The present study was conducted to study whether a temporary consumption of a soy protein isolate (SPI) based diet as compared to a casein (CAS) based diet results in persistent effects on oxidative stress response and wether these alterations are associated with changes in hepatic gene expression patterns and growth in juvenile pigs. The SPI diet is characterized by an imbalanced amino acid pattern, the CAS control diet has a balanced amino acid composition.
Transcription analysis showed that temporary feeding of SPI diet to growing pigs resulted in prolonged up-regulation of genes involved in oxidative stress response as compared to the CAS diet. The persistent diet associated elevation of oxidative stress response was significantly associated with growth retardation observed here. These findings identified the SPI diet associated prolonged oxidative stress response as a molecular mechanism of persistent growth retardation in juvenile pigs.
DNA methylation is a possible mechanism for diet associated persistent alterations of hepatic expressions of genes involved in oxidative stress response. In promotor regions DNA methylation at CpG dinucleotides is negative correlated with gene expression. The entire hepatic DNA methylation of CpG islands was significant lower in SPI fed animals. These indicates a methylation associated broad induction of hepatic gene expression as a result of the SPI diet. To study DNA methylation associated with oxidative stress response we have quantified methylation rate at 21 CpG sites within the porcine GSTA1 (Glutathione-S-Transferase) promotor region [-1812 to -1] an indicator for oxidative stress response. Statistical analysis of the individual CpG site methylation rate reveal that increased hepatic transcript levels are due to insignificantly decreased predominantly below-average methylation rates of all CpG-sites within the GSTA1 promotor region. The diet associated influence of the promotor CpG methylation state on hepatic GSTA1 expression reveals the DNA methylation as a potential molecular mechanism for persistent changes in gene expression.
The significantly lower hepatic DNMT1 (DNA Methyl Transferase) transcript level in pigs permanently fed SPI diet in comparison to CAS fed pigs could be the general reason of the overall slightly lower GSTA1 CpG methylation.
The deficient methionine supply due to the SPI diet could explain the observed hepatic long term gene expression alterations in growing pigs. Methionine is the precursor for S-adenosylmethionine (SAM), the principal biological methyl donor.