Bryostatin 1 acts synergistically with interlearkin-1α to induce secretion of G-CSF and other cytokines from marrow stromal cells

The protein kinase C (PKC) activator bryostatin 1 (bryo) has substantial antileukemic and hematopoietic actions. Bryo promotes the in vitro growth of normal hematopoietic progenitors by inducing the release of growth factors from accessory cells. We have examined the effects of bryo on the expression and release of certain myeloid growth factors from fibroblast-like marrow stromal cells (MSC). Substantial release of granulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage colony-stimulating factor (GM-CSF), or interleukin-6 (IL-6) following bryo treatment was seen only in MSC cultures contaminated with macrophages. Bryo alone was ineffective in inducing release of the cytokines from MSC cultures containing only fibroblast-like stromal cells. When MSC were treated with IL-1α, substantial quantities of the cytokines (G-CSF, GM-CSF, IL-6) were released. Bryo acted synergistically with IL-1α to significantly increase cytokine release two- to nine-fold compared to IL-1α alone (p < 0.016). Neither IL-1α nor bryo, alone or in combination, induced release of stem cell factor (SCF) from MSC. The synergistic interaction between IL-1α and bryo was dose- and schedule-dependent, requiring simultaneous application of IL-1α and bryo for optimum effect. Bryo alone induced no G-CSF mRNA accumulation but increased the level seen with IL-1α treatment by 50%. The synergistic interaction of bryo and IL-1α required PKC, since it was antagonized by agents which depleted or inhibited PKC but not by a protein kinase A antagonist. The increase in GCSF mRNA was associated with a marked increase in mRNA stability. Bryostatin may promote the release of cytokines from several accessory cell populations, including MSC, to accomplish its in vivo hematopoietic effects.