Growth of Chinese hamster ovary (CHO) cells expressing the 5-HT2 serotonin receptor in suspension culture: an efficient method for large-scale acquisition of membrane protein for drug evaluation.

Chinese hamster ovary (CHO) cells expressing the rat 5-HT2 serotonin receptor were grown and evaluated in suspension culture to provide an efficient method of producing membrane-bound receptors for drug discovery. Expression of the 5-HT2 receptor in cells grown in batch suspension culture fluctuated as a function of culture age. Both receptor expression and receptor G-protein coupling were the highest early (1.9 pmol/mg membrane protein) but declined rapidly as the culture increased in age. However, addition of fresh serum-containing medium to stationary-phase cells reversed the decline and, after 24 h of growth, resulted in maximal receptor density and G-protein coupling. This serum response was found to be reproducible and lead to the establishment of a semi-continuous batch culture system in which cells were maintained in a growth state that supported high levels of membrane-incorporated and G protein-coupled receptors. In this system, 50% of the culture volume could be removed at 24-h intervals for membrane preparation and the lost volume replenished with fresh medium, resulting in a continuous supply of high-quality membrane preparations.