Background
Idiopathic pulmonary fibrosis (IPF) is the most frequent interstitial lung disease with chronic and progressive features that lead to decline in lung function and ultimately lung failure [
1,
2]. The fibrotic lesions observed in IPF are thought to be primarily driven by fibroblasts, particularly myofibroblasts, which are the major cell types responsible for fibrotic progression. This process is mediated through formation of focal lesions of active fibroblasts, exuberant production and deposition of extracellular matrix (ECM), which ultimately leads to thickening of the interstitium and distortion of the lung architecture [
3‐
5]. The disease exhibits a median survival of less than 5 years from the time of diagnosis due to a lack of effective therapies [
6,
7]. While two currently available treatment options, nintedanib and pirfenidone, can improve the lung function in patients, both drugs fail to effectively halt lung fibrosis, with little to no effect on overall mortality [
7,
8]. For these reasons, there is an urgent need to develop new and better therapeutic options for those diagnosed with IPF.
Recent studies demonstrate that dysregulated expression of metabolic enzymes and altered levels of metabolites are involved in IPF pathogenesis [
9‐
11] where metabolic reprogramming modulates the fibrotic activities of lung fibroblasts [
12]. Inhibition of glycolysis, glutaminolysis or arginine biosynthesis has been shown to reduce pulmonary fibrosis in animal models, suggesting targeting metabolic reprogramming of fibrotic lung fibroblasts as a viable therapeutic avenue for IPF [
13‐
16]. We have previously identified downregulation of argininosuccinate synthase 1 (ASS1), a rate-limiting enzyme in
de novo arginine biosynthesis, in IPF lung fibrotic tissues and fibroblasts. As IPF lung fibroblasts with ASS1 deficiency require the uptake of extracellular arginine for survival due to an inability to endogenously synthesize arginine, arginine deprivation strategies, including arginine-free diet and arginine deiminase (ADI) which degrades extracellular arginine, demonstrate antifibrotic effects on reducing collagen content and fibrotic lesions in the lung of mice exposed to bleomycin [
16]. Although ADI therapy exhibited promising results, several studies in cancer have reported that this therapy inevitably triggers ADI resistance through induction of ASS1 expression and activation of alternate signaling pathways in ASS1-negative cells [
17,
18]. Given these shortcomings, the development of novel therapeutic strategies for arginine-dependent pulmonary fibrosis is greatly needed.
Myo-inositol (or inositol) is a natural compound provided by dietary uptake and/or endogenously synthesized through the glycolysis pathway [
19]. As a precursor of second messengers, inositol regulates an array of signal transduction pathways and metabolic circuits in cells [
20,
21]. One of the most common inositol-derived metabolites, phosphatidylinositol 4,5-bisphosphate (PIP
2), hydrolyzes into diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3) to mediate Ca
2+ signaling and activation of the conventional protein kinase C isoforms (cPKCs) [
21,
22]. Since inositol metabolism plays a critical role in different aspects of physiological conditions, inositol has been considered for treating a diverse range of conditions from newborn respiratory distress syndrome to lung cancer [
23,
24]. Despite these findings, our understanding of inositol function in other lung diseases such as lung fibrosis remains incomplete. In this study, we not only characterized the metabolomic profiles of fibrotic lung fibroblasts derived from IPF patients but also identified fibrogenic metabolic pathways driven by ASS1 deficiency. Furthermore, we determined the therapeutic potential of inositol in IPF through in-vitro cell-based assays and an in-vivo animal model of lung fibrosis.
Discussion
Targeting altered fibrometabolism is an emerging therapeutic strategy for halting pulmonary fibrosis. The metabolic reprogramming that accompanies the development of fibrogenesis creates targetable differences between IPF (fibrotic or activated) lung fibroblasts and normal fibroblasts, which may be exploited for therapy [
13‐
16]. Our prior studies revealed dysregulated expression of argininosuccinate synthase 1 (ASS1), a rate-limiting enzyme in the urea cycle, in IPF lung fibroblasts. Although we previously developed an arginine starvation strategy and demonstrated the therapeutic potential of arginine deiminase (ADI) in pulmonary fibrosis [
16], many challenges remain to be resolved, especially the problem of induced ADI resistance [
17,
18]. To develop a novel strategy to target arginine-dependent pulmonary fibrosis, we have conducted metabolomics studies and characterized the metabolic landscape of fibrotic lung fibroblasts mediated by ASS1. This current work identifies inositol as an antifibrotic metabolite in lung fibroblasts. In addition, we present a proof of concept for the use of inositol to suppress inositol-driven signaling and subsequent fibrotic activities in IPF lung fibroblasts as well as in bleomycin-induced pulmonary fibrosis in mice.
