ATF4, the osteoblast accumulation of which is determined post-translationally, can induce osteoblast-specific gene expression in non-osteoblastic cells.

Based on the analysis of a loss-of-function model, we recently showed that ATF4 regulates osteoblast terminal differentiation and function and is implicated in the pathophysiology of Coffin-Lowry syndrome. That study, however, did not address whether forced expression of Atf4 in non-osteoblastic cells would lead to osteoblast-specific gene expression, one of the most important features of a cell differentiation factor. To address this question we searched for cell lines that would not express Atf4. Contrasting with the restricted pattern of its protein accumulation, Atf4 mRNA was found in all cell lines and mouse tissues tested. Treatment of non-osteoblastic cells with MG115, a proteasome inhibitor, induced ATF4 accumulation and resulted in activation of an Osteocalcin promoter luciferase construct as well as expression of endogenous Osteocalcin, a molecular marker of differentiated osteoblasts and a target gene of ATF4. Eliminating the expression of beta-TrCP1, an ubiquitin-protein isopeptide ligase interacting with ATF4 by RNA interference, led to ATF4 accumulation and to endogenous Osteocalcin expression in fibroblasts. These results indicate that the absence of ATF4 in most cell types is determined, at least in part, by an ubiquitination-dependent process. To our knowledge ATF4 is the first cell-specific transcription factor in which cell-specific distribution is achieved post-translationally. This study also establishes that ATF4, like other osteoblast differentiation factors, such as Runx2 and Osterix, has the ability to induce osteoblast-specific gene expression in non-osteoblastic cells.