Transforming growth factor β1 (TGF-β1) regulates a wide variety of events in adult bone marrow (BM), including quiescence of hematopoietic stem cells, via undefined mechanisms. Because megakaryocytes (MKs)/platelets are a rich source of TGF-β1, we assessed whether TGF-β1 might inhibit its own production by comparing mice with conditional inactivation of Tgfb1 in MKs (PF4Cre;Tgfb1flox/flox) and control mice. PF4Cre;Tgfb1flox/flox mice had ∼30% more MKs in BM and ∼15% more circulating platelets than control mice (P < .001). Thrombopoietin (TPO) levels in plasma and TPO expression in liver were approximately twofold higher in PF4Cre;Tgfb1flox/flox than in control mice (P < .01), whereas TPO expression in BM cells was similar between these mice. In BM cell culture, TPO treatment increased the number of MKs from wild-type mice by approximately threefold, which increased approximately twofold further in the presence of a TGF-β1-neutralizing antibody and increased the number of MKs from PF4Cre;Tgfb1flox/flox mice approximately fourfold. Our data reveal a new role for TGF-β1 produced by MKs/platelets in regulating its own production in BM via increased TPO production in the liver. Additional studies are required to determine the mechanism.
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