Molecular changes underlying pulmonary emphysema and chronic bronchitis in Chronic Obstructive Pulmonary Disease: An updated review

. The aim of this review is to update and synthesize the molecular mechanisms that lead to the heterogeneous effect on tissue remodeling observed in the two most important clinical phenotypes of chronic obstructive pulmonary disease (COPD), pulmonary emphysema (PE) and chronic bronchitis (CB). Clinical and experimental evidence suggests that this heterogeneous response to promote PE, CB, or both, is related to differentiated genetic, epigenetic, and molecular conditions. Specifically, a tendency toward PE could be related to a variant in the DSP gene, SIRT1 downregulation, macrophage polarization to M1, as well as the involvement of the noncanonical Wnt5A signaling pathway, among other alterations. Additionally, in advanced stages of COPD, PE development is potentiated by dysregulations in autophagy, which promotes senescence and subsequently cell apoptosis, through exacerbated inflammasome activation and release of caspases. On the other hand, CB or the profibrotic phenotype could be potentiated by the downregulated activity of HDAC2, the activation of the TGF-beta/Smad or Wnt/beta-catenin signaling pathways, macrophage polarization to M2, upregulation of TIMP1, and/or the presence of the epithelial-mesenchymal transition (EMT) mechanism. Interestingly, the upregulated activity of MMPs, especially MMP-9, is widely involved in the development of both phenotypes. Furthermore, MMP-9 and MMP-12 enhance the severity, perpetuation, and exacerbation of COPD, as well as the development of autoimmunity in this disease.