Hepatic gene expression of apolipoprotein B100 during early lactation in underfed, high producing dairy cows

The hepatic gene expression of apolipoprotein B, the major protein of very low density lipoproteins in plasma, was studied using 8 Holstein x Friesian cows during the first 12 wk of lactation. Cows were fattened during gestation and were underfed just after parturition to increase fat mobilization and subsequent hepatic steatosis. Intracellular concentrations of apolipoprotein B and apolipoprotein B mRNA and control parameters (albumin, total lipids, RNA, and proteins) were determined in liver samples obtained by biopsy from each cow on four occasions at 1, 2, 4, and 12 wk after calving. Results were compared with those obtained from 5 dry cows in late pregnancy and 4 dry nonpregnant cows. The hepatic concentration of apolipoprotein B was lower (approximately 25%) during wk 1, 2, and 4 after calving, a period of intense liver steatosis (44.2 to 95.7 mg of triglycerides/g of fresh tissue), than for nonsteatotic dry cows (pregnant or nonpregnant); hepatic concentrations were also lower than those during wk 12. In contrast, hepatic concentrations of mRNA coding for apolipoprotein B, total proteins, RNA, and albumin did not vary significantly during early lactation. These results suggested that synthesis of apolipoprotein B during early lactation is specifically regulated at a posttranscriptional level by a decrease in the rate of translation, or by a higher rate of intracellular degradation of apolipoprotein B, or both.