The role of inducible cAMP early repressor (ICER) in bone

Inducible cAMP early repressor (ICER) is a member of the CREM transcription factor family and is a dominant negative regulator of cAMP-mediated transcription. PTH has been shown to induce ICER expression potently and transiently in cultured osteoblasts and in calvariae in vivo. To establish the role of ICER in bone, we previously developed transgenic mice overexpressing ICER in vivo by cloning ICER cDNA with an N-terminal FLAG epitope downstream of a 3.6 kb fragment of the rat Col1a1 promoter (pOBCol3.6-ICER), targeting ICER overexpression to cells of the osteoblast lineage. ICER transgenic mice showed reduced body size and weight, a dramatic reduction in trabecular bone parameters and disruption of cortical bone integrity. Serum osteocalcin from ICER transgenic mice was dramatically reduced compared to wild type littermates. Femurs showed a marked reduction in osteocalcin (OC) expression while Col1a1 and bone sialoprotein (BSP) expression was less affected. ^ Ex vivo analysis of osteogenic bone marrow stromal cell cultures showed reduced expression of Col1a1, BSP and OC as well as reduced mineralization. In addition, studies of bone marrow stromal cells cultured with RANKL and m-CSF showed an increase in osteoclastogenesis in ICER transgenic cultures compared to wild type cells. Therefore, the low bone mass phenotype seen in ICER transgenic mice is likely due to an uncoupling of bone remodeling involving both osteoblasts and osteoclasts. ^ The loss of osteocalcin gene expression suggested that ICER may regulate its expression in osteoblasts. Activating transcription factor-4 (ATF4) has been shown to induce osteocalcin expression at the OSE1 site. To determine if ICER regulated ATF4-induced osteocalcin gene expression, MC3T3-E1 cells were transfected with a promoter-reporter construct containing four multimerized OSE1 sites cloned into the pGL3-basic vector (4OSE1-Luc). Cells were co-transfected with pCR3.1-ICER and/or pCMV-ATF4 expression. ATF4 induced 4OSE1-Luc activity 30-fold over control. ICER DNA was shown to potently repressed ATF4 induction at the OSE1 site. ^ We conclude that ICER overexpression in osteoblasts results in impaired osteoblast differentiation and function with a concomitant increase in osteoclastogenesis leading to a low bone mass phenotype in ICER transgenic mice. Furthermore, ICER represses osteocalcin gene expression likely due to disruption of ATF4 function.