Bone morphogenic protein antagonist Drm/gremlin is a novel proangiogenic factor

Gremlin-mediated decrease in bone morphogenetic protein signaling promotes pulmonary fibrosis

RATIONALE

Members of the transforming growth factor (TGF)-beta superfamily, including TGF-betas and bone morphogenetic proteins (BMPs), are essential for the maintenance of tissue homeostasis and regeneration after injury. We have observed that the BMP antagonist, gremlin, is highly up-regulated in idiopathic pulmonary fibrosis (IPF).

OBJECTIVES

To investigate the role of gremlin in the regulation of BMP signaling in pulmonary fibrosis.

METHODS

Progressive asbestos-induced fibrosis in the mouse was used as a model of human IPF. TGF-beta and BMP expression and signaling activities were measured from murine and human fibrotic lungs. The mechanism of gremlin induction was analyzed in cultured lung epithelial cells. In addition, the possible therapeutic role of gremlin inhibition was tested by administration of BMP-7 to mice after asbestos exposure.

MEASUREMENTS AND MAIN RESULTS

Gremlin mRNA levels were up-regulated in the asbestos-exposed mouse lungs, which is in agreement with the human IPF biopsy data. Down-regulation of BMP signaling was demonstrated by reduced levels of Smad1/5/8 and enhanced Smad2 phosphorylation in asbestos-treated lungs. Accordingly, analyses of cultured human bronchial epithelial cells indicated that asbestos-induced gremlin expression could be prevented by inhibitors of the TGF-beta receptor and also by inhibitors of the mitogen-activated protein kinase kinase/extracellular signal-regulated protein kinase pathways. BMP-7 treatment significantly reduced hydroxyproline contents in the asbestos-treated mice.

CONCLUSIONS

The TGF-beta and BMP signaling balance is important for lung regenerative events and is significantly perturbed in pulmonary fibrosis. Rescue of BMP signaling activity may represent a potential beneficial strategy for treating human pulmonary fibrosis.

Conditional deletion of gremlin causes a transient increase in bone formation and bone mass

Gremlin is a glycoprotein that binds bone morphogenetic proteins (BMPs) 2, 4, and 7, antagonizing their actions. Gremlin opposes BMP effects on osteoblastic differentiation and function in vitro and in vivo, and its overexpression causes osteopenia. To define the function of gremlin in the skeleton, we generated gremlin 1 (grem1) conditional null mice by mating mice where grem1 was flanked by lox(P) sequences with mice expressing the Cre recombinase under the control of the osteocalcin promoter. grem1 null male mice displayed increased trabecular bone volume due to enhanced osteoblastic activity, because mineral apposition and bone formation rates were increased. Osteoblast number and bone resorption were not altered. Marrow stromal cells from grem1 conditional null mice expressed higher levels of alkaline phosphatase activity. Gremlin down-regulation by RNA interference in ST-2 stromal and MC3T3 osteoblastic cells increased the BMP-2 stimulatory effect on alkaline phosphatase activity, on Smad 1/5/8 phosphorylation, and on the transactivation of the BMP/Smad reporter construct 12xSBE-Oc-pGL3. Gremlin down-regulation also enhanced osteocalcin and Runx-2 expression, Wnt 3a signaling, and activity in ST-2 cells. In conclusion, deletion of grem1 in the bone microenvironment results in sensitization of BMP signaling and activity and enhanced bone formation in vivo.