Our metabolomics studies demonstrated that a number of amino acid metabolic pathways (e.g., arginine and proline metabolism, inositol phosphate metabolism) were dysregulated in IPF and/or ASS1-deficient lung fibroblasts. Given that
de novo arginine biosynthesis is suppressed in fibrotic lung fibroblast cells due to ASS1 deficiency [
16], fibrotic lung fibroblasts rely on extracellular arginine to continuously supply the urea cycle and produce ornithine. Since ornithine has been revealed to positively regulate glucose-6-phosphatase (G6Pase) transcription [
36], we believe that higher ornithine levels in fibrotic lung fibroblasts increase G6Pase expression, which in turn promotes hydrolysis of glucose 6-phosphate (G6P) and production of glucose. Our metabolomics data showing that elevated ornithine was concomitant with decreased G6P and increased glucose in IPF lung fibroblasts with ASS1 loss (Fig.
1D) further supports this notion. As noted above, given a crucial role of the G6P pathway for inositol biosynthesis, it is reasonable to expect that ASS1 deficiency contributes to dysregulation of inositol metabolism in IPF fibroblasts through its influence on the G6P pathway.
In general, ornithine breakdown feeds into the putrescine or proline pathway. A previous metabolomic study on IPF and normal lung tissues showed an increase in both putrescine and 4-hydroxyproline levels in lung tissues from IPF patients [
11]. In contrast to IPF lung tissues, we found that IPF lung fibroblasts exhibited a reduction of putrescine and an elevation of 4-hydroxyproline, suggesting that ornithine metabolism favors proline synthesis in fibrotic lung fibroblasts. We also discovered dysregulation of the inositol-related metabolic pathways (inositol phosphate metabolism and phosphatidylinositol signaling system) and a shift in metabolite abundance from myo-inositol to inositol-4-monophosphate in ASS1-deficient cells, suggesting dysregulated inositol metabolism in fibrotic lung fibroblasts. These data support the notion of the ASS1-inositol axis in fibrometabolism, where inositol-associated metabolic pathways are rewired in response to ASS1 loss and modulate lung fibroblast activation and pulmonary fibrosis.
While we did observe a general downregulation of glycolytic intermediates in IPF fibroblasts, several studies on metabolomics of human lung tissues have demonstrated augmentation of glycolysis in IPF lung tissues compared to normal tissues [
37,
38]. Lactic acid, also known as lactate, in particular was found to be elevated in IPF lungs [
11,
37,
38]. Kottmann et al’s investigation of the mechanism points to the effect of increased lactic acid levels in lung tissues on TGF-β activation, myofibroblast differentiation, and pulmonary fibrosis [
37]. The findings from the Liu group also support the notion of an increase in glycolysis in IPF lung tissues and upregulation of key glycolytic enzymes in lung myofibroblasts [
38]. In Bernard et al’s study, an induction of mitochondrial biogenesis and aerobic glycolysis was demonstrated to occur in TGF-β-induced myofibroblast differentiation [
39]. In contrast, our data showed a decrease of lactic acid in IPF lung fibroblasts. This is likely due to intrinsic differences between tissues and cells. In our study, we analyzed the metabolic profiling of lung fibroblasts rather than that of whole lung tissues. Lung fibroblasts derived from IPF patients are heterogeneous and consist of several subpopulations of fibroblasts including proliferative/aggressive fibroblasts and myofibroblasts [
33‐
35]. Since most IPF lung fibroblasts used in this work lack ASS1 expression and the aforementioned studies mainly focused on myofibroblast differentiation [
37‐
39], it would not be illogical that our observations in the metabolomics data differ from prior observations by other groups. Our findings of reduced lactic acid in IPF lung fibroblasts suggest that the products of glycolysis in ASS1-deficient lung fibroblasts may be shuttled towards other pathways, instead of lactic acid production. Despite a general decline of glycolysis in IPF fibroblasts, an increase in sorbitol and fructose may be alternative carbon sources for these fibrotic cells.
As dysregulated inositol was noted in IPF lung fibroblasts, most of which are deficient in ASS1, inositol-derived metabolites and critical components of inositol-mediated pathways were likely altered. As expected, we found that IPF fibroblasts with ASS1 deficiency display downregulated inositol level and upregulated expression of several enzymes (i.e., MIOX and CDIPT) involved in inositol catabolism and phosphatidylinositol metabolism, suggesting that ASS1-deficient lung fibroblasts tend to consume and/or eliminate most of cellular inositol. Given that inositol catabolism and phosphatidylinositol metabolism participate in generation of second messengers such as PIP2, IP3, and DAG as well as activate the downstream signaling cascades, it was not surprising to observe the activation of EGFR and PKC-mediated signaling molecules in ASS1-deficient cells due to an increase in inositol breakdown and elimination. Of these signaling molecules, EGFR, AKT, and STAT3 have been reported to participate in TGF-β-induced myofibroblast differentiation [
40‐
42] and are being considered as potential therapeutic targets for IPF [
27,
42‐
44]. Furthermore, our prior research revealed that targeting MARCKS activity in IPF lung fibroblasts with elevated α-SMA expression led to a reduction in COL1A1 and FN1 expression, AKT activity, and PIP3 levels [
26]. Therefore, we propose that MARCKS, a major substrate of cPKCs, might potentiate COL1A1 and α-SMA expression in IPF fibroblasts. Based on these findings, it is plausible that the pathway underlying the facilitation of COL1A1 and α-SMA expression in ASS1 deficiency (with lower inositol) involves the activation of EGFR, AKT, STAT3, and MARCKS (cPKCs) pathways. However, further research is needed to explore the specific association between these pathways and COL1A1 and α-SMA expression in the context of inositol reduction.
