Thursday, November 21, 2024
11/21/2024
PROJECT SUMMARY/ABSTRACT The current understanding of vitamin A metabolism in extrahepatic tissues is that hepatic retinoid stores are mobilized as retinol bound to retinol-binding protein 4 (RBP4) and delivered through the circulation to the cells where they are taken up and used for all-trans-retinoic acid (ATRA) synthesis and ATRA-mediated transcriptional regulation. Our preliminary data challenge this paradigm and indicate that retinoids are delivered to the lung predominantly via circulating lipoproteins, a previously ignored pathway for retinoid delivery involving the action of lipoprotein-metabolizing proteins. We found from our novel studies employing single-cell RNA sequencing (scRNA- seq) of cells possessing vitamin A fluorescence that lipofibroblasts (LFs), endothelial (ECs), and epithelial cells (AECs) all accumulate retinoids in form of retinyl esters (REs) and all express key genes needed to allow for postprandial lipoprotein-derived vitamin A uptake, storage, and intercellular transport. Moreover, our mouse model studies compellingly establish that the mice lacking locally stored retinoids in the lung develop more severe acute lung injury resulting in death due to the loss of alveolar barrier integrity and impaired surfactant production. This occurs even though mice are fed a nutritional complete chow diet, circulating vitamin A levels are normal, and the animals are not otherwise vitamin A-insufficient. These findings lead us to hypothesize that lipoprotein-derived and locally stored cellular vitamin A is preferentially processed, as opposed to circulating or systemic retinol, for ATRA synthesis and signaling via nuclear retinoic acid receptors (RARs). We are proposing in-depth studies to explore the metabolic consequences of vitamin A transport and how local vitamin A stores are acquired, metabolized, and processed to mount protective cellular responses. Aim 1 will delineate the mechanisms that allow ECs, LFs, and AEC2s to take up, accumulate, and intercellularly transfer retinoids. We will determine how retinoids are taken up by these cells from circulating lipoproteins and redistributed among them. This will be achieved by kinetic studies employing tracer labeling of retinoids in mice lacking cell-specific expression of Lpl, Cd36, and Lrp1 as well as in cultured mouse and human cells coupled with lipidomics analyses. Aim 2 will establish how alterations in vitamin A stores, which are primarily located in intracellular lipid droplets of the LFs, define the transcriptional and phenotypic identity of the LF population. Using scRNA-seq molecular profiling coupled with immunohistochemical spatial analysis of labeled retinoid-containing LFs, we will characterize how exposure to lipoprotein-derived retinoids defines quiescent and profibrotic LF identity in vivo. Aim 3 will determine the crosstalk mechanisms by which LF-derived retinoids regulate ATRA-RAR-mediated cellular functions of ECs and AEC2s. Employing genetic and pharmacological manipulations of retinoid signaling via RARs in AEC2s and ECs coupled with lipidomics analyses, we will dissect the crosstalk mechanisms underlying the role of LF-derived retinoids in cell-specific ATRA signaling to regulate endothelial barrier integrity and surfactant lipid production. To further define underlying ATRA- RAR-mediated mechanisms, we will employ human AEC2s and ECs co-cultured with retinoid-containing LFs.
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