Down-regulation of prostaglandin E receptors and homologous desensitization of isolated adipocytes

Adipocytes are known to contain prostaglandin E (PGE) binding sites and PGE is known to be antilipolytic. These studies were performed to ascertain whether PGE binding sites in isolated adipocytes can be down-regulated and whether down-regulation (DR) decreases the sensitivity of the fat cell to the antilipolytic effects of PGE₂, adenosine, and insulin. Treatment in vivo of Sprague-Dawley rats with 16,16-dimethyl-PGE₂, a PGE analog, induced DR of PGE-specific binding site density in both intact fat cells (175 vs. 307 fmol/mg protein) and triglyceride-free broken fat cell preparations (148 vs. 360 fmol/mg protein). There were no changes in binding affinities. DR was associated with diminished antilipolytic potency of PGE on basal glycerol production by intact fat cells in the presence of adenosine deaminase (IC₅₀/control = 0.31 ± 0.03 x 10^-9 M vs. DR = 1.6 ± 0.03 x 10^-9 M; P < 0.01). In contrast, there was no desensitization of the adipocytes to the antilipolytic effects of phenylisopropyladenosine (IC₅₀/control = 4.65 ± 0.96 x 10^-10 M vs. DR = 4.90 ± 0.82 x 10^-10 M; P = NS) or insulin (IC₅₀/control = 4.40 ± 0.57 x 10^-11 M vs. DR = 2.70 ± 0.24 x 10^-11 M; P = NS). PGE desensitization was also observed during studies of isoproterenol-stimulated lipolysis. These data uniquely demonstrate that the adipocyte PGE receptor can be down-regulated and that this decrease in PGE receptor density is associated with homologous desensitization of the fat cell to the antilipolytic effect of PGE and no adenosine or insulin. These findings suggest that a PGE-specific receptor may be involved in regulation of lipolysis by PGE.