# Retinol Binding Protein 4 Promotes Chondrocyte and Osteoclast Differentiation

## TL;DR

This study shows that RBP4, a metabolic protein, promotes cartilage and bone changes linked to osteoarthritis, especially in people with diabetes, and suggests it could be a new treatment target.

## Contribution

The study reveals a new role for RBP4 in regulating osteoclast pathways and joint tissue changes in metabolic conditions like diabetes.

## Key findings

- RBP4 levels increase in cartilage cells under inflammation and high glucose, linked to tissue breakdown.
- Inhibiting RBP4 reduces inflammatory responses and cartilage/bone remodeling markers in lab models.
- RBP4 inhibition also decreases osteoclast differentiation markers in macrophage cells.

## Abstract

This study investigated whether the metabolic protein RBP4 contributes to inflammation and joint tissue changes linked to osteoarthritis, particularly in people with type 2 diabetes. Laboratory experiments showed that inflammation and high glucose conditions increased RBP4 levels in cartilage-related cells along with markers of tissue breakdown. Inhibiting RBP4 reduced inflammatory responses and changes associated with cartilage and bone remodeling. These findings suggest that RBP4 could be a future target for preventing or slowing osteoarthritis, especially in people with diabetes.

Retinol-binding protein 4 (RBP4), an adipokine secreted by adipose tissues, has been implicated in metabolic inflammation and insulin resistance. Type 2 diabetes (T2D) is a recognized risk factor for osteoarthritis, with both conditions characterized by chronic low-grade inflammation, suggesting potential links between metabolic disorder and joint degeneration. This study aimed to investigate whether inflammatory and metabolic stresses regulate RBP4 expression and function in joint-related cells. Murine immature chondrocyte cells (iMACs) and the mouse AT805 teratocarcinoma cell line, clone 5, that differentiates into chondrogenic cells (ATDC5), were used as in vitro models for chondrocyte cells. Rbp4 mRNA expression increased during differentiation of iMACs, with 3.6- and 2.2-fold elevations observed on days 7 and 14, respectively (p < 0.01 vs. undifferentiated controls). Inflammatory stimulation with interleukin-6 (IL-6) significantly increased Rbp4 mRNA expression in ATDC5 cells (p < 0.05 vs. vehicle), along with elevated expression of catabolic and inflammatory mediators, including monocyte chemoattractant protein-1 (Mcp1), cyclooxygenase-2 (Cox2), and matrix metalloproteinase-3 (Mmp3) (p < 0.05 vs. vehicle). Pharmacological inhibition of RBP4 using fenretinide (FEN) attenuated chondrogenic differentiation marker expression, reduced glycosaminoglycan synthesis during chondrogenic differentiation, and mitigated high-glucose-induced catabolic responses, as indicated by reduced Mcp2 (p = 0.04) and Mmp13 (p = 0.01) expression in ATDC5 cells treated with FEN compared with cells treated with the vehicle under high-glucose conditions. Furthermore, in RAW 264.7 cells, a murine macrophage cell line commonly used as an in vitro model for osteoclastogenesis, FEN significantly reduced the expression of osteoclast differentiation markers, dendritic cell-specific transmembrane protein (DC-Stamp), nuclear factor of activated T-cells, cytoplasmic 1 (Nf-atc1), cathepsin k (Cath.k), and tartrate-resistant acid phosphatase (Trap) under osteoclastogenic conditions (p < 0.01 vs. vehicle). Collectively, these findings suggest that RBP4 functions as a metabolic–inflammatory mediator influencing both cartilage and bone-remodeling processes. This study reveals a previously unrecognized role of RBP4 in regulating osteoclast-associated pathways. Targeting RBP4 may, therefore, represent a promising therapeutic strategy for delaying or preventing osteoarthritis progression, particularly in metabolically compromised conditions.