Starvation of a clonal osteoblast-like cell line, MOB 3-4-F2, down-regulates prostaglandin E2 receptors but increases cAMP response to prostaglandin E2.

Prostaglandin E2 (PGE2) stimulated cAMP production in the MOB 3-4-F2 cell line, a subclone of the osteoblast-like MOB 3-4 cell line. After being cultured in alpha-minimum essential medium supplemented with 10% heat-inactivated foetal calf serum (HIFCS), cells responded to PGE2 (greater than or equal to 50 ng/ml) with a small, but significant, increase in cAMP production. This response did not vary with duration of culture. In 2% HIFCS-containing medium, despite their lower basal cAMP level, cells responded to PGE2 (greater than or equal to 5 ng/ml) with strikingly increased cAMP production. In addition, prolonged culture in this serum-deficient medium enhanced this response. On the other hand, culture of cells in 2% HIFCS-containing medium decreased the apparent number of PGE2 receptors, which was also enhanced by prolonged culture, without effect on their apparent affinity. Their number in 10% HIFCS-containing medium, more than that in 2% HIFCS-containing medium, was almost constant, independent of the culture period. Starvation of MOB 3-4-F2 cells in serum-deficient medium, therefore, appeared to down-regulate PGE2 receptors but increase the cAMP response to PGE2. Moreover, prolonged starvation of cells appeared to facilitate these phenomena. Our findings suggest that cAMP response to PGE2 does not always reflect the number of available PGE2 receptors in the cells.