INTERLEUKIN-1 INHIBITS BONE-RESORPTION AND ACUTELY INCREASES CYTOSOLIC CA2+ IN MURINE OSTEOCLASTS

Interleukin-4 (IL-4) is an immune cytokine recently shown to inhibit b one resorption. To determine whether IL-4 directly acts on osteoclasts , we have analyzed its effect on cytosolic calcium concentration [Ca2](i) and bone resorptive function of murine osteoclastic cells generat ed from bone marrow/stromal cell co-cultures. IL-4 exposure induced an immediate and sustained increase in [Ca2+](i) that remained elevated for at least 10 min. This IL-4 effect was dose-dependent, with the max imal effect (209 +/- 15% of baseline, n = 16) at 200 units/ml and an a pparent ED(0.5) of 60 units/ml. The IL-4-induced [Ca2+](i). rise requi red extracellular Ca2+ influx, since the response was prevented by LaC l3, and voltage-gated Ca2+ channel blockers, although the IL-4 effect was more sensitive to nicardipine and nifedipine than to diltiazem. De polarization by high extracellular K+ concentration also raised [Ca2+] (i), and, under these conditions, osteoclasts failed to respond to IL- 4. On the other hand, when intracellular Ca2+ stores were depleted by thapsigargin, IL-4 still induced an increase in [Ca2+](i), although sm aller in amplitude and transient. Calcitonin also produced [Ca2+](i) i ncreases in osteoclasts, yet it only slightly desensitized these cells to IL-4. Furthermore, IL-4 was much less effective on osteoclasts pre treated (5-10 min) with either forskolin or 8-bromo-cAMP. Both IL-4 an d calcitonin were effective even when [Ca2+](i) had been increased by exposure to high extracellular Ca2+. Finally, IL-4 dose dependently in hibited the bone resorptive activity of mature osteoclasts. Therefore, IL-4 signal transduction in osteoclasts involves a rapid and sustaine d elevation of [Ca2+](i) mediated by a voltage dependent Ca2+ influx, in combination with Ca2+ release from intracellular stores. Modulation of osteoclast [Ca2+](i) represents a potential mechanism by which IL- 4 inhibits bone resorption.