Down-regulation of hepatic ApoB100 expression during hot season in transition dairy cows

Heat stress is recognized as one of the main factors adversely affecting yield, reproduction and health of dairy ruminants. The peripartum period is the most critical phase for dairy cows. The interaction between heat stress and physiological stress imposed by the peripartum, might worsen the metabolic status of transition dairy cows. Therefore, the objective of the present study was to investigate the effect of hot season on metabolic status and on hepatic apolipoprotein B100 (ApoB100) expression during the peripartum period in Holstein dairy cows. Twenty-four multiparous Holstein cows were utilized in the study. Twelve cows gave birth in spring (SP: 28 March to 30 April). The remaining 12 cows gave birth in summer (SU: 15 June to 2 July). The two groups were balanced for parity and body condition score (BCS) and monitored from −30 to 35 d relative to calving. Values of air temperature (°C) and relative humidity (%) were recorded at 1 h intervals and were utilized to calculate the temperature-humidity index (THI). Milk production and BCS were recorded weekly. Blood samples were taken at 0800 h from the jugular vein −30, −18, and −4 d prepartum and 3, 17, and 35 d postpartum. Plasma was analyzed for glucose, urea, cholesterol, β-OH butyrate (BHBA), non-esterified fatty acids (NEFA) and ApoB100. Liver biopsies were performed on d −30, 3, and 35 relative to day of calving, and tissue samples were used for histopathological examination, and for ApoB100 mRNA and protein determination. Milk yield in SU cows was lower than in their SP counterparts. During summer, transition cows showed lower glucose and cholesterol, higher NEFA and BHBA, had lower liver ApoB100 gene and protein expression and higher lipid accumulation in liver compared with transition cows during spring. The current study provides the first evidence about the negative effects of hot season on ApoB100 gene and protein expression in transition dairy cows. The down-regulation of ApoB100 expression together with higher circulating NEFA and altered lipid metabolism are likely to be responsible for the higher risk of liver lipidosis in transition SU cows. These metabolic conditions make transition cows more susceptible to metabolic diseases during hot season.