to the same extent in membranes from control and Cbz-Gly-Phe-NH(2)-treated cells or when Cbz-Gly-Phe-NH(2) was added directly to the assay. The inhibitory effects of suboptimal amounts of Cbz-Gly-Phe-NH(2) and BFA were not additive pointing to a similar mode of action of the two substances. However, Madin-Darby canine kidney cells of which the Golgi structure is BFA-resistant were not resistant to the inhibitory action of Cbz-Gly-Phe-NH(2) on CT cytotoxicity. Several lines of evidence indicate that a perturbation of intracellular Ca(2+) homeostasis by Cbz-Gly-Phe-NH(2) is not responsible for the inhibitory effect of the dipeptide. The dipeptide had also no effect on the binding of (125)I-CT to cells and even increased its intracellular internalization. In contrast with BFA, Cbz-Gly-Phe-NH(2) did not completely suppress the formation of the catalytically active A(1) fragment from bound CT. The data are compatible with a role of metalloendoprotease activity in the intracellular trafficking and processing of CT, although other mechanisms of action of Cbz-Gly-Phe-NH(2) cannot be excluded.

Regulation of translational initiation during cellular responses to stress

Chemicals and conditions that damage proteins, promote protein misfolding, or inhibit protein processing trigger the onset of protective homeostatic mechanisms resulting in "stress responses" in mammalian cells. Included in these responses are an acute inhibition of mRNA translation at the initiation step, a subsequent induction of various protein chaperones, and the recovery of mRNA translation. Separate, but closely related, stress response systems exist for the endoplasmic reticulum (ER), relating to the induction of specific "glucose-regulated proteins" (GRPs), and for the cytoplasm, pertaining to the induction of the "heat shock proteins" (HSPs). Activators of the ER stress response system, including Ca(2+)-mobilizing and thiol-reducing agents, are discussed and compared to activators of the cytoplasmic stress system, such as arsenite, heavy metal cations, and oxidants. An emerging integrative literature is reviewed that relates protein chaperones associated with cellular stress response systems to the coordinate regulation of translational initiation and protein processing. Background information is presented describing the roles of protein chaperones in the ER and cytoplasmic stress response systems and the relationships of chaperones and protein processing to the regulation of mRNA translation. The role of chaperones in regulating eIF-2 alpha kinase activities, eIF-2 cycling, and ribosomal loading on mRNA is emphasized. The putative role of GRP78 in coupling rates of translation to processing is modeled, and functional relationships between the HSP and GRP chaperone systems are discussed.