FIAT inhibition increases osteoblast activity by modulating Atf4-dependent functions

Area of Science:

• Molecular Biology
• Bone Biology
• Cellular Biology

Background:

• The transcription factor ATF4 regulates osteoblast differentiation, controlling key genes like osteocalcin.
• Factor-inhibiting ATF4-mediated transcription (FIAT) is identified as a protein that inhibits ATF4 activity by forming inactive dimers.
• Overexpression of FIAT in mice leads to osteopenia, indicating its inhibitory role in bone formation.

Purpose of the Study:

• To investigate the hypothesis that inhibiting FIAT enhances ATF4 activity.
• To determine if FIAT inhibition increases osteocalcin transcription, type I collagen synthesis, and mineralization.
• To evaluate the therapeutic potential of targeting FIAT in osteoblast function.

Main Methods:

• Utilized small interfering RNAs (siRNA) to knockdown FIAT expression in MC3T3-E1 cells.
• Employed a luciferase reporter assay driven by the mouse osteocalcin gene promoter to measure transcriptional activity.
• Assessed endogenous osteocalcin gene expression, bone sialoprotein expression, type I collagen synthesis, and mineralization in primary osteoblasts.

Main Results:

• FIAT siRNA successfully inhibited FIAT expression without altering total or phosphorylated ATF4 levels.
• Knockdown of FIAT led to increased ATF4 binding to the osteocalcin promoter.
• FIAT inhibition resulted in earlier onset and increased osteocalcin promoter activity, enhanced endogenous osteocalcin expression, increased bone sialoprotein and type I collagen synthesis, and improved mineralization.

Conclusions:

• Inhibition of FIAT expression significantly enhances ATF4-mediated transcription of osteocalcin.
• FIAT plays a crucial role in regulating osteoblast differentiation and function.
• Targeting FIAT represents a potential strategy to promote bone formation and treat bone loss disorders.