The objective of this study was to determine the effects of dietary energy and recombinant bovine somatotropin (bST) injection to identify genes that might control mammogenesis. Total RNA was extracted from the parenchymal tissue of 32 heifers randomly assigned to one of four treatments: two diets (a standard diet and a high energy, high protein diet), each with or without bST. To perform microarray experiments, RNA samples were pooled (2 animals/pool) before reverse transcription and labeling with Cy3 or Cy5. A 4-node loop design was used to examine the differential gene expression among treatments using a bovine-specific cDNA microarray (National Bovine Functional Genomics Consortium Library, NBFGC) containing 18,263 unique expressed sequence tags (EST). Significance levels of differential gene expression among treatments were assessed using a mixed model approach. Injection of bST altered the expression of 12 % of the genes on NBFGC slide related to tissue development, whereas 6% were altered by diet. Administration of bST increases the expression of genes positively related to cell proliferation and mammary parenchyma to a greater extent than a high energy diet.

Objetivou-se com este trabalho avaliar os efeitos de uma dieta de alto nível de energia e proteína combinada com a aplicação de bST no perfil de expressão dos genes da leptina e de seu receptor Ob-Rb no parênquima mamário de novilhas leiteiras. Foram utilizadas amostras de parênquima mamário de 32 novilhas holandesas distribuídas aleatoriamente em quatro tratamentos (n=8): dieta com alto ou baixo teor de energia e proteína combinada ou não com a aplicação de bST. O delineamento utilizado foi em blocos casualizados com arranjo de tratamentos em esquema fatorial 2 × 2. A extração do RNA total das amostras de tecido foi feita e o nível de expressão gênica foi analisado por qRT-PCR utilizando-se o gene da glicuronidase β como controle, pelo método 2-ΔΔCt. Animais que receberam a dieta com alto conteúdo de energia e proteína apresentaram maior expressão de mRNA de leptina, com aumento de 56%, e menor expressão de mRNA do receptor Ob-Rb, com redução de 18%. Por outro lado, a aplicação de bST resultou em diminuição da expressão do mRNA de leptina e do receptor Ob-Rb em 74% e 23%, respectivamente. Não houve interação entre dieta e aplicação de bST. O aumento na expressão de leptina pode explicar, ao menos em parte, os efeitos negativos da dieta de alta energia e proteína, oferecida no período pré-púbere, sobre a produção de leite de novilhas leiteiras.

The objective of this study was to examine the effects of a diet with high level of energy and protein, combined with bST injections, on leptin and leptin-receptor (Ob-Rb) gene expression profile in the mammary parenchyma of dairy heifers. Mammary parenchyma samples from 32 Holstein heifers, randomly assigned to one of four treatments (n=8), were utilized: high or low energy and protein diet, with or without bST injection. The experiment was designed in randomized blocks and arranged in a 2 × 2 factorial arrangement. Total RNA was extracted from tissue samples and gene expression profile was tested in qRT-PCR, against glucuronidase β as a control, using the 2-ΔΔCt method. Animals receiving the diet with high level of energy and protein had higher leptin mRNA expression, with an increase of 56%, and lower leptin receptor Ob-Rb mRNA expression, with a decrease of 18%. On the other hand, bST injection decreased leptin mRNA and receptor Ob-Rb mRNA expression in 74% and 23%, respectively. There was no interaction between diet and bST injection. The increase in leptin expression can explain, at least in part, the detrimental effects caused by diets rich in energy and protein, offered during the prepubertal period, in future milk production of dairy heifers.