Interleukin-1 alpha upregulates tumor necrosis factor receptors expressed by a human bone marrow stromal cell strain: implications for cytokine redundancy and synergy

To explore the biochemical and physiologic basis of the overlapping effects of interleukin-1 alpha (IL-1 alpha) and tumor necrosis factor alpha (TNF-alpha) on myeloid cytokine production, we have studied the dynamics of granulocyte colony-stimulating factor (G-CSF) and granulocyte-monocyte colony-stimulating factor (GM-CSF) production as well as IL-1 receptor and TNF receptor expression in a clonally derived bone marrow stromal cell strain (CDCL). IL-1 alpha and TNF alpha act in a synergistic manner to stimulate G-CSF and GM-CSF production by CDCL, resulting in an increase in CSF secretion that is 250-fold greater than that observed with either cytokine alone. This synergism in protein secretion is paralleled by synergistic increases the steady-state level of GM- and G-CSF mRNA, with supra-additive levels achieved by 24 hours. Coincident with this synergistic induction of myeloid CSFs, treatment of CDCL cells with IL-1 alpha induces a 300% increase in the expression of TNF receptors. IL-1 alpha induction of TNF receptors reaches a peak after 6 hours and gradually returns to baseline level by 24 hours. IL-1 alpha does not affect TNF receptor ligand binding affinity. A kinetic study comparing IL-1/TNF synergistic induction of growth factor secretion with IL-1 alpha induction of TNF receptors shows that these events occur in parallel. In contrast with the induction of TNF receptors by IL-1 alpha, treatment with TNF alpha has no effect on either the number of IL-1 receptors expressed by CDCL cells or IL-1 receptor ligand binding affinity. Brief treatment of IL-1 alpha/TNF alpha-stimulated CDCL cells with cycloheximide before receptor induction reduces the synergistic increase in growth factor mRNA by 40% to 60% compared with cells not treated with CHX. Taken together, these results raise the possibility that IL-1 alpha cross-induction of TNF receptors may contribute to the biochemical mechanisms underlying the synergistic stimulation of G-CSF and GM-CSF production by IL-1 alpha and TNF alpha.