Abstract
During avian embryonic development, endodermal epithelial cells (EECs) absorb yolk through the yolk sac membrane. Sterol O-acyltransferase (SOAT) is important for esterification and yolk lipid utilization during development. Because the major enzyme for yolk sac membrane cholesteryl ester synthesis is SOAT1, we cloned the avian SOAT1 promoter and elucidated the cellular functions of SOAT1. Treatments with either glucagon, isobutylmethylxanthine (IBMX), an adenylate cyclase activator (forskolin), a cAMP analog (dibutyryl-cAMP), or a low glucose concentration all increased SOAT1 mRNA accumulation in EECs from Japanese quail, suggesting that SOAT1 is regulated by nutrients and hormones through a cAMP-dependent pathway. Activity of protein kinase A (PKA) was increased by IBMX, whereas co-treatment with the PKA inhibitor, H89 negated the increase in PKA activity. Cyclic AMP-induced EECs had greater cholesterol esterification than untreated EECs. By promoter deletion and point-mutation, the cAMP-response element (-349 to -341 bp) was identified as critical in mediating transcription of SOAT1. In conclusion, expression of SOAT1 was regulated by a cAMP-dependent pathway and factors that increase PKA will increase SOAT1 to improve the utilization of lipids in the EECs and potentially modify embryonic growth.
Highlights
During avian embryonic development, nutrients are mainly absorbed from yolk by the endodermal epithelial cells (EECs) in the yolk sac membrane (YSM)
We confirmed that SOAT1 was in YSM by real-time PCR, and that the SOAT1 mRNA was expressed in YSM during entire embryonic development (Fig 1A)
Using the PepTag1 Assay, we found that protein kinase A (PKA) phosphorylation was induced by IBMX, and this effect was neutralized by H89 (Fig 2G and 2H), both cyclic AMP (cAMP) and IBMX up-regulated the expression of SOAT1 by activating PKA
Summary
Nutrients are mainly absorbed from yolk by the endodermal epithelial cells (EECs) in the yolk sac membrane (YSM). 68% of lipids in yolk are absorbed during the late stages of egg incubation. Sterol Oacyltransferase (SOAT), called acyl-coenzyme A: cholesterol acyltransferase (ACAT), is responsible for the esterification of cholesterol with a long-chain fatty acid, which is important for lipid utilization during embryonic development [2,3,4]. The concentration of CE increases from 3.3% to 6.9% of total lipids in the chicken YSM during ED 13 to ED21; the CE level in yolk remains constant [5, 6]. The CE levels in embryonic liver increased from 33.9 to 70.2% of total lipids during ED 13 to ED21 [2, 5, 6]
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