Expression of genes regulating protein metabolism in Atlantic cod (Gadus morhua L.) was altered when including high diet levels of plant proteins

Atlantic cod were fed five different diets consisting of a plant protein mixture replacing fishmeal in a regression design up to total replacement. Growth was high and maintained equal up to 75% plant protein inclusion, while a total plant protein diet resulted in significantly reduced appetite, feed conversion, and growth. To reveal if the transcription of stress responsive genes or any metabolic pathways were affected by plant proteins, a microarray consisting of 744 EST library-clones from cod was used on liver samples after the fish doubled its weight. Liver mRNA-expression of insulin-like growth factor II (IGF-II), insulin-like growth factor receptors I and II (IGF-IR, IGF-IIR), genes coding for proteolytic enzymes; cathepsinD (CatD), cathepsinF (CatF), calpain 2, and seven proteins involved in protein-biosynthesis and energy-turnover, were studied by means of quantitative real-time PCR. Transcriptional levels of IGF-IIR and CatD were reduced in the plant fed fish. With few exceptions, dietary inclusion of plant protein did not affect the expression of genes related to cellular stress, protein-biosynthesis or energy turnover when the fish was fed up to 75% plant protein inclusion. The data on down-regulation of the IGF receptors and CatD coincided with the reduced growth in the high plant inclusions, indicating that IGF and CatD responses were closely related to protein turnover.