SUMMARYIn the present study we investigated the signal transduction cascades triggered by acute thermal stress in Mytilus galloprovincialis gills. This particular species has been reported to exhibit a significant tolerance to high temperatures; thus, it was intriguing to examine the molecular mechanisms responsible for this extraordinary trait. In particular, exposure to 30°C was found to cause a significant and sustained stimulation of p38-MAPK phosphorylation while the activation profile of JNKs was transient and relatively moderate. We also observed that hyperthermia induced apoptosis as a delayed response, with both MAPK subfamilies rapidly translocating to the nucleus. The phosphorylation of cJun, ATF2 and NFB was detected next. Using selective inhibitors, phosphorylation of these transcription factors was established to be dependent on p38-MAPK or JNKs. Subsequently, potential changes in gene expression were assessed. In this context, hyperthermia resulted in the transcriptional upregulation of Hsp70 and MT20 genes with a widely known salutary effect, preserving mussel fitness and performance under adverse environmental conditions. Interestingly, p38-MAPK and JNKs were found to mediate the hyperthermia-induced Hsp70 and MT20 upregulation as well as the delayed induction of apoptosis under the interventions studied. Overall this is, to our knowledge, the first time that an insight into the compensatory survival 'programme' initiated in Mytilus galloprovincialis gills, contributing to this organism's exceptional tolerance to thermal stress, has been gained. In particular, we provide evidence demonstrating the principal role of p38-MAPK and JNKs in transducing the stress signal via mobilization of specific transcription factors and the transcriptional upregulation of cytoprotective genes.

The exact physiological role of oxidative stress as a primary cause for skeletal muscle pathological conditions involving muscle degeneration remains elusive. Therefore, the present study was performed so as to decipher the signalling pathways orchestrating the potential cytoprotective role of heme oxygenase 1 (HOX-1) as well as cyclooxygenase 2 (COX-2) in skeletal myoblasts exposed to H(2)O(2). Cell treatment with H(2)O(2) (0.5 mM) resulted in a time- and dose-dependent response of HOX-1 and COX-2 mRNA and protein levels, with ERK1/2, p38-MAPK and MSK1 found to mediate these effects. Furthermore, Src and JNKs blockade attenuated COX-2 response. Collectively, these novel findings highlight for the first time HOX-1 and COX-2 fundamental contribution to skeletal myoblast tolerance under oxidative stress, since their inhibition significantly attenuated viability of skeletal myoblasts. The data also delineate the various effectors regulating HOX-1 and COX-2 expression, probably alleviating muscle degeneration in related disorders.

One of the most significant insults that jeopardize cardiomyocyte homeostasis is a surge of reactive oxygen species (ROS) in the failing myocardium. Early growth response factor-1 (Egr-1) has been found to act as a transcriptional regulator in multiple biological processes known to exert deleterious effects on cardiomyocytes. We thus investigated the signaling pathways involved in its regulation by H2O2. Egr-1 mRNA levels were found to be maximally induced after 2 h in H2O2-treated H9c2 cells. Egr-1 respective response at the protein level, was found to be maximally induced after 2 h of treatment with 200 μM H2O2, remaining elevated for 6 h, and declining thereafter. H2O2- induced upregulation of Egr-1 mRNA and protein levels was ablated in the presence of agents inhibiting ERKs pathway (PD98059) and JNKs (SP600125, AS601245). Immunofluorescent experiments revealed H2O2-induced Egr-1 nuclear sequestration to be also ERK- and JNK-dependent. Overall, our results show for the first time the fundamental role of ERKs and JNKs in regulating Egr-1 response to H2O2 treatment in cardiac cells at multiple levels: mRNA, protein and subcellular distribution. Nevertheless, further studies are required to elucidate the specific physiological role of Egr-1 regarding the modulation of gene expression and determination of cell fate.

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