A new study by researchers from the University of South Carolina-USA and Xiangya Hospital, Central South University, Hunan-China has found that the spike proteins of the SARS-CoV-2 coronavirus typically impairs the lipid metabolic and autophagic pathways in human host cells, leading to increased susceptibility to lipotoxicity via ferroptosis, which often results in the damage of cells and tissues of the heart, liver and kidneys.
The study also found that an Nrf2 inhibitor such as the phytochemical Trigonelline which is an alkaloid derived from Coffee, can be used to suppress this impairment of the lipid metabolic and autophagic pathways and susceptibility to lipotoxicity.
The study findings were published on a preprint server and are currently being peer reviewed. https://www.biorxiv.org/content/10.1101/2022.04.19.488806v1
The data in this study has demonstrated that the Spike protein alone can directly impair lipid metabolic and autophagic pathways in host cells, leading to increased lipotoxicity through ferroptosis.
This result has shown a direct and evident role of the Spike protein in exaggeration of pre-existing lipotoxicity, revealing a mechanistic insight into the clinical manifestations of high susceptibility and mortality rate of obese patients with COVID-19.
Furthermore, we have shown that the Spike protein-induced necrosis can be suppressed by PI3K pan inhibitor Wortmannin, ferroptosis inhibitor ferrostatin 1, and Nrf2 inhibitor trigonelline. Trigonelline, an alkaloid enriched in coffee, is among those effective inhibitors, providing a potential and feasible preventive strategy to mitigate COVID-19-associated cardiometabolic pathologies associated with obesity.
Numerous studies have reported lipidomic dysregulation in COVID-19 patients. For example, Shen et al. showed a strong downregulation of over 100 lipids including sphigolipids, glycerophospholipids, fatty acids, and various apolipoproteins in COVID-19 patients.22 Increased levels of sphingomyelins (SMs), non-esterified fatty acids (NEFAs), and free polyunsaturated fatty acids (PUFAs) have been shown in COVID-19 patients as well.23–25
Increases in PLA2 activation, which results in long-chain PUFAs, may be associated with the COVID-19 disease deterioration.26,27 Our previous studies together with other reports have shown the downregulation of serum LDL-c and HDL-c levels in COVID-19 patients.12–14,28 These lines of accumulated evidence have revealed a central role of lipids and lipid metabolism in the development of COVID-19 disease.
In this study, we demonstrate that the Spike protein executes a direct function in altering lipidome via upregulation of a panel of genes involving lipid metabolism and resulting in enhanced lipid deposition on the cell membrane. This data provides direct evidence showing that the Spike protein modulates lipid metabolism in host cells and is an important independent factor contributing to the altered lipidome in COVID-19 patients.
PI3Ks play important roles in autophagy formation during early stages of viral infection for both canonical and non-canonical endocytic pathways.19–21
They are also critical downstream components of growth factor receptor (GFR) signaling cascades which drive phosphorylation of viral proteins upon SARS-CoV-2 infection.29
Indeed, inhibition of class I or class III PI3K prevents viral replication,29,30 probably through distinct mechanistic actions on different stages of SARS-CoV-2 viral life cycle. In a distinct mechanism, our data shows that the Spike protein alone can dysregulate expression of various PI3Ks in host cells including upregulation of class I PIK3CA, PIK3CD, and PIK3R3, but downregulation of class 3 PIK3C3 which is required for autophagosome and lysosome fusion31.
In addition, pan-PI3K inhibitor wortmannin shows a potent inhibition of the Spike-protein exaggerated PA-induced lipotoxicity, suggesting that increasing autophagosome formation while decreasing autophagosome fusion with lysosomes thereby leading to accumulation of autophagosomes may be a cause of Spike protein-exaggerated lipotoxicity.
The transcription factor Nrf2 controls the basal and induced expression of more than 1,000 genes in cells that can be clustered into several groups with distinct functions, such as antioxidative defense, detoxification, protein degradation, iron and lipid metabolism.18
Thus, the functions of Nrf2 spread rather broadly from antioxidative defense to protein quality control and metabolism regulation. Studies have demonstrated that Nrf2 is required for cardiac adaptation when cardiac autophagy is intact; however, it operates a pathological programme to exacerbate maladaptive cardiac remodeling and dysfunction when myocardial autophagy is inhibited in the settings of sustained pressure overload32 and chronic type 1 diabetes.33
Notably, chronic obesity, a pre-type 2 diabetic setting, results in inhibition of myocardial autophagy, thereby leading to cardiac pathological remodeling and dysfunction.34,35 In this study, the protein level of Nrf2 is upregulated in the Spike cells in response to PA treatment. Furthermore, TRG, a Nrf2 inhibitor, can attenuate Spike-protein-exaggerated PA-induced necrosis in the cell with impaired autophagy.
Collectively, it is reasonable to posit a central role of Nrf2 in PA-induced Spike protein-exaggerated lipotoxicity in host cells. However, the detailed pathological mechanism and molecular interactions mediated by impaired Nrf2 pathways need further validation, which will be the goal of our future studies.
There are several questions yet to be answered, which will be the focus of our future studies. First, the types of altered lipids or lipid metabolites caused by the Spike protein have not been identified. Second, the molecular mechanism underlying the Spike protein-induced lipidomic dysregulation is not well understood.
Our data indicate that Nrf2 may play a central role in the transcriptional level for this pathological process. However, this needs further elucidation and validation.
Third, the current emerging variants, omicron strains, present multiple mutations in the Spike protein; whether these omicron versions of Spike protein variants can enhance or attenuate their functions in lipid metabolic alteration as compared with the alpha version of the Spike protein is unknown. Fourth, the Spike protein-induced impairments in both autophagic and lipid metabolic pathways in host cells are evident.
However, whether autophagic impairment is a consequence of, or in parallel to lipid metabolic impairments is unknown. Most likely, autophagic impairment is intertangled with lipidomic alterations not only as a result but also an independent factor for the deterioration in response to lipotoxicity.
In conclusion, this study has demonstrated that the Spike protein can cause lipid deposition, impair lipid metabolic and autophagic pathways in host cells, ultimately leading to increased susceptibility to lipotoxicity via ferroptosis. The Spike protein-enhanced lipotoxicity can be suppressed by the Nrf2 inhibitor TRG, indicating a central role of Nrf2 in COVID-19-associated cardiac complications involving obesity.