Skeletal development is a complex event that requires a delicate balance between bone formation and bone resorption. Multiple transcription factors expressed in the bone-forming cells, osteoblasts, play crucial roles during the process of bone formation. Among them, ATF4 (Activating Transcription Factor 4) is a basic domain-leucine zipper transcriptional activator that is responsible for osteoblast differentiation, osteoblast-specific genes expression, synthesis of type I collagen, and osteoclast differentiation. Mice deficient for ATF4 are runted and exhibit severe skeletal dysplasia. Our laboratory has discovered Factor Inhibiting ATF4-mediated Transcription (FIAT), whose name was coined for its interaction with ATF4 and subsequent repression of ATF4-mediated osteocalcin gene transcription. FIAT is a leucine zipper nuclear molecule lacking a basic domain for DNA binding. We hypothesize that FIAT suppresses the bone-forming activities of osteoblasts by interacting with ATF4 and thereby blocking ATF4 attachment to the DNA to mediate downstream signalling pathways. To prove this hypothesis, we monitored the expression profiles of FIAT in parallel with ATF4 during osteoblastogenesis. Mechanism of FIAT repression of ATF4 was investigated through structure-function and mutation analysis. The physiological significance of FIAT expression in osteoblasts was studied through silencing FIAT in osteoblasts by RNA interference, as well as through characterization of two genetic mouse models: FIAT transgenic mice which overexpress FIAT in osteoblasts, and osteoblast-specific FIAT knockout mice. These studies showed that FIAT and ATF4 are co-expressed in osteoblasts, and that FIAT inhibition of matrix mineralization requires dimerization with ATF4 through the second leucine zipper. Furthermore, transgenic mice overexpressing FIAT exhibited osteopenia whereas FIAT knockout mice showed enhanced bone formation. These results support our hypothesis and demonstrate that FIAT is a key transcriptional repressor that modulates osteoblast function.

No description available.