TGF-β1 increases sialidase 3 expression in human lung epithelial cells by decreasing its degradation and upregulating its translation.

We previously found extensive desialylation of glycoconjugates and upregulation of the sialidase enzyme NEU3 in fibrotic lesions in human and mouse lungs. However, studies using microarray analysis of whole lung tissue mRNA and single cell RNA-seq found no significant difference in levels of mRNA between IPF patients and controls. This study aimed to elucidate how NEU3 was upregulated in fibrotic lungs. Transforming growth factor-β1 (TGF-β1), a key driver of fibrosis, was added to A549 human alveolar basal epithelial adenocarcinoma cells and human small airway epithelial cells (HSAEpC). NEU3 expression in A549 cells and HSAEpC was detected by immunofluorescence staining. NEU3 translation and degradation were assessed by polysome profiling (polysomes efficiently translate mRNAs; monosomes poorly translate mRNAs) and cycloheximide chase after treating cells with or without TGF-β1 for 48 h. TGF-β1 increased NEU3 expression and secretion in A549 cells and HSAEpC but did not change total (nuclear + cytosolic) mRNA levels. TGF-β1 decreased the degradation rate of NEU3 in A549 cells. TGF-β1 decreased mRNA levels in monosomes and increased mRNA level in polysomes. TGF-β1 upregulates levels of NEU3 in epithelial cells by both decreasing NEU3 degradation and by increasing the translation of mRNA, explaining the apparent paradox of high levels of NEU3 protein in pulmonary fibrosis without a concomitant increase in the expression of mRNA.