Protein Markers in Osteoporosis.

Osteoporosis is a systemic skeletal disease characterized by low bone mass and microarchitectural deterioration of bone tissue. Biomarkers of bone turnover have been used for years in bone disease management, especially to determine response to treatment. They are substances found in biological fluids, produced during the bone remodelling process. Recently, new approaches for the detection of bone physiology and pathology biomarkers have been proposed, among which proteomics, with particular interest in osteoporosis. The objective of this manuscript is to review current knowledge on proteomics applied to osteoporosis in vivo. The analysis of the 14 studies published to date showed a range of proteins whose expression is altered in patients with osteoporosis. The relatively small number of papers depends mainly on high costs and technical limitations; due to the difficulty to collect osteoclasts, most of the studies performed proteomics on peripheral blood monocytes (PBMs), already accepted as an excellent osteoporosis cell model in vivo. Among the identified proteins, the most promising are represented by Gelsolin (GSN), Annexin A2 (ANXA2), and Prolyl 4-hydroxylase (P4HB). They have been related to bone mineral density (BMD), sometimes in apparent disagreement (some upregulated and others downregulated in patients with low BMD). Finally, worthy of mention is the application of proteomics in the emerging field of microvesicles (MVs); they are important messengers, widely present in body fluids, and have recently emerged as novel targets for the diagnosis of multiple diseases, among which musculoskeletal diseases. In conclusion, the proteomic field is relatively novel in osteoporosis and has a considerable but theoretical potential; further investigations are needed in order to make proteome-derived markers applicable to clinical practice.