Iron regulation mechanisms of bone morphogenetic protein pathway in tissue and plasma

Koštani morfogenetski protein 6 (BMP6) je ključni regulator hepcidina, proteina koji sudjeluje u održavanju homeostaze željeza. Istražili smo utjecaj željeza na BMP6 u tkivima i plazmi, te zašto ostale BMP molekule nisu u mogućnosti spriječiti hematokromatozu u Bmp6 -/- miševa. Kod miševa divljeg tipa, 4 sata nakon tretmana željezom dolazi do povećanja izražaja gena za BMP6 i hepcidin u jetri, dok je izražaj ostalih BMP molekula nepromijenjen. BMP6 je detektiran u cirkulaciji miševa i štakora, te je pružen prvi dokaz utjecaja željeza na povećanje razine BMP6 u plazmi. Naprotiv, u Bmp6 -/- miševa, tretman željezom dovodi do zakašnjele i slabije aktivacije Smad signalnog puta i sinteze hepcidina u jetri nakon 24 sata, zajedno s povećanim izražajem gena za BMP2, 4, 5 i 9 u duodenumu i BMP5 i BMP7 u jetri. Povećana sinteza BMP7 u jetri potencijalno doprinosi indukciji izražaja gena za hepcidin, što je dodatno podržano BMP7 terapijom koja dovodi do djelotvorne sinteze hepcidina i normalizacije serumskog željeza. No, sinteza BMP7 nije povezana s metabolizmom željeza, već s uzročnim oštećenjima jetre gdje BMP7 predstavlja mogući oblik obrambenog mehanizma od viška nakupljenog željeza. Rezultati pokazuju da akutna doza željeza aktivira kompenzacijske mehanizme s različitim BMP molekulama, ali nedovoljnog intenziteta da bi se spriječila hematokromatoza u Bmp6 -/- miševa. Učinkovita terapija s BMP7 sugerira da je BMP6 bitan, ali ne isključiv regulator homeostaze željeza in vivo.Bone morphogenetic protein 6 (BMP6) is a key regulator of hepcidin expression, a key iron regulatory hormone. We investigated the effect of iron on Bmp6 expression in tissues and plasma, and why other BMPs could not prevent hemochromatosis in Bmp6 -/- mice. In WT mice, 4 hours following iron challenge liver Bmp6 and Hamp expression were increased, while expression of other Bmps was not affected. In parallel, we provided the first evidence that iron increased the circulating BMP6 level. On the contrary, in Bmp6 -/- mice, iron challenge lead to a 24 hours delayed and blunted activation of liver Smad signaling and hepcidin expression, that was accompanied by an increased expression of Bmp2, 4, 5 and 9 in the duodenum and Bmp5 and Bmp7 in the liver. High liver Bmp7 expression potentially contributed to the late hepcidin response, which was further supported by BMP7 therapy resulting in an effective hepcidin expression and normalization of serum iron. However, BMP7 synthesis was not associated with iron metabolism, but the causal hepatic injuries where BMP7 represented a form of defense mechanism to the excessive iron accumulation. Our data indicate that iron activated BMP compensatory mechanisms that were not sufficient for preventing hemochromatosis in Bmp6 -/- mice. However, the effective BMP7 therapy suggested that BMP6 is an essential but not exclusive in vivo regulator of iron homeostasis