Agouti regulation of intracellular calcium. role of melanocortin
receptors.
Kim, J. H., L. L. Kiefer, R. P. Woychik, W. O. Wilkison, A. Truesdale,
O. Ittoop, D. Willard, J. Nichols, and M. B. Zemel.
The Department of Nutrition and Physiology Program, University of
Tennessee, Knoxville, TN 37996-1900, Biology Division, Oak Ridge
National Laboratory, Oak Ridge, TN 37831-8077, Division of
Biochemistry, Glaxo Research Institute, Research Triangle Park, NC
27709
APStracts 3:0168E, 1996.
Several dominant mutations at the murine agouti locus cause a syndrome
of marked obesity and insulin resistance. We recently reported that
intracellular free calcium ([Ca2+]i is elevated in viable yellow
mice. Since [Ca2+]i has a key role in the pathogenesis of insulin
resistance, obesity and hypertension, the role of the purified agouti
gene product in regulating [Ca2+]i was evaluated in a number of cell
types. Purified murine agouti induced slow, sustained increases in
[Ca2+]i in A7r5 vascular smooth muscle cells and 3T3-L1 adipocytes in
a dose-dependent fashion. In L6 skeletal myocytes, agouti stimulated
an increase in [Ca2+]i with an apparent EC50 of 62 nM. This response
was substantially inhibited by Ca2+ entry blockade with nitrendipine.
To determine if melanocortin receptors may play a role in agouti
regulation of [Ca2+]i, we examined the effect of melanocortin
peptides and agouti in cells stably transfected with human
melanocortin receptors. Human embryonic kidney cells (HEK-293 cells)
transfected with either the human melanocortin 1 (MC1-R) or
melanocortin 3 (MC3-R) receptor responded to human agouti with slow,
sustained increases in [Ca2+]i, while non-transfected HEK-293 cells
with no melanocortin receptors did not respond to agouti. Dose
response curves in the MC1-R line showed that agouti had an EC50 of
18 nM, which is comparable to that for agouti antagonism of 125I-NDP
-MSH binding in the same cell line. This direct effect of agouti on
stimulating increases in [Ca2+]i suggests a potential mechanism for
agouti-induced insulin resistance.
Received 4 March 1996; accepted in final form 26 July 1996.
APS Manuscript Number E113-6.
Article publication pending Am. J. Physiol. (Endocrinol. Metab.).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 29 August 1996