The systemic renin-angiotensin system in COVID-19

Abstract

SARS-CoV-2 gains cell entry via angiotensin-converting enzyme (ACE) 2, a membrane-bound enzyme of the "alternative" (alt) renin-angiotensin system (RAS). ACE2 counteracts angiotensin II by converting it to potentially protective angiotensin 1-7. Using mass spectrometry, we assessed key metabolites of the classical RAS (angiotensins I-II) and alt-RAS (angiotensins 1-7 and 1-5) pathways as well as ACE and ACE2 concentrations in 159 patients hospitalized with COVID-19, stratified by disease severity (severe, n = 76; non-severe: n = 83). Plasma renin activity (PRA-S) was calculated as the sum of RAS metabolites. We estimated ACE activity using the angiotensin II:I ratio (ACE-S) and estimated systemic alt-RAS activation using the ratio of alt-RAS axis metabolites to PRA-S (ALT-S). We applied mixed linear models to assess how PRA-S and ACE/ACE2 concentrations affected ALT-S, ACE-S, and angiotensins II and 1-7. Median angiotensin I and II levels were higher with severe versus non-severe COVID-19 (angiotensin I: 86 versus 30 pmol/L, p < 0.01; angiotensin II: 114 versus 58 pmol/L, p < 0.05), demonstrating activation of classical RAS. The difference disappeared with analysis limited to patients not taking a RAS inhibitor (angiotensin I: 40 versus 31 pmol/L, p = 0.251; angiotensin II: 76 versus 99 pmol/L, p = 0.833). ALT-S in severe COVID-19 increased with time (days 1-6: 0.12; days 11-16: 0.22) and correlated with ACE2 concentration (r = 0.831). ACE-S was lower in severe versus non-severe COVID-19 (1.6 versus 2.6; p < 0.001), but ACE concentrations were similar between groups and correlated weakly with ACE-S (r = 0.232). ACE2 and ACE-S trajectories in severe COVID-19, however, did not differ between survivors and non-survivors. Overall RAS alteration in severe COVID-19 resembled severity of disease-matched patients with influenza. In mixed linear models, renin activity most strongly predicted angiotensin II and 1-7 levels. ACE2 also predicted angiotensin 1-7 levels and ALT-S. No single factor or the combined model, however, could fully explain ACE-S. ACE2 and ACE-S trajectories in severe COVID-19 did not differ between survivors and non-survivors. In conclusion, angiotensin II was elevated in severe COVID-19 but was markedly influenced by RAS inhibitors and driven by overall RAS activation. ACE-S was significantly lower with severe COVID-19 and did not correlate with ACE concentrations. A shift to the alt-RAS axis because of increased ACE2 could partially explain the relative reduction in angiotensin II levels.

Figure 1

Study cohort.

Figure 2

RAS metabolite levels. (A) Classical and alternative RAS metabolite levels in healthy individuals (n = 16), COVID-19 patients of different severity (non-severe [n = 83], severe [n = 76]; subgroup of mechanically ventilated [n = 44] as well as patients with severe influenza pneumonia [n = 27]; Diameter of the spheres represent median levels of angiotensins, values are provided on the side. (B) Angiotensin levels stratified for severe and non-severe COVID-19 and time after hospitalization (5-day intervals); all angiotensin concentrations in pmol/L.

Figure 3

Key RAS enzymes. Key RRAS enzymes over time stratified for severe and non-severe COVID-19. PRA-S is reported as a unitless ratio; ACE and ACE 2 are reported in µg/mL and ng/mL, respectively.

Figure 4

Alternative RAS. (A, B) The alternative RAS ratio (ALT-S) increased in severe COVID-19 compared with less severe cases over time. (C) The alternative RAS ratio was highly correlated with ACE2 levels in the circulation. There was a non-linear correlation between ACE2 and the alternative RAS-ratio in ex vivo experiments using pooled serum samples containing defined amounts of recombinant human ACE2 and renin (black squares, C). (D) Angiotensin 1-7 and angiotensin II (product and substrate of ACE2) showed a strong correlation that was modified by systemic ACE2 concentration.

Figure 5

Ratio of angiotensin II to angiotensin I (A, B) The ratio of angiotensin II to angiotensin I (ALT-S) was reduced in severe COVID-19 throughout hospitalization. (C) ACE-S showed only a poor correlation with ACE concentration in the circulation. (D) Angiotensin II and angiotensin I showed a high correlation that was modified by systemic ACE2 concentration, resulting in a relative decrease of angiotensin II in relation to angiotensin I. (E,F) Both RAS activation (assessed by PRA-S, angiotensin-based plasma renin activity) and ACE2 showed moderate overall correlation with intrinsic ACE activity.

Figure 6

Mixed effects models. $R_m^2$, i.e., marginal R squared for linear mixed effects regression, modeling angiotension 1-7, angiotension II, the angiotension II:I ratio (ACE-S), and alternative RAS ratio (ALT-S); all dependent variables were modeled using enzyme plasma renin activity (PRA-S) and enzyme concentration of ACE and ACE2 as single covariates, as well as with a combined model. All models included a random intercept per patient. $R_m^2$ denotes the variance explained by the fixed effects of the respective model.

Figure 7

RAS and patient survival in severe COVID-19. ACE2 and ACE-S over time in severe COVID-19 patients stratified for patient survival.