Rediscovering Lung Lipofibroblast: A Lipid Wizard on the Alveolar Fibroblast Spectrum

Lipid metabolism and signaling are indispensable for maintaining the health of the adult lung (1, 2). Among the different lung cells (3), several are known for their highly specialized actions in lipid metabolism that go beyond fulfilling their own cellular needs, rather ones that assure the functional health of the whole lung. These cells include alveolar epithelial type II (AT2) cells serving as the lung’s power stations for surfactant lipid production, alveolar macrophages (AM) maintaining alveolar lipid homeostasis, and lung endothelial cells (EC) facilitating lipid uptake from the circulation (1, 2, 4). Besides these prominent examples, there is another fascinating cell population – lung lipofibroblasts (lipid interstitial cells, lipid body-containing interstitial cells, lipid-laden lung fibroblasts, or vitamin-A-storing lung cells) – a subgroup of PDGFRα⁺ lipid droplet-containing alveolar fibroblasts (AFs) that are characterized by their exceptional ability to metabolize lipids (2, 4–8). In the human lung, lipofibroblasts (LFs) reside in the alveolar wall between capillary ECs and alveolar epithelial cells (3, 9, 10). This unique topological location, coupled with LF’s ability to acquire and transfer lipids, allows them to be extensively involved in lipid-mediated paracrine communication among the alveolar cells. Most importantly, LFs have been recognized to be crucial for maintaining the alveolar stem cell niche by supporting AT2 cells (11) and so much needed alveolar regeneration and resolution of the lung disease in both animals (5, 7, 12) and humans (9, 13, 14).

LFs efficiently handle lipids due to the robust expression of genes encoding proteins of lipogenic transcription (PPARγ, C/EBPα, LXRs), lipolysis (LPL, LIPA, CES1), lipid binding (FABPs, RBP1, RBP4), storage (PLIN2, DGAT1, LRAT), and trafficking (APOE) (3, 13, 14). That’s why PLIN2 (perilipin 2) or adipose differentiation-related protein (ADRP, also known as ADFP or adipophilin), a lipid droplet-associated protein, is perhaps the most recognizable gene/protein marker used for the LF identification among different AFs (3, 7, 9, 10). Moreover, the inclusion of lipid markers into the molecular identification revealed the multifaceted nature of LFs within the AF spectrum, possessing diverse transcriptional signatures (7). Given the highly specialized function of lung LFs and recent technical advances in cellular characterization (15, 16), questions regarding LF identity, developmental origin, relation to other lung fibroblasts, and their fate in lung injury and disease resolution remain to be fully understood (17).

In this issue of the Journal, Boyer and colleagues (18) further advance our understanding of the human LF cell identity and reiterate the critical role of the fibroblast lipogenic program in protecting and maintaining the function of the adult human lung. The authors addressed the unmet need for finding strategies targeting alveolar stem cells by inducing existing endogenous mechanisms for alveolar regeneration to reverse alveolar destruction in emphysema. By leveraging the publicly available sequencing datasets of the human lung cells from healthy and diseased individuals, the authors identified a significant decline in PLIN2⁺ LFs in the lungs of individuals with chronic obstructive pulmonary disease (COPD). The authors further corroborate the LF loss by detecting significantly fewer cells co-expressing vimentin and ADFP in immunostained lung sections from individuals with emphysema and COPD. In addition, differentially expressed gene analysis of fibroblasts from COPD lungs identified a downregulation of genes important for lipid-mediated cell communications, including the ones encoding lipid transfer proteins (APOD, APOC1, and APOL2) and phospholipid-metabolizing enzymes generating signaling lipids (PLA2G2A and PLCB1).

