BRYOSTATIN-1 ACTS SYNERGISTICALLY WITH INTERLEUKIN-1-ALPHA TO INDUCE SECRETION OF G-CSF AND OTHER CYTOKINES FROM MARROW STROMAL CELLS

The protein kinase C (PKC) activator bryostatin 1 (bryo) has substanti al 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 b ryo on the expression and release of certain myeloid growth factors fr om fibroblastlike marrow stromal cells (MSG). Substantial release of g ranulocyte colony-stimulating factor (G-CSF), granulocyte-macrophage c olony-stimulating factor (GM-CSF), or interleukin-6 (IL-6) following b ryo treatment was seen only in MSC cultures contaminated with macropha ges. Bryo alone was ineffective in inducing release of the cytokines f rom MSC cultures containing only fibroblastlike stromal cells. When MS C were treated with IL-1 alpha, substantial quantities of the cytokine s (G-CSF, GM-CSF, IL-6) were released. Bryo acted synergistically with IL-1 alpha to significantly increase cytokine release two- to nine-fo ld compared to IL-1 alpha alone (p < 0.016). Neither IL-1 alpha nor br yo, alone or in combination, induced release of stem cell factor (SCF) from MSG. The synergistic interaction between IL-1 alpha and bryo was dose- and schedule-dependent, requiring simultaneous application of I L-1 alpha and bryo for optimum effect. Bryo alone induced no G-CSF mRN A accumulation but increased the level seen with IL-1 alpha treatment by 50%. The synergistic interaction of bryo and IL-1 alpha required PK C, since it was antagonized by agents which depleted or inhibited PKC but not by a protein kinase A antagonist. The increase in G-CSF mRNA w as associated with a marked increase in mRNA stability. Bryostatin may promote the release of cytokines from several accessory cell populati ons, including MSC, to accomplish its in vivo hematopoietic effects.