Influence of PI-3K/Akt pathway on Wnt signalling in regulating myeloid progenitor cell proliferation. Evidence for a role of autocrine/paracrine Wnt regulation.

The regulation of myeloid progenitor cell (granulocyte-macrophage colony-forming units, CFU-GM) proliferation/differentiation by the Wnt and phosphatidylinositol-3 kinase (PI-3K) pathways was investigated using a colony-replating assay. The PI-3K pathway promoted differentiation of interleukin-3 (IL-3)-stimulated myelopoiesis via Akt, because inhibition of the PI-3K/Akt pathway with LY294002 or SH-5 increased proliferation. The involvement of canonical and non-canonical Wnt pathways was investigated using Wnt3a and Wnt5a respectively. Addition of the recombinant Wnts to IL-3 increased CFU-GM proliferation. Dkk-1, when combined with the Wnt proteins, abrogated the effects of Wnt3a but not Wnt5a. Surprisingly, the addition of Dkk-1 to LY294002 or SH-5 blocked their proliferative effects. We hypothesized that increased proliferation induced by PI-3K/Akt inhibitors was not mediated by downstream activation of the Wnt pathway but by induced endogenous production/release of Wnt proteins. The addition of SH-5 to IL-3 created an autocrine Wnt loop in CD34(+) cells, resulting in the phosphorylation of lipoprotein-receptor-related-protein 6. Furthermore, the addition of medium conditioned by CD34(+) cells cultured in IL-3 + SH-5 to IL-3 increased CFU-GM proliferation. This effect was abrogated by Dkk-1, suggesting that a Wnt in the conditioned medium increased proliferation. In summary, IL-3 via the PI-3K pathway promoted differentiation of myeloid progenitor cells through a decrease of endogenous Wnt production/release.