Based on the observations of LF loss and an altered lipogenic program, the authors hypothesized that restoration of the LF phenotype could be a strategy to restore the properties of a healthy lung stem cell niche and to improve alveolar regeneration in emphysema. Indeed, alterations in LF functions have been shown in the literature to result in lung pathologies, while the restoration of LF identity via fibroblast lipogenic stimulation has been a successful strategy to improve lung function (9, 13, 14). To test their hypothesis, the authors employed an in vitro approach of lipogenic stimulation of primary human fibroblasts derived from emphysematous lung with T0901317, a synthetic liver X receptor (LXRα and LXRβ) pan-agonist. LXRs are nuclear receptors whose lipogenic activity is largely mediated by the induction of the genes encoding sterol regulatory element binding proteins 1 and 2 (SREBP1 encoded by the SREBPF1 gene and SREBP2 encoded by the SREBPF2 gene) that are central transcriptional activators of fatty acid biosynthesis and cholesterol metabolism (19). SREBP1 and SREBP2 activation during lipogenic differentiation of lung fibroblasts has been identified by this group previously in an animal model (20). Now, using the in vitro approach, the authors showed the responsiveness of primary human lung fibroblasts to T0901317 with upregulated expression of lipogenic LXR-target genes (SREBPF1, SREBPF2, FASN, and APOE), enhanced production of major structural and signaling lipid species, and restored ADFP⁺ lipid droplet-containing LF phenotype. It is generally believed that the lipid content of LF lipid droplets, comprised of neutral lipids, phospholipids, and retinoids (vitamin A and its metabolites), can be mobilized and transferred to the neighboring alveolar cells to support their function (7, 21). Indeed, the authors elegantly showed the functional consequences of the acquired LF phenotype. By undertaking lung organoid co-culture studies using primary human fibroblasts and primary human AT2 cells, the authors proved experimentally that stimulation of LXRs and their downstream targets SREBPs in lung fibroblasts from emphysematous lung restores their stem niche properties and the regenerative capacity of primary human AT2 cells.

The study by Boyer and colleagues provides new knowledge on the lipid metabolism in a specific lung cell population in healthy and diseased lungs. The significance of the study is a new set of lipid markers that expands the array of molecular features for use in human LF identification and characterization in vivo. More importantly, the study possesses high translational value as it gives essential information regarding the cell-specific proteins and pathways that can be therapeutically targeted in human lung disease, including COPD. By demonstrating that fibroblast LXR stimulation followed by SREBP activation is a valid target for enhancing alveolar regeneration, the authors have expanded the existing arsenal of molecular tools that can be explored in more depth for restoring normal lung structure and function. The authors reiterate the undeniable benefits of stimulating the lung fibroblast lipogenic program. However, future studies are needed to dissect the cell-specific and isoform-specific contributions of LXRs and/or SREBPs to driving the beneficial regenerative effects. Moreover, the study opens new avenues for future explorations to better understand whether and how the lipids (or lipid-derived signals) from LFs reach the neighboring target cell. What are the molecular mechanisms behind the lipid-mediated paracrine communication within the alveolar niche? And what molecular responses in the receiver AT2 cells enhance their regenerative capacity?

Undoubtedly, this is a sound and inspiring study that provides optimism in science’s ability to combat human lung disease with molecular precision and represents a step forward for improving the quality of life of people with lung disease.

Figure. 1. LXR activation in emphysematous lung fibroblasts restores a lipofibroblast-like niche and supports AT2 cell regeneration.

Lung LFs reside in the alveolar wall between capillary endothelial cells and alveolar epithelial cells and communicate with these cells employing lipids and lipid-derived signaling molecules. LFs can acquire lipids from the neighboring cells, followed by intracellular lipid processing and de novo lipid synthesis in the endoplasmic reticulum, storage in the characteristic lipid droplets, and extracellular lipid transfer. These processes are regulated by specific lipogenic transcription factors, including PPARγ, LXRs, and SREBPs, that control the expression of downstream target genes of lipid metabolism. LF lipogenic program is downregulated in emphysematous lung fibroblasts, however it can be reactivated by the ligand-dependent stimulation of LXRs, followed by SREBP activation, resulting in the restoration of fibroblast stem niche properties and the regenerative capacity of AT2 cells. (created with Biorender)