L-Dopa decarboxylase modulates autophagy in hepatocytes and is implicated in dengue virus-caused inhibition of autophagy completion.

The enzyme L-Dopa Decarboxylase (DDC) synthesizes the catecholamine dopamine and the indolamine serotonin. Apart from its role in the brain as a neurotransmitter biosynthetic enzyme, DDC has been detected also in the liver and other peripheral organs, where it is implicated in cell proliferation, apoptosis, and host-virus interactions. Dengue virus (DENV) suppresses DDC expression at the later stages of infection, during which DENV also inhibits autophagosome-lysosome fusion. As dopamine affects autophagy in neuronal cells, we investigated the possible association of DDC with autophagy in human hepatocytes and examined whether DDC mediates the relationship between DENV infection and autophagy. We performed DDC silencing/overexpression and evaluated autophagic markers upon induction of autophagy, or suppression of autophagosome-lysosome fusion. Our results showed that DDC favored the autophagic process, at least in part, through its biosynthetic function, while knockdown of DDC or inhibition of DDC enzymatic activity prevented autophagy completion. In turn, autophagy induction upregulated DDC, while autophagy reduction by chemical or genetic (ATG14L knockout) ways caused the opposite effect. This study also implicated DDC with the cellular energetic status, as DDC silencing reduced the oxidative phosphorylation activity of the cell. We also report that upon DDC silencing, the repressive effect of DENV on the completion of autophagy was enhanced, and the inhibition of autolysosome formation did not exert an additive effect on viral proliferation. These data unravel a novel role of DDC in the autophagic process and suggest that DENV downregulates DDC expression to inhibit the completion of autophagy, reinforcing the importance of this protein in viral infections.

Significance of Catecholamine Biosynthetic/Metabolic Pathway in SARS-CoV-2 Infection and COVID-19 Severity.

The SARS-CoV-2 infection was previously associated with the expression of the dopamine biosynthetic enzyme L-Dopa decarboxylase (DDC). Specifically, a negative correlation was detected between DDC mRNA and SARS-CoV-2 RNA levels in in vitro infected epithelial cells and the nasopharyngeal tissue of COVID-19 patients with mild/no symptoms. However, DDC, among other genes related to both DDC expression and SARS-CoV-2-infection (ACE2, dACE2, EPO), was upregulated in these patients, possibly attributed to an orchestrated host antiviral response. Herein, by comparing DDC expression in the nasopharyngeal swab samples of severe/critical to mild COVID-19 cases, we showed a 20 mean-fold reduction, highlighting the importance of the expression of this gene as a potential marker of COVID-19 severity. Moreover, we identified an association of SARS-CoV-2 infection with the expression of key catecholamine biosynthesis/metabolism-related genes, in whole blood samples from hospitalized patients and in cultured cells. Specifically, viral infection downregulated the biosynthetic part of the dopamine pathway (reduction in DDC expression up to 7.5 mean-fold), while enhanced the catabolizing part (increase in monoamine oxidases A and B expression up to 15 and 10 mean-fold, respectively) in vivo, irrespectively of the presence of comorbidities. In accordance, dopamine levels in the sera of severe cases were reduced (up to 3.8 mean-fold). Additionally, a moderate positive correlation between DDC and MAOA mRNA levels (r = 0.527, p < 00001) in the blood was identified upon SARS-CoV-2-infection. These observations were consistent to the gene expression data from SARS-CoV-2-infected Vero E6 and A549 epithelial cells. Furthermore, L-Dopa or dopamine treatment of infected cells attenuated the virus-derived cytopathic effect by 55% and 59%, respectively. The SARS-CoV-2 mediated suppression of dopamine biosynthesis in cell culture was, at least in part, attributed to hypoxia-like conditions triggered by viral infection. These findings suggest that L-Dopa/dopamine intake may have a preventive or therapeutic value for COVID-19 patients.