Identification of differential melanocortin 4 receptor agonist profiles at natively expressed receptors in rat cortical astrocytes and recombinantly expressed receptors in human embryonic kidney cells.
Using cAMP accumulation as a functional readout, we pharmacologically characterized the response of native melanocortin receptors in cultured rat astrocytes, and found this response to be mediated by the melanocortin 4 receptor (MC4R). Melancortin agonists stimulate cAMP in a concentration-dependent manner in both astrocytes and human embryonic kidney cells recombinantly expressing rat MC4R (HEK-rMC4R), however, the relative potency and intrinsic activity of both small molecule and peptide agonists are reduced in the native system. As such, the small molecules THIQ, NBI-702 and MB243 display 43, 30 and 18% of the maximal response elicited by alpha-MSH in astrocytes. Likewise, the peptides MTII and ACTH display 55 and 72% of the maximal response elicited by alpha-MSH in these cells. In contrast, all of these compounds elicit full agonist responses with similar intrinsic activity to alpha-MSH in HEK-rMC4R cells. MC4R mRNA was detected in astrocytes, however radioligand binding experiments failed to detect measurable MC4R in astrocyte membranes, in contrast to membranes from HEK-rMC4R cells that display a binding site density of 18.1+/-1.5 fmol/mg. We propose that the divergent observations in functional activity between the cell types reflect differences in receptor expression and that caution should be exercised when interpreting agonist activity in over-expression systems for the purposes of drug discovery.
Selkirk
J V
JV
Department of Neuroscience, Neurocrine Biosciences Inc., 12790 El Camino Real, San Diego, CA 92130, USA. julie.selkirk@pfizer.com
Nottebaum
L M
LM
Lee
J
J
Yang
W
W
Foster
A C
AC
Lechner
S M
SM
D000818:Animals; D000831:Animals, Newborn; D001253:Astrocytes; D001665:Binding Sites / Q000187:drug effects; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology; D000242:Cyclic AMP / Q000378:metabolism; D017362:Cyclic AMP Response Element-Binding Protein / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004622:Embryo, Mammalian; D005786:Gene Expression Regulation / Q000187:drug effects* / Q000502:physiology; D006728:Hormones / Q000494:pharmacology*; D006801:Humans; D020128:Inhibitory Concentration 50; D007668:Kidney; D011485:Protein Binding / Q000187:drug effects; D051381:Rats; D044105:Receptor, Melanocortin, Type 4 / Q000819:agonists* / Q000235:genetics / Q000378:metabolism*; D013997:Time Factors; D000521:alpha-MSH / Q000494:pharmacology*
D017362:Cyclic AMP Response Element-Binding Protein; D006728:Hormones; D044105:Receptor, Melanocortin, Type 4; D000521:alpha-MSH; D000242:Cyclic AMP