Type 1 diabetes and lung diseases: Investigating the effects of hyperglycemia in the lung

Abstract

Type 1 diabetes (T1D) is a metabolic disease characterized by hyperglycemia, resulting from decreased insulin secretion by the pancreatic b cells. In the long term, T1D can affect multiple organs, and lead to life-threatening complications. Increased prevalence of pulmonary abnormalities and respiratory diseases is observed in T1D patients. However, the effects of T1D on the lung are not well defined and the exact mechanisms underlying diabetic complications in the lung remain elusive. The objective of the study was to determine whether T1D exacerbates the progression of lung damage in the presence of chronic lung diseases and to delineate the mechanisms leading to lung pathologies. We performed pulmonary function test (PFT) in streptozotocin (STZ)-induced T1D mouse model, which demonstrated a restrictive pulmonary pattern in the lungs of STZ-injected mice, corresponding to fibrotic changes in the lung. In line with this, lungs from STZ-injected mice showed an increase in collagen accumulation as well as in gene expression of the fibrotic markers Acta2 and Fn1. We studied the effect of T1D in two mouse models of emphysema: a cigarette smoke (CS) exposure model and an alpha-1 antitrypsin (AAT) deficiency model. CS exposure alone did not impair pulmonary function but increased collagen accumulation in the lung. CS-exposed STZ mice showed similar pulmonary function patterns and collagen deposition in the lung as room air (RA)-exposed STZ mice. STZ injection in AAT-deficient mice accelerated the accumulation of collagen and the development of emphysema in the lung. To determine the molecular mechanisms for fibrotic remodeling in STZ-injected mouse lungs, we performed RNA-sequencing of primary human bronchial epithelial cells (HBECs) cultured in physiologically normal (5 mM) and high (12.5 mM) glucose conditions. Pathways associated with metabolism, oxidative stress and cellular senescence were overexpressed in primary HBECs response to high glucose. We investigated these pathways to define mechanisms that may lead to high glucose-induced fibrotic remodeling in the lungs using HBE cell line cultured in normal (5 mM) and high (12.5 mM) glucose media. HBECs increased the uptake of glucose in response to high glucose treatment for 24 hours and showed a metabolic shift in which the excess glucose was routed towards the pentose phosphate pathway, lactate synthesis and glycogen synthesis. These metabolic shifts were not associated with changes in the cell proliferation rate of HBECs. In contrast to our RNA-seq data, oxidative stress and cellular senescence pathways were not altered in HBECs cultured in high glucose media. To explore whether HBECs cultured in high glucose media can directly activate fibroblasts, primary human lung fibroblasts were cultured in normal and high glucose media conditioned by HBECs. Fibroblasts cultured in high glucose media conditioned media showed an increase in the gene expression of COL1A1 and COL1A2 along with increased protein expression of a-SMA and COL1A1. Protein array using media conditioned by HBECs demonstrated high glucose treatment increased secretion of proteins associated with pulmonary fibrosis and ECM remodeling into the media by HBECs, suggesting a potential mechanism of high glucoseinduced pulmonary fibrosis. In this study, we demonstrated that STZ-injected mice exhibit functional, histological and gene expression changes in the lung that correspond to the restrictive pulmonary pattern. Our findings from the study suggest that T1D exacerbates the progression of lung damage in AAT deficiency by increasing collagen accumulation in the lung and accelerating the development of emphysema. Our in vitro study also showed that HBECs undergo metabolic reprogramming in high glucose conditions and that high glucose-exposed HBECs increase the secretion of profibrotic mediators to the media and are capable of activating fibroblasts. These data expand primary HBECs response to high glucose. We investigated these pathways to define mechanisms the existing knowledge on the mechanisms for hyperglycemia exerting deleterious effects on pulmonary function and reveal new comorbidities for AAT deficiency. In addition, this work presents new knowledge on the metabolic use of glucose by pulmonary epithelium and suggests potential mechanisms of high glucose-induced pathogenesis of pulmonary fibrosis. that may lead to high glucose-induced fibrotic remodeling in the lungs using HBE cell line cultured in normal (5 mM) and high (12.5 mM) glucose media. HBECs increased the uptake of glucose in response to high glucose treatment for 24 hours and showed a metabolic shift in which the excess glucose was routed towards the pentose phosphate pathway, lactate synthesis and glycogen synthesis. These metabolic shifts were not associated with changes in the cell proliferation rate of HBECs. In contrast to our RNA-seq data, oxidative stress and cellular senescence pathways were not altered in HBECs cultured in high glucose media. To explore whether HBECs cultured in high glucose media can directly activate fibroblasts, primary human lung fibroblasts were cultured in normal and high glucose media conditioned by HBECs. Fibroblasts cultured in high glucose media conditioned media showed an increase in the gene expression of COL1A1 and COL1A2 along with increased protein expression of a-SMA and COL1A1. Protein array using media conditioned by HBECs demonstrated high glucose treatment increased secretion of proteins associated with pulmonary fibrosis and ECM remodeling into the media by HBECs, suggesting a potential mechanism of high glucoseinduced pulmonary fibrosis.