Schematic representation showing proposed role of TMPRSS6 regulation by the BMP6-SMAD signaling pathway and iron via ID1. We propose that, in addition to being stimulated by several signals that inhibit hepcidin, such as iron deficiency, erythropoeitic drive, and hypoxia, TMPRSS6 expression is also stimulated indirectly by the hepcidin activators BMP6 and iron. Stimulation by BMP6 and/or iron induces an increase of the BMP6-HJV-SMAD pathway activity, possibly through a mechanism involving HFE and transferrin receptor 2 (TFR2), leading to binding of SMAD complexes to BMP-responsive elements (BMP-REs) on the hepcidin promoter and up-regulation of hepcidin transcription. In parallel, BMP6-SMAD pathway directly up-regulates SMAD7 and ID1 transcription. ID1 induction leads to the up-regulation of TMPRSS6 expression. TMPRSS6 then serves as a negative feedback inhibitor of BMP-SMAD pathway activity and hepcidin expression by cleaving the BMP coreceptor HJV. Inhibitory SMAD7 can also act as a negative feedback inhibitor by blocking SMAD activation. By acting as negative feedback inhibitors, TMPRSS6 and SMAD7 are important to prevent excessive hepcidin increases in response to BMP6 and iron, thereby maintaining tight control of iron homeostasis. Abbreviations: BMPR indicates BMP receptor; TF-Fe, holotransferrin; sHJV, soluble hemojuvelin; and BMP-RE, BMP responsive element.