Our current study has confirmed that ASS1 deficiency acted in parallel with decreased cellular inositol in IPF lung fibroblasts, showing the antifibrotic potential of cellular inositol abundance in cells. In cell-based studies, we found downregulation of inositol-related signalosomes (e.g., EGFR and PKC signaling) when “extracellular” inositol was added to increase “intracellular” levels of inositol in IPF fibroblasts and/or ASS1-knockdown normal fibroblasts. As mentioned earlier, fibrotic lung fibroblasts with ASS1 loss favor the state of lower cellular inositol, and there is a theoretical possibility that ASS1-deficient fibroblasts decrease inositol-associated signaling activity in order to reduce and/or avoid additional inositol uptake from outside the cell and sustain cell survival in response to high levels of extracellular inositol. Although downregulation of inositol-associated signaling pathways may act as a protective mechanism to overcome excess inositol entry into cells, we cannot completely rule out the possibility of the pathway crosstalk effect by inositol supplementation. In addition to binding to inositol transports, “extracellular” inositol may interact with receptors and/or kinases which in part regulate the inositol-associated signaling molecules.
Consistent with our observations, a handful of reports have confirmed that inositol supplementation suppresses the PI3K/AKT pathway in cancer cells [
45‐
48]. Inositol plays a crucial role as a precursor of second messengers and regulates Ca
2+ signaling through the PIP2 to IP3 circuit. An increase in calcium levels has been reported to activate calcium-dependent proteases such as calpains, which can induce apoptosis by activating caspase-3 in cancer cells [
49,
50]. Inositol hexaphosphate (IP6), a phosphate metabolite of inositol, has demonstrated anti-cancer properties by inducing apoptosis in various cancers [
46,
51]. However, the role of inositol in fibroblast apoptosis remains unknown and requires further investigation. Moreover, inositol has been documented to hinder epithelial-mesenchymal transition (EMT), a known fibrotic process, through upregulation of E-cadherin and downregulation of metalloproteinase-9 and snail family transcriptional repressor 1 levels [
45]. Most significantly, we have demonstrated that inositol not only inhibited fibrotic molecules induced by ASS1 loss (e.g., COL1A1 and α-SMA) but also repressed cell invasiveness in IPF fibroblasts. In addition, our bleomycin-exposed mice treated with inositol had a significant improvement on histologic fibrosis and collagen deposition. Impressively, inositol supplementation supported body weight recovery in diseased mice with lung fibrosis. Weight loss is a critical issue in IPF patients as approximately 20% of IPF patients experience more than 5% unintended weight loss, and this weight loss is associated with worse IPF prognosis over time [
52]. Despite mild efficacy in relieving the disease, adverse effects are common with the approved IPF therapies (nintedanib and pirfenidone) and both drugs are limited in halting lung fibrosis or in improving overall mortality [
7,
8,
53]. In this work, inositol supplementation shows an inhibitory effect on lung fibrosis, weight loss, and overall mortality in bleomycin-exposed mice, indicating the possibility of inositol supplementation as a viable antifibrotic therapeutic strategy for IPF.
Inositol has been evaluated in both pre-clinical and clinical studies for various diseases, and many toxicity studies suggest that taking lower dosages of inositol are relatively safe [
54]. Given the extensive safety data in clinical trials and our findings on preclinical tests, inositol presents itself as a promising antifibrotic agent, all of which will facilitate quicker translation to clinical trials and potential regulatory approval. In addition to its antifibrotic activity, the effect of inositol in mitigating lung fibrosis of bleomycin-exposed may be mediated partly through anti-inflammatory effects. In the following phase II trial of smokers with bronchial dysplasia, 6-month inositol intervention was reported to reduce IL-6 level in bronchoalveolar lavage fluid of treated subjects, and the treatment significantly decreased PI3K activation among those with complete response [
23]. A decrease in IL-6 promotes macrophage polarization and subsequently lowers STAT3 activity, thereby reducing tumor development in cancer-prone mice fed with inositol diet [
55]. Identifying whether inositol modulates the cytokine profile and immune cell populations during fibrogenesis is a promising area of future study. However, work in understanding how inositol influences the immune landscape within the fibrotic microenvironment is beyond the scope of this study but merits further investigation.
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