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J. Cell Biol.,
Volume 142, Number 5, September 7, 1998 1347-1356


* Center for Osteoporosis and Skeletal Aging, Veterans Affairs Medical Center, and Department of Medicine, Medical College
of Pennsylvania-Hahnemann School of Medicine, Philadelphia, Pennsylvania 19104; We describe a physiologically significant
mechanism through which interleukin-6 (IL-6) and a
rising ambient Ca2+ interact to regulate osteoclastic
bone resorption. VOXEL-based confocal microscopy
of nonpermeabilized osteoclasts incubated with anti-
IL-6 receptor antibodies revealed intense, strictly peripheral plasma membrane fluorescence. IL-6 receptor
expression in single osteoclasts was confirmed by in situ
reverse transcriptase PCR histochemistry. IL-6 (5 ng/l
to 10 µg/l), but not IL-11 (10 and 100 µg/l), reversed the
inhibition of osteoclastic bone resorption induced by
high extracellular Ca2+ (15 mM). The IL-6 effect was
abrogated by excess soluble IL-6 receptor (500 µg/l).
Additionally, IL-6 (5 pg/l to 10 µg/l) inhibited cytosolic
Ca2+ signals triggered by high Ca2+ or Ni2+. In separate
experiments, osteoclasts incubated in 10 mM Ca2+ or
on bone released more IL-6 than those in 1.25 mM
Ca2+. Furthermore, IL-6 mRNA histostaining was
more intense in osteoclasts in 10 or 20 mM Ca2+ than
cells in 1.25 mM Ca2+. Similarly, IL-6 receptor mRNA
histostaining was increased in osteoclasts incubated in 5 or 10 mM Ca2+. Thus, while high Ca2+ enhances IL-6
secretion, the released IL-6 attenuates Ca2+ sensing
and reverses inhibition of resorption by Ca2+. Such an
autocrine-paracrine loop may sustain osteoclastic activity in the face of an inhibitory Ca2+ level generated
locally during resorption.
Division of Endocrinology, University of
Arkansas for Medical Sciences, Little Rock, Arkansas 72205; and § School of Dentistry, University of Southern California, Los
Angeles, California 90089
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