Since 2019, COVID-19 has swept the world, causing many deaths due to ALI. Research on its risk factors and prognosis is proliferating, and many studies have confirmed the role of VitD in COVID-19 [21, 22]. Some scholars gradually found that the VDBP also played a crucial role in COVID-19. Previous studies have found that the VDBP has independent biological functions, including macrophage activation, fatty acid transport, neutrophil chemotaxis, and actin clearance [23–25]. Although VDBP plays a vital role in many diseases, its role in ALI remains uncertain. This study aimed to investigate the correlation between VDBP and the severity of COVID-19 and further explore the function of VDBP in ALI mouse and alveolar epithelial cell models.
In this study, we found that the VDBP concentration of severe COVID-19 was higher than that of non-severe COVID-19 and common pneumonia, but the VDBP was not an independent risk factor for the severity of COVID-19. Multiple studies have confirmed a significant correlation between VDBP gene polymorphism and COVID-19[7–12], but there is no consistent conclusion between VDBP and the severity and mortality of COVID-19. Alabdullatif W. found that the VDBP of the critical COVID-19 was significantly higher than that of the non-critical group[13], which is consistent with our findings. Povaliaeva A. also found that VDBP in COVID-19 patients was higher than in healthy people, but the correlation between VDBP and the severity was insignificant[14]. Besides, a proteomic study found no significant difference between VDBP and COVID-19 of different severity[15], and another study found that after adjusting for 25(OH)D, age, gender, and significant comorbidities did not show a correlation between VDBP and mortality in COVID-19 [16]. Thus, it can be speculated that VDBP may be closely related to the occurrence and development of COVID-19 but not an independent risk factor for the severity and mortality of COVID-19.
Previous studies in critically ill patients have found that low serum VDBP levels are associated with a higher risk of respiratory failure and sepsis [25–29], and VDBP levels are negatively correlated with septic shock mortality [30, 31]. Although these previous studies suggested VDBP as a protective factor for septic shock in critically ill patients, research on VDBP and ALI is few and has small sample sizes. There is still a lack of studies on the relevant mechanism of VDBP in ALI, especially in COVID-19 patients. It is unclear why our sample's VDBP of severe COVID-19 is significantly higher than that of non-severe patients. The mechanism is not yet fully understood.
To further explore the role of VDBP in ALI of COVID-19, we established the mouse model of ALI and discovered the expression of VDBP was significantly increased in the lung tissue of ALI mice than in controls. In a previous study, elevated levels of VDBP were also detected in bronchoalveolar lavage fluid of ARDS patients. Release of VDBP from endothelial injury sites has a chemotactic effect on complement-derived C5α, which might lead to attraction, aggregation, and activation of monocytes and neutrophils, resulting in an oxidative burst[26]. Another study found that VDBP gene knockout mice significantly reduced lung neutrophil recruitment[27]. Thus, we speculate that VDBP may play an essential role in the pathogenesis of ALI by promoting the aggregation and activation of inflammatory cells.
Previous research discovered that VDBP exacerbates diffuse alveolar injury during the acute phase of inflammation[23–25, 28], but the exact mechanism is unclear. To explore the effect of VDBP on alveolar epithelial cells during ALI, we cultured human alveolar epithelial cells in vitro and stimulated them with LPS for inflammation. This study found that VDBP expression in alveolar epithelial cells was significantly increased after alveolar epithelial cell injury. To further understand the cause of its occurrence, we silenced the VDBP gene. In the VDBP gene silencing group, we found a significant decrease in the inflammatory factors IL-1β and TNF-α, when alveolar epithelial cells were subjected to acute injury. Therefore, this study further confirms the critical regulatory role of VDBP in the inflammatory response to ALI.
In addition, we found that in the VDBP gene silencing group, the proliferation ability of alveolar epithelial cells improved, and apoptosis decreased. Previous studies have found that VDBP can activate macrophages in the early stage of infection or inflammation and induce macrophage apoptosis by up-regulating cysteine protease activity via the p38 and JNK1/2 pathways[29]. However, the metabolic way of VDBP in alveolar epithelial cells is currently unknown, and further research is needed. This study confirms that VDBP accelerates apoptosis of alveolar epithelial cells in ALI. Silencing the VDBP gene has a particular protective effect on alveolar epithelial cells. This provides more research data for understanding the pathogenesis of ALI, screening target genes, and providing new therapeutic targets.
Although many scholars emphasize the vital role of VDBP in COVID-19, this article is one of the few studies to evaluate the correlation between VDBP and COVID-19. Furthermore, it analyzes the effect of VDBP on alveolar epithelial cells during ALI. Despite efforts to reduce the risk of confounding factors, this study still has many limitations. Firstly, this study is a single-center observational study. Secondly, the included subjects were all older adults over 65 years old, and their universality may be limited due to various comorbidities and other factors, resulting in inconsistent research conclusions. Thirdly, due to the limitations of experimental conditions, the animal and cell models of ALI in this study were constructed through LPS, not directly caused by SARS-CoV-2 infection.
In conclusion, this study found that the level of VDBP in severe COVID-19 was significantly increased. VDBP may play an inflammatory factor in ALI and promote apoptosis of bronchial epithelial cells.