# Osteocytes regulate osteoprotegerin expression via the p38-MAPK-CREB pathway in rheumatoid arthritis

## TL;DR

This study shows that osteocytes in rheumatoid arthritis increase OPG production through a specific signaling pathway, which may help explain bone loss in the disease.

## Contribution

The study identifies a novel p38-MAPK-CREB pathway in osteocytes that upregulates OPG under inflammatory conditions in rheumatoid arthritis.

## Key findings

- RA patients have higher OPG levels in femoral neck cortical bone compared to OA patients.
- TNF-α activates p38-MAPK and CREB signaling to increase OPG expression in osteocytes.
- Osteocyte-derived factors modulate osteoclast maturation, with OPG playing a key role.

## Abstract

Rheumatoid arthritis (RA) is accompanied by systemic and local bone loss, yet the regulation of osteoclast-modulating factors within human osteocytes under inflammatory conditions remains incompletely understood. As an exploratory study, we analyzed surgically resected femoral heads from patients with RA and osteoarthritis (OA). Micro-CT of standardized cancellous regions showed no significant differences in trabecular microarchitectural parameters between groups. In contrast, immunoblot analysis of cortical bone from the femoral neck demonstrated higher osteoprotegerin (OPG) protein levels in RA than in OA. To investigate the underlying mechanism, we examined osteocytes under inflammatory stimulation using the mouse osteocyte-like cell line MLO-Y4 and osteocyte-enriched bone fractions (OEBFs). Among tested cytokines, TNF-α most strongly induced osteoclast-regulatory transcripts and increased OPG protein levels and secretion. TNF-α activated p38-MAPK and cyclic adenosine monophosphate (AMP) response element-binding protein (CREB) signaling, and pharmacologic inhibition of either p38-MAPK activity or CREB activity reduced TNF-α-induced OPG protein accumulation. Chromatin immunoprecipitation (ChIP) further showed enhanced recruitment of CREB to a responsive region within the OPG promoter after TNF-α stimulation. Finally, osteocyte-conditioned medium altered osteoclast morphology and reduced expression of an osteoclast lineage marker during RANKL-driven differentiation, while OPG neutralization did not produce a dominant reversal of these effects. Together, these findings indicate that OPG is upregulated in femoral neck cortical bone from patients with RA and that TNF-α can enhance osteocytic OPG expression via a p38-MAPK-CREB axis, alongside additional osteocyte-derived factors that modulate osteoclast maturation. This study provides evidence from human femoral head specimens of increased OPG in RA cortical bone and suggests that the p38-MAPK-CREB axis may represent an underappreciated regulatory component of osteocyte-derived OPG under inflammatory conditions.

Graphical Abstract