Dottorato in SCIENZE ENDOCRINOLOGICHE

Titolo della tesi: Growth Differentiation Factor 15 As A Novel Biomarker In Type 2 Diabetes And The Effect Of Exercise Training

Background: The identification of patients with T2D diabetes at increased risk of fracture is challenging because they usually have normal or high bone mass associated with low bone turnover. Therefore, we explored the use of novel biomarkers that could help in the evaluation of bone health in this setting and focused our attention on GDF15, a marker of cellular senescence, which is associated with several age-related diseases, such as T2D. Aim of the study: Evaluate whether GDF15 could have a role in bone metabolism and fracture risk in people with T2D, and whether it could be modulated by exercise training, which is known to exert beneficial effects on the skeleton. Methods: Serum GDF15 was measured using an ELISA (R&D Systems). In study protocol n. 1, we measured GDF15, bone density (BMD by DXA), bone turnover markers (BTMs) and HRpQCT in 180 non-diabetic individuals, divided into three age groups (16-18, 30-32, and over 70 years), each consisting of 30 women and 30 men. In study protocol n. 2, we investigated the relationship of GDF15 with the risk of hip fracture in 1174 community-dwelling older adults (age 70-79 years) participating in the Health ABC study. We measured: BTMs, glucose metabolism and kidney function, inflammation markers, BMD by DXA, muscle strength (handgrip), muscle mass and physical performance (gait speed, standing balance test time). In study protocol n. 3, we assessed GDF15 levels in 64 trained (following a specific 2-year exercise training program), 54 untrained (following usual care) people with T2D and 48 non-diabetic controls. We measured: BTMs, glucose and kidney metabolism, bone outcomes (BMD by DXA scan, pQCT at radius and tibia, calcaneal QUS), body composition (total-body DXA and bioimpedance scale), muscle function (muscle mass, muscle strength, physical performance by SPPB, physical fitness, muscle quality), falls assessments. A longitudinal analysis was also performed in a small group of patients with T2D. Statistical analyses were performed using SPSS version 27.0 (SPSS Inc., Chicago, Illinois, USA). A critical p value < 0.05 was considered to indicate statistical significance. Results: The first study confirmed that GDF15 significantly increases with age, with the highest levels observed in older men. In the second study, we found that increasing GDF15 levels were associated with an increased risk of hip fracture in older adults. This relationship seemed to be mediated by muscle strength. In both studies, we did not find any relevant associations between GDF15 and bone outcomes. In the third analysis, GDF15 was higher in people with T2D compared to controls, with the highest levels observed in the untrained group, and negatively associated with muscle strength and physical performance. In particular, exercise training in people with T2D was associated with a reduction of GDF15, which in turn correlated with better physical performance, physical fitness, body composition and kidney function. These results were supported by the longitudinal analysis, which demonstrated a significant decrease in GDF15 levels along with an improvement in physical fitness, physical performance, and kidney function after the 2-year exercise training program. In the untrained group instead, a significant GDF15 increase was observed at the end of follow-up, a trend which is not unexpected in ageing persons. Conclusions: Our findings support the hypothesis that exercise training counteracts the ageing process in T2D and that GDF15 may act as a mediator of this response. In our study, due to the cross-sectional design and the low number of fractures, we were not able to determine whether GDF15 could be significantly related to fracture risk in T2D. However, we could speculate that GDF15 might influence fracture risk by exerting its actions on muscle function.