Considerable attention has been paid to the

Considerable attention has been paid to the role of p38 MAPK against bacterial and viral infections [42,43]. Cai et al. (2011) showed that p38 MAPK is activated by bacterial and viral infections in grouper [44]. Also, enhanced expression of p38 MAPK in immune tissues after infection with gram-negative (Edwardsiella tarda) and gram-positive (Streptococcus iniae) bacteria has been reported in Oplegnathus fasciatus [45]. There is accumulating evidence that viral infection activates p38 MAPK signaling pathway [10,26]. Overexpression of p38 MAPK delayed the occurrence of cytopathic effect in fathead minnow epithelial SBHA supplier after Singapore grouper iridovirus infection [10]. Furthermore, the viral gene transcription and protein synthesis were reduced by overexpression of p38 MAPK in cells [10]. In the present study, p38 MAPK expression in head kidney, gill and intestine was up-regulated and reached its peak after 24 h of A. hydrophila challenge, while expression of p38 MAPK in spleen was up-regulated almost at 48 h. A clear time-dependent expression pattern was observed as p38 MAPK expression significantly increased and then returned to normal levels. The expression profiles suggested that p38 MAPK contributes to immunity against pathogens. This may give us a clue to understand the relationship between p38 MAPK and pathogen infection. However, the mechanism of action of p38 MAPK during infection remains unknown and needs further research. In other animals, the importance of the p38 MAPK pathway in pro-inflammatory genes expression has been demonstrated [46,47]. There has been accumulating evidence that p38 MAPK plays a crucial role in macrophage-mediated inflammation [48]. p38α participates in the expression of pro-inflammatory mediators in macrophages such as IL-1β, TNF-α, PGE2, and IL-12 [[49], [50], [51]] as well as COX-2, IL-8, IL-6, IL-3, IL-2, and IL-1, all of which contain AU-rich elements in their 3ˊ untranslated domain to which p38 attaches [52]. Accordingly, in this study we investigated the role of the p38 MAPK pathway in pro-inflammatory cytokines (IL-1β and IL-6) expression in hepatocytes following LPS stimulation. The findings showed that the pro-inflammatory genes expression was enhanced after LPS stimulation and this was along with up-regulated expression of p38 MAPK. To confirm whether p38 MAPK is essential for induction of inflammation by LPS-stimulation, we used RNA interference (RNAi) technology to knockdown p38 MAPK expression. The results indicated that pro-inflammatory responses were mediated through the p38 MAPK pathway as knockdown of p38 MAPK decreased the expressions of both IL-1β and IL-6. It has been reported that the p38 MAPK pathway can regulate gene expression by one of two mechanisms, activation of transcription and stabilization of mRNA transcripts [53,54]. This study provides evidence for the use of p38 MAPK inhibitors as potential therapeutics for the treatment of inflammatory responses in fish. In conclusion, a p38 MAPK was identified and characterized in M. amblycephala. It is ubiquitously expressed in various tissues and its expression can be up-regulated in immune organs after ammonia stress and A. hydrophila challenge. Moreover, our results indicated that the inflammatory response induced by LPS in hepatocytes is p38 MAPK dependent.
Acknowledgments This work was funded by Nature Science Foundation of Fujian Province (2017J05056), Central Public-interest Scientific Institution Basal Research & Key Laboratory of Sustainable Development of Marine Fisheries, Ministry of Agriculture, P. R. China, CAFS (NO. 2018HY-XKQ01) and PHD Support Program from Henan University of Science and Technology (China, 13480074). Also, the financial support of the Ministry of Education, Youth and Sports of the Czech Republic – projects “CENAKVA” (No. CZ.1.05/2.1.00/01.0024), “CENAKVA Center Development” (No. CZ.1.05/2.1.00/19.0380), and “CENAKVA II” (No. LO1205 under the NPU I program) and by NAZV project QJ1510119 is gratefully acknowledged.