Role of TNF alpha in bryostatin-induced inhibition of human hematopoiesis.
In previous studies bryostatin has been shown to cause dose-dependent stimulatory or inhibitory effects on colony formation in acute myeloid leukemias. In this study we investigated the inhibitory effect of high dose bryostatin-1 (bryo-1) on normal human bone marrow mononuclear cells (BMNC) colony-forming capacity. Preculturing BMNC for 24 h with 250 nM bryo-1 reduced colony formation by 66 +/- 11% whereas this treatment did not reduce clonogenic capacity of highly purified CD34+ BMNC. When precultures with bryo-1 were performed in the presence of several cytokine neutralizing antibodies abrogation of the inhibitory effect could only be demonstrated by anti-TNF alpha. However, preculturing of BMNC or CD34+ cells with a wide range of TNF alpha concentrations as well as TNF alpha neutralization in supernatant of bryo-1-stimulated BMNC failed to affect the inhibitory effect on CD34+ cells. Both indicate that TNF alpha was not the only factor responsible for the inhibitory effect. Depletion of CD14+ cells from BMNC cultures showed that upon bryo-1 exposure the monocytes served as the main source of TNF alpha but not as a source of the inhibitory cytokine(s): in CD14+-depleted cultures the combination of exogenous added TNF alpha and bryo-1 resulted in an inhibition of colony formation comparable to that found in crude BMNC. In contrast, purified CD34+ cultures were not directly affected by bryo-1 and TNF alpha. However, clonogenic growth of purified CD34+ cells was inhibited if mononuclear cells were preincubated with TNF alpha alone for 24 h, and the supernatant of these cultures was used together with bryo-1. These results show that bryo-1-induced inhibition of clonogenic cell growth is not mediated by a direct effect of bryo-1 on CD34 cells but is a result of a process involving production of TNF alpha by CD14+ cells upon bryo-1 stimulation together with the induction of (a) secondary factor(s) by TNF alpha, which together with bryo-1 itself is inhibitory towards clonogenic cell growth.