Abstract
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 the 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 the 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 2 to 4 days, respectively, to 1 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 1 (mRNA expression) or 2 days (protein expression) up until 1 week after transfer. These results suggested potential roles of regucalcin in maintaining calcium homeostasis and its correlation to differential physiological responses in the livers of milkfish when they were acclimated to FW and SW.
Highlights
Regucalcin was discovered in 1978 as a calcium (Ca2+) binding protein in the liver of rats (Yamaguchi and Yamamoto, 1978) and is known as the senescence marker protein-30 (SMP30) that decreases expression with ageing in rat livers (Fujita et al, 1992)
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
Summary
Regucalcin was discovered in 1978 as a calcium (Ca2+) binding protein in the liver of rats (Yamaguchi and Yamamoto, 1978) and is known as the senescence marker protein-30 (SMP30) that decreases expression with ageing in rat livers (Fujita et al, 1992). Being a calcium-binding protein without EF-hand motif, several functions of regucalcin have been reported in mammals including intracellular calcium homeostasis, modulation of Ca2+- or Ca2+/calmodulin-dependent enzymes, participation of the biosynthesis process of ascorbic acid, and transcriptional regulation for several hormones (e.g., insulin and estrogen) (Fujita et al, 1998; Kondo et al, 2006; Yamaguchi, 2011; Yamaguchi, 2013). As a calcium binding protein, regucalcin was found to have inhibitory effects on several Ca2+-dependent kinase and enzymes (i.e., cAMP phosphodiesterase, caspase-3, and nitric oxide synthase) which inhibited apoptosis under stress (Matsutama et al, 2004; Izumi and Yamaguchi, 2004; Yamaguchi and Kurota, 1997).
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