Transforming Growth Factor 18: Is It a Downregulator of Stem Cell Inhibition by Macrophage Inflammatory Protein lcr

Transforming growth factor fll (TGF-/30 and macrophage inflammatory protein lot (MIP-lot) have recently been identified as potent inhibitors of hemopoietic stem cell proliferation. From previous studies, these molecules appear to have similar functions in the control of stem cell proliferation. This study was designed to investigate the relationship, if any, between these two negative regulators in an attempt to elucidate possible distinctive roles for each within the hemopoietic system. We report here that both MIP-la and TGF-/3 are capable of inhibiting the same stem cell population (colony-forming unit [CFU]-A/CFU-S) with similar potencies. We further show that TGF-fl potently inhibits MIP-lot gene expression in bone marrow-derived macrophages, the presumed source of MIP-lt~ in the bone marrow. This inhibition is not specific to MIP-Iot in that expression of MIP-lfi, a related molecule that does not exhibit potent stem cell inhibitory properties, is inhibited in a similar manner. The inhibition of MIP-lot gene expression is also seen as a reduction in MIP-lc~ protein production, which markedly decreases 24 h after treating RAW 264.7 cells, a murine macrophage cell line, with TGF-fl. These in vitro results suggest that in the presence of active TGF-fi in vivo, and in the absence of upregulators of MIPlot transcription, very little MIP-lot will be produced. To address how MIP-I&s target cells, the stem cells, would respond to TGF-fi, and the consequently low levels of MIP-lot produced, we analyzed the effect of TGF-fi on MIP-lc~ receptor levels on FDCP-MIX cells, a murine stem cell line. We show that TGF-fl (100 pM) reversibly downregulates MIP-lo~ receptor levels on these cells to a maximum of 50-70% after 24 h. This level of downregulation does not change upon increasing the concentration of TGF-~ or the length of exposure of the cells to TGF-fl. Scatchard analysis shows that TGF-fl downregulates MIP-lot receptor numbers with no change in affinity of the remaining receptors. These results suggest that TGF-~ may be capable of interfering with MIP-I&s role as a stem cell inhibitor. Indeed, they suggest that in the presence of active TGF-fl in vivo, MIP-lot is at best a weak contributor to the overall physiological inhibition of stem cells.