INTERLEUKIN-4 ENHANCES OSTEOBLAST MACROPHAGE-COLONY-STIMULATING FACTOR, BUT NOT INTERLEUKIN-6, PRODUCTION

To determine if interleukin 4's (IL-4) recently discovered skeletal effects could be explained by its effects on osteoblasts, we have examined IL-4's impact on macrophage colony stimulating factor (M-CSF) and interleukin 6 (IL-6) secretion by the murine osteoblastic cell line MC3T3-E1. Interleukin-4 increased colony-forming activity in MC3T3 supernatants two-threefold with colony cytomorphology, cytohistochemistry, and blockade of the effect by anti-M-CSF antibody, indicating that the IL-4-induced activity was M-CSF. MC3T3 M-CSF supernatant activity increased in a time-dependent manner with positive IL-4 effects seen after a 24-hour exposure. The maximal IL-4 effective dose was 100 U/ml where conditioned media from IL-4-treated cells contained twofold more M-CSF than control cells (400 U/ml versus 200 U/ml M-CSF) as detected by a sandwich M-CSF ELISA. Northern blots showed that IL-4 (200 U/ml) rapidly increased steady-state M-CSF mRNA levels with maximal induction observed by 2 hours followed by a decline to near basal levels by 24 hours. IL-4 also dose dependently increased M-CSF mRNA levels with maximal induction (fourfold) seen at 100 U/ml IL-4. In contrast to its impact on MC3T3 M-CSF production, IL-4 (200 U/ml) did not stimulate MC3T3 IL-6 secretion whereas IL-1 (1 pM) stimulated a 500-fold increase in MC3T3 IL-6 release. When utilized to treat newborn calvarial osteoblast-enriched cultures, IL-4 dose dependently augmented M-CSF production, with the maximal effect seen at 200 U/ml where IL-4-treated, osteoblast-conditioned media contained almost 500 U/ml M-CSF, compared with 200 U/ml M-CSF in control-conditioned media. These observations indicate that the range of IL-4 cellular targets in skeletal tissues include osteoblastic cells, and that this cytokine increases osteoblast expression of M-CSF, a hematopoietic cytokine pivotal for monocyte/macrophage differentiation. Furthermore, IL-4's impact on osteoblast-produced M-CSF levels is selective because IL-6 levels were unaltered by IL-4 treatment.