Hypothermal Effects on Expression of Regucalcin, A Calcium Binding Protein, in Livers of Seawater- and Fresh Water-Acclimated Milkfish, Chanos Chanos

Regucalcin (RGN) is a calcium-binding protein mainly expressed in the liver. It functions in regulating activities of several calcium-dependent enzymes related to energy metabolism, antioxidant mechanisms, and apoptotic pathways. Previous proteomics analyses revealed downregulation of regucalcin in milkfish livers when acclimated to low temperature (18°C) from normal temperature (28°C). This study first identified the full-length sequence of milkfish regucalcin from livers with high similarity in the protein structure and calcium-binding function compared to the regucalcin of other animals. The mRNA and protein expression of regucalcin in livers of fresh water (FW)- and seawater (SW)-acclimated milkfish under hypothermal acclimation were further analyzed. In FW milkfish, upregulation of regucalcin was found in mRNA and protein levels from two and four days, respectively, to one week after transfer to 18°C for the two. However, in SW milkfish, upregulation of regucalcin occurred quickly and returned to the basal levels in one (mRNA expression) or two days (protein expression) up until one week after transfer. These results suggested potential roles of regucalcin in maintaining calcium homeostasis and its correlation to differential physiological responses in livers of milkfish when they were acclimated to FW and SW.