SPP1 and obesity due to melanocortin 4 receptor deficiency: Concentrations of serum MMP-2 (Derosa et al. 2008) and OPN are increased in patients with obesity and type 2 diabetes mellitus (T2D) (Daniele et al. 2014), while OPN has been implicated in mediating the relationship between inflammation, obesity and diabetes (Kahles et al. 2014).