Differential expression and hormonal regulation of alternatively spliced IGF-I mRNA transcripts in salmon.
Salmon have been shown to express alternatively spliced IGF-I mRNA transcripts coding for four different IGF-I prohormones. These transcripts, now designated Ea-1, Ea-2, Ea-3 and Ea-4, differ in size due to the inclusion of additional sequences in the E domain-coding region of the molecule. In this study, the tissue distribution and hormonal regulation of expression of alternatively spliced IGF-I mRNA transcripts were investigated in coho salmon. IGF-I mRNAs were detected by solution hybridization/RNase protection assay in all tissues examined. GH treatment significantly increased hepatic IGF-I mRNA content. Hepatic IGF-I mRNA levels were not influenced by prolactin or somatolactin. Heart, fat, brain, kidney, spleen and ovary IGF-I mRNA levels were not affected by GH, prolactin or somatolactin. Ea-1, Ea-3 and Ea-4 mRNA transcripts were detectable in the liver, and Ea-1 and Ea-3 levels increased dramatically in response to GH treatment, whereas the amount of Ea-4 mRNA was unchanged. Most non-hepatic tissues expressed only the Ea-4 transcript, and expression was not influenced by GH, prolactin or somatolactin. Ea-1 and Ea-3 transcripts were visible in gill samples from fish treated with GH. The ovaries of juvenile fish expressed Ea-1, Ea-2 and Ea-4. The amounts of these transcripts were not changed by gonadotrophin treatment. During smoltification of juvenile coho salmon, liver and gill IGF-I mRNA levels increased with increasing plasma GH and thyroxine concentrations. Muscle, brain and ovary IGF-I mRNA levels were unchanged during this period. These data suggest that the liver is a major site of IGF-I production in response to GH. Heart, fat, brain, kidney, spleen and ovary did not show increased IGF-I mRNA levels in response to GH treatment. GH and prolactin had inconsistent effects on muscle IGF-I mRNA levels. Somatolactin and a gonadotrophin preparation did not stimulate IGF-I expression in tissues of juvenile fish. Differences in tissue GH responsiveness can be partially explained by the expression of alternatively spliced IGF-I mRNAs. Of the four hepatic IGF-I mRNA transcripts, Ea-1 and Ea-3 are GH-responsive, while Ea-2 and Ea-4 are not. Most non-hepatic tissues express only the Ea-4 transcript, and IGF-I mRNA levels do not increase after GH treatment. The increased IGF-I mRNA levels observed in gill tissue during smoltification suggest that other factors, in addition to GH, may regulate IGF-I expression.(ABSTRACT TRUNCATED AT 400 WORDS)
Duguay
S J
SJ
School of Fisheries, University of Washington, Seattle 98195.
Swanson
P
P
Dickhoff
W W
WW
D017398:Alternative Splicing; D000704:Analysis of Variance; D000818:Animals; D029941:Fish Proteins; D005786:Gene Expression Regulation / Q000502:physiology*; D006023:Glycoproteins / Q000502:physiology; D006065:Gonadotropins, Pituitary / Q000502:physiology; D013006:Growth Hormone / Q000502:physiology; D007334:Insulin-Like Growth Factor I / Q000235:genetics*; D017684:Oncorhynchus kisutch / Q000235:genetics* / Q000378:metabolism; D010907:Pituitary Hormones / Q000502:physiology*; D011388:Prolactin / Q000502:physiology; D012333:RNA, Messenger / Q000378:metabolism; D014158:Transcription, Genetic
D029941:Fish Proteins; D006023:Glycoproteins; D006065:Gonadotropins, Pituitary; D010907:Pituitary Hormones; D012333:RNA, Messenger; C064448:somatolactin protein, fish; D007334:Insulin-Like Growth Factor I; D011388:Prolactin; D013006:Growth Hormone