Esterification of plasma membrane cholesterol and triacylglycerol-rich lipoprotein secretion in CaCo-2 cells: possible role of p-glycoprotein.

Acylcoenzyme A:cholesterol acyltransferase (ACAT) and/or cholesteryl esters have been implicated as important factors in the normal assembly of apolipoprotein (apoB)-containing lipoproteins. The predominant substrate for ACAT is believed to originate from cholesterol contained within the plasma membrane. To investigate a possible role of intestinal plasma membrane cholesterol in triacylglycerol-rich lipoprotein synthesis and secretion, CaCo-2 cells were incubated with agents that are known to interfere with cholesterol transport from the plasma membrane to the ER. Progesterone, verapamil, and trifluoperazine significantly decreased the movement of cholesterol from plasma membrane to endoplasmic reticulum (ER) in CaCo-2 cells. Without altering the synthesis of apoB and independent of their effects on cellular cholesterol esterification, progesterone, verapamil, and trifluoperazine decreased the basolateral secretion of triacylglycerols, cholesteryl esters, and immunoreactive and newly synthesized apoB. The three agents also interfered with the esterification of cholesterol absorbed from taurocholate micelles. As progesterone, verapamil, and trifluoperazine are recognized inhibitors of p-glycoprotein, a variety of agents that have been shown to interfere with p-glycoprotein function were tested to investigate their effects on cholesterol transport and apoB secretion. All the agents significantly decreased in parallel both cholesterol transport and apoB secretion. In contrast, methotrexate, an antimetabolite that does not interact with p-glycoprotein, had no effect. Nigericin, a potassium ionophore, which causes alkalinization of intracellular vesicles, also caused a profound inhibition of cholesterol transport and apoB secretion. Preventing plasma membrane cholesterol from arriving at the ER, or inhibiting the esterification of plasma membrane cholesterol, does not alter apoB secretion. However, the results suggest a possible role for p-glycoprotein in normal cholesterol trafficking and triacylglycerol-rich lipoprotein secretion in CaCo-2 cells. It is postulated that p-glycoprotein might function to maintain the acidic environment of transport vesicles, and therefore, could play a role in the transport of lipids by the intestine.