MMP9 and obesity due to melanocortin 4 receptor deficiency: Level 3 confirmationa. Matrix metalloproteinases (MMPs) have multiple functions, including tissue remodeling in response to injury. MMP-9 is mainly expressed in lymphoid tissue, blood and adipose tissue and involved in several biological processes, including inflammation (Nagase et al., 2006). Circulating levels of MMP-9 are increased with obesity, metabolic syndrome (MetS) and cardiovascular disease (Hopps et al., 2016; Jaoude and Koh, 2016; Ritter et al., 2017).