Cholesterol esterification by mammary gland microsomes from the lactating rat

Area of Science:

• Biochemistry
• Cell Biology
• Lactation Biology

Background:

• Cholesterol homeostasis is crucial in biological systems.
• Fatty acyl-CoA:cholesterol acyltransferase (ACAT) plays a key role in cellular cholesterol metabolism.
• The presence and regulation of ACAT in the lactating mammary gland are not fully understood.

Purpose of the Study:

• To demonstrate and characterize cholesterol esterification activity in lactating rat mammary gland microsomes.
• To investigate the regulation of mammary gland ACAT by cholesterol availability and specific inhibitors.
• To explore the potential role of ACAT in milk cholesterol synthesis and its distinction from other acyltransferases.

Main Methods:

• Microsomal preparations from lactating rat mammary glands were used.
• Cholesterol esterification was measured by the incorporation of [1-14C]oleoyl-CoA into cholesteryl[14C]oleate.
• ACAT activity was assessed under varying cholesterol concentrations and in the presence of inhibitors like progesterone, estradiol, retinol, and N-acylamides.

Main Results:

• ACAT activity was confirmed in mammary gland microsomes and was found to be significantly increased with higher unesterified cholesterol levels.
• Progesterone markedly inhibited ACAT activity (93% decrease), while estradiol and retinol showed lesser inhibition.
• N-acylamides, which inhibit ACAT in other systems, did not significantly affect retinol esterification or triglyceride synthesis in mammary microsomes, indicating specificity.

Conclusions:

• The lactating mammary gland possesses ACAT activity with properties similar to ACAT in other tissues.
• Mammary gland ACAT activity is regulated by cholesterol availability and can be inhibited by progesterone.
• The presence of ACAT provides a mechanism for cholesteryl ester synthesis in milk and suggests it is distinct from retinol and glyceride acyltransferases.