Altered Expression of Cell Cycle Regulators and Factors Released by Aged Cells in Skeletal Muscle of Patients with Bone Fragility: A Pilot Study on the Potential Role of SIRT1 in Muscle Atrophy

<b>Background/Objectives</b>: Cellular aging represents a crucial element in the progression of musculoskeletal diseases, contributing to muscle atrophy, functional decline, and alterations in bone turnover, which promote fragility fractures. However, knowledge about expression patterns of factors potentially involved in aging and senescence at the tissue level remains limited. Our pilot study aimed to characterize the expression profile of cell cycle regulators, factors released by aged cells, and sirtuin 1 (SIRT1) in the muscle tissue of 26 elderly patients undergoing hip arthroplasty, including 13 with low-energy fracture and 13 with osteoarthritis (OA). <b>Methods</b>: The mRNA expression levels of cyclin-dependent kinase inhibitor 1A (<i>CDKN1A</i>), cyclin-dependent kinase inhibitor 1B (<i>CDKN1B</i>), cyclin-dependent kinase inhibitor 2A (<i>CDKN2A</i>), <i>p53</i>, tumor necrosis factor alpha (<i>TNF-α</i>), interleukin-1 beta <i>(IL-1β</i>), interleukin-6 (<i>IL-6</i>), interleukin-15 (<i>IL-15</i>), chemokine (C-C motif) ligand 2 (<i>CCL2</i>), chemokine (C-C motif) ligand 3 (<i>CCL3</i>), growth differentiation factor 15 (<i>GDF15</i>), and <i>SIRT1</i> were evaluated in muscle tissue by qRT-PCR. In addition, immunohistochemistry and Western blotting analysis were conducted to measure the protein levels of SIRT1. <b>Results</b>: A marked muscle atrophy was observed in fractured patients compared to the OA group, in association with an up-regulation of cell cycle regulators and factors released by the aged cells. The expression of matrix metallopeptidase 3 (<i>MMP3</i>), plasminogen activator inhibitor 1 (<i>PAI-1</i>), and fas cell surface death receptor (<i>FAS</i>) was also investigated, although no significant differences were observed between the two experimental groups. Notably, SIRT1 expression was significantly higher in OA patients, confirming its role in maintaining muscle health during aging. <b>Conclusions</b>: Further studies will be needed to clarify the role of SIRT1 in the senescence characteristic of age-related musculoskeletal disorders, counteracting the muscle atrophy that predisposes to fragility fractures.