Antibodies to cyclic citrullinated peptide and angiopoietin-like protein type 4 as markers of immune inflammation and osteoporotic processes in rheumatoid arthritis patients

Low-energy fractures in rheumatoid arthritis (RA) are more common in patients with high activity and long duration of disease, and with high titers of anti-citrullinated antibodies (ACPA). Increased expression of angiopoietin-like protein type 4 (ANGPTL4) in bone tissue has also been noted in inflammatory arthritis. The purpose of the present study was to analyze the effect of ACPA and ANGPTL4 on systemic bone mineral density in RA patients. Antibodies to ACPA and ANGPTL4 content were detected in blood serum of 96 RA patients (women, 91.7%) by enzyme immunoassay. Mineral density of the lumbar vertebrae (BMD L1-L4 ), hip neck, and entire femur (BMD total ) was measured by dual-energy X-ray absorptiometry (DXA). In study group, the ACPA and ANGPTL4 tests were positive in 61.5% and 41.7% of patients, respectively. Negative correlations were shown between ACPA and BMD total , and of ANGPTL4 with BMD L1-L4 (p < 0.05). Separation of the patients into groups with low (n = 34) and high (n = 62) DAS28 activity demonstrated a significant increase in ACPA with increasing RA activity (p = 0.042). ACPA and ANGPTL4 scores were also significantly higher in the group of 45 RA patients with osteoporosis (OP) compared to the RA group without OP (n = 51) showing significant difference at p = 0.002 and p = 0.028, respectively. Patients’ age, body mass index (BMI), duration and activity of the disease had no significant effect on ACPA in the general group of RA patients. However, the correlation between ACPA and DAS28 proved to be significant in the group of RA patients with OP (b = 0.31, p = 0.039). Among all presented variables, the disease duration was the only significant factor for ANGPTL4 in the total group of RA patients (b = 0.31, p = 0.039). In the regression model, BMD total showed similar correlations with patients’ age (b = -0.28), BMI (b = 0.25), and ACPA level (b = -0.26). A search for association between BMD L1-L4 and various RA characteristics demonstrated a strong correlation with ANGPTL4 only (b = -0.74; R 2 = 0.57). The revealed correlation between ANGPTL4 and decreased BMD specifically in the spongy layer of bone allows us to identify the RA patients with high ANGPTL4 levels as a risk group specifically for spinal fractures thus considering ANGPTL4 as a potential target for treatment of osteoporotic disorders.