SFRP1 Is Estrogen Inducible in Bone Marrow Stromal Cells and Suppresses the Earliest Events in Lymphopoiesis.

It has long been known that lymphopoiesis is transiently suppressed during pregnancy, and that this can be experimentally simulated by treatment with estrogen. Indeed, sensitivity to estrogen made it possible to identify very primitive lymphoid cells in the Lin- c-kitHi Sca-1+ fraction that is enriched with hematopoietic stem cells (HSC) and multipotent progenitors of mouse bone marrow. However, analyses showing how the earliest events in lymphopoiesis is suppressed in those circumstances have not been performed. Hematopoietic and environmental cells are both potential hormone targets and, because of this complexity, very little has been known regarding mechanisms. In the present study, we performed high resolution analysis using RAG-1/GFP reporter mice and confirmed that early lymphoid progenitors (ELP) in the Lin-c-kitHi Sca-1+ fraction are particularly depressed in pregnancy or after estrogen injection. The Lin- c-kitLo fraction containing common lymphoid progenitors (CLP)/pro-lymphocytes was very sensitive to estrogen in stroma-free serum-free culture, but earlier stages of lymphopoiesis seemed to be blocked in vivo. Thus, we focused on identifying mechanisms involving bone marrow environment. Two independent strategies with macroarrays or differential display-PCR were used to isolate stromal cell genes that were both estrogen regulated and associated with inability to support B lymphopoiesis. We have identified secreted frizzled related protein 1 (sFRP1) as an estrogen inducible gene, and its expression corresponded to inability to support lymphopoiesis. This member of the complex Wnt family has been previously described as both an agonist and an antagonist of canonical, β-catenin dependent signaling. We found that sFRP1, like recombinant Wnt3a, stimulated the canonical pathway in the Lin- c-kitHi Sca-1+ fraction, and blocked progression in the B lymphocyte lineage even when exogenous Wnt ligands were not provided. The suppressive effects of sFRP1 and Wnt3a on B lymphopoiesis were canceled out when both were present simultaneously. To better understand the vulnerability of early progenitors to the Wnt signaling, we examined the expression pattern of Frizzled receptors. Interestingly, the HSC-enriched fraction (Lin- c-kitHi Sca-1+ IL-7Rα-RAG-1/GFP-) highly expressed 7 out of 9 Frizzled receptors, which markedly declined as they differentiated to ELP (Lin- c-kitHi Sca-1+ IL-7Rα- RAG-1/GFP+) and CLP (Lin- c-kitLo Sca-1Lo/- IL-7Rα+). Myelo-erythroid progenitors were less affected by sFRP1 in culture, suggesting that it is similar to estrogen with respect to lineage specificity. Bone-lining stromal cells express sFRP1 and the transcripts were elevated in bone marrow by pregnancy or estrogen injection. In summary, these observations implicate sFRP1 as a possible mediator in hormone regulation of the earliest events in lymphopoiesis.