Transcriptional Responses Of HSP70 Genes In The Liver Of Three Freshwater Fishes Intraperitoneally Injected With Microcystin-LR

Silver carp(Hypophthalmichthys molitrix),grass carp(Ctenopharyngodon idellus), and Nile tilapia(Oreochromis nilotica) are three warm freshwater fishes with differential tolerance to microcystin-LR(MC-LR):Nile tilapia has a higher tolerance than silver carp, and both have higher tolerance than grass carp.Full-length cDNAs encoding the heat shock protein 70(HSP70) were cloned and sequenced from the livers of the three fishes. The silver carp,grass carp,and Nile tilapia HSP70 cDNAs were 2356,2348,and 2242 bp in length,and contained an open-reading frame(ORF) of 1950 bp(encoding a polypeptide of 649 amino acids),1950 bp(encoding a polypeptide of 649 amino acids),1917 bp (encoding a polypeptide of 638 amino acids),respectively.The partial cDNA sequences encoding HSP70 genes were cloned from orange-spotted grouper(Epinephelus coioides), bighead carp(Aristichthys mobilis),crucian carp(Carasslius auratus),white cloud mountain minnow(Tanichthys albonubes),common sturgeon(Acipenser sturio Linnaeus), senegal bichir(Polypterus senegalus Cuvier),Taiwan snakehead(Channa maculata). Using beta-actin as an external control,semiquantitative RT-PCR was conducted to determine the constitutive and inducible expression level of HSP70 genes among the three fishes(6-13)g intraperitoneally injected with MC-LR(50μg kg^-1 body weight),mRNA expression level of HSP70 gene in Nile tilapia liver raised significantly after exposure to MC-LR(50μg kg^-1 bwt) for 8 h,whereas the mRNA expression level of HSP70 in silver carp just tended to raise after exposure to MC-LR(50μg kg^-1 bwt) for 8 h.The expression level of grass carp has no change in substance.The constitutive mRNA expression level of heat shock protein 70 in silver carp control groups seems to be twice as much as that in grass carp control groups.The inducible expression of the liver HSP70 gene showed a close relationship with their tolerance to MC-LR:high-resistant fish(Nile tilapia) is the most significant upregulation,followed by resistant fish(silver carp),and high-sensitive fish(grass carp) has no change in substance.We suggest that inducible expression of the liver HSP70 gene should play an important role in the tolerance to microcystin exposure, as well as detoxification of MC-LR by protecting the cell from the damage of ROS.