Expression of leukemia inhibitory factor (LIF)/interleukin-6 family cytokines and receptors during in vitro osteogenesis: differential regulation by dexamethasone and LIF.

The leukemia inhibitory factor/interleukin-6 (LIF/IL-6) family of cytokines is known to play a major role in bone physiology. Although much work has focused on the regulation of bone resorption by IL-6 and related cytokines, their effects on osteoblast development and bone formation have not been as well studied. Previously, we reported that LIF inhibits, in a non-IL-6-dependent manner, osteoblast differentiation and bone nodule formation in the rat calvaria (RC) model, an effect that is antagonized by dexamethasone (Dex). The culture time-sensitive window suggested that LIF targets late preosteoblasts or early osteoblasts, and that this stage-specific effect coincided with a period of low endogenous production of LIF and IL-6. To detect potential crosstalk between members of this family, we have extended these observations by assessing the expression levels of other LIF/IL-6 cytokines (CNTF, OSM, IL-11, CT-1) and their receptors in the same RC cell model treated with or without LIF or Dex. Semiquantitative reverse transcription-polymerase chain reaction (RT-PCR) analysis showed that IL-11 and its receptor, CNTF and its receptor, LIFR, and gp130 were constitutively expressed throughout the culture period. Expression of CT-1 and OSM increased with culture time - that is, with osteoblast differentiation - whereas the specific receptor for OSM (OSMR) was highly expressed at early timepoints and either plateaued or decreased thereafter. Continuous treatment with Dex (10(-8) mol/L) inhibited the endogenous production of IL-6, LIF, OSM, IL-11R, and OSMR, but had no detectable effect on the expression of IL-11, CT-1, CNTF, CNTFR, LIFR, or gp130. Finally, treatment with exogenously added LIF stimulated IL-6, LIF, LIFR, and OSMR, but had no other detectable effects. These data indicate that multiple members of the LIF/IL-6 family and their receptors are expressed in RC cell cultures, and are differentially regulated by Dex and LIF, suggesting that these cytokines play a complex and interdependent role, further modulated by glucocorticoid levels, in osteoprogenitor differentiation and bone nodule formation.