Predominant expression of ACE2 and TMPRSS2 in well-differentiated HCCs
We first searched for relationships ofACE2 and TMPRSS2 mRNA expression levels with HCC aggressiveness. To this end, overall and disease-free survival analyses were carried out in the TCGA dataset (n= 370). After applying the exclusion criteria described in Supplementary Fig. 1A, the survival dataset consisted of 256 patients. In consistency with previous reports21-23,higher ACE2and TMPRSS2 mRNA expression were associated with better overall and disease-free survival (Supplementary Fig. 1B and 2A). ACE2 and TMPRSS2mRNAs were also detected in 47 non-tumor livers in the TCGA dataset. WhereasACE2 mRNA expression was 1.7-fold higher in non-tumors livers than in HCCs, TMPRSS2 expression did not differ between both groups (Supplementary Fig. 2B).
In HCCs, the expression of genes associated with relatively good prognosis is frequently preserved in well-differentiated tumors because they maintain the phenotype of adult hepatocytes24,25.To verify if this applied to ACE2 and TMPRSS2,we measured their expression levels in HCCs classified according to their likeness to normal hepatocytes. Thus classified, HCCs are divided into two major classes, namely “low-proliferation” and “high-proliferation”25. Four HCC subclasses result from the interaction of, on one hand, the metabolic phenotype and, on the other hand, the proliferation/differentiation ratios of tumor cells24,26. Hence, in the “low-proliferation” class of well-to-moderately differentiated HCCs, the periportal-type (PP) and perivenous-type (PV) subclasses refer to the preservation of the metabolic zonation phenotypes of normal hepatocytes24,26. At the opposite end of the spectrum, in the “high-proliferation” class of moderately-to-poorly differentiated HCCs, the extracellular-matrix-type (ECM) and STEM-type subclasses, refer to tumors developing an important stromal and vascular support and expressing cancer stem cell markers24,26. In this context, we found higher ACE2 mRNA expression in perivenous-type HCCs than in the other subclasses (Fig. 1A; Supplementary Fig. 2C)in both the Désert’s cDNA microarray meta-dataset (1133 HCCs)26 (Fig.1A) and in the TCGA dataset (370 HCCs) (Supplementary Fig.2C). Of note, ACE2 mRNA expression was higher in HCCs expressing previously validated β-catenin pathway activationsignatures26,27(Fig. 1B) and in HCCs carryingsequenced β-catenin (CTNNB1)activating mutations (Supplementary Fig.2C). In turn, TMPRSS2 was expressed at higher levels in periportal-type HCCs (Supplementary Fig. 2C, D), which we previously demonstrated to carry wild-type CTNNB126. The MERS-CoV receptor, DPP4, was also related to β-catenin pathway activation (Supplementary Fig. 2E-G). ACE2 DNA was hypomethylatedin tumors carrying CTNNB1 mutations (Fig. 1C), which is consistent with transcriptional activation of ACE2.By contrast, TMPRSS2was hyper-methylated in HCCs carrying CTNNB1 mutations (Supplementary Fig. 3A; Supplementary Table 2), which is consistent with transcriptional repression.Neither ACE2 nor DPP4 mRNA expression were related TERT and TP53 gene mutations, which are the two most frequent ones in HCCs (Supplementary Fig. 3C, D). In turn, TMPRSS2mRNA levels were higher in HCCs carrying wild-type TP53(Supplementary Fig. 3D), which is consistent with the above-described higher expression of TMPRSS2 in the well-differentiated, non-proliferative class of HCCs,where TP53 gene mutations are rare26,28. Indeed, TP53 mutations are most frequent in poorly-differentiated HCCs (S229; G330), showing an aggressive, cancer stem cell phenotype26,31.
In line with the higher ACE2and DPP4mRNA expression in HCCs showing increased β-catenin pathway activation and CTNNB1 activating mutations, in silico analysis of 5,000 base pairs upstream of the transcription start sites of ACE2 and DPP4 DNAs revealed putative T-cell factor 4/LEF-1 consensus transcription factor binding sites, respectively (Fig. 1D; Supplementary Fig. 4A, B).
ACE2is immunodetected in moderately-to-well-differentiated HCCs preserving hepatocyte polarization and carrying CTNNB1 mutations.
In consistency with the above findings, immunohistochemistry followed by quantitative image analysis in tissue microarray of 41 HCCs revealed that the expression of ACE2 and of the marker of β-catenin pathway activation GLUL (glutamine synthetase) were correlated (Spearman’s R= 0.42; p= 0.00001). Both ACE2 and GLUL were 3.6 folds higher in HCCs carrying activating CTNNB1mutations than in tumors carrying wild-type CTNNB1 (Fig. 1, E-H).
HCCs with mutated CTNNB1 are frequently cholestatic and contain pseudo-glandular structures with bile plugs, where hepatocytes preserve the basal and apical poles31. Co-immunolabeling for ACE2 and ABCC2 (a.k.a. MRP2, a marker of the biliary pole in hepatocytes) in HCCs, detected ACE2 at the biliary pole and within the lumen of pseudo-glandular tumor structures and in slit-like bile canaliculi (Fig. 2, A,B).By contrast, ACE2 was not detected at the basal hepatocyte pole, which was highlighted by the capillary endothelial cell marker CD34 (Fig. 2C). As expected, the antibodies used detected high levels of ACE2 in the apical intestinal epithelium, Bowman’s capsule and convoluted tubes in the kidney; in turn, TMPRSS2 was detected in the apical compartment of the epithelial lining of prostatic glands (Supplementary Fig. 5). In HCCs, TMPRSS2 was also detected at the apical pole of pseudo-glandular formations or within slit-like trabecular structures and in cell-cell borders (Supplementary Fig. 6). Neither ACE2 (Supplementary Fig. 6) nor TMPRSS2 (not shown) were detected in myofibroblasts.
In five non-tumor samples from patients undergoing resection of colon cancer metastases and with minimal inflammatory changes in the liver, ACE2 was co-detected with the sinusoidal endothelial cell marker CLEC4M (a.k.a. DC-SIGNR) (Fig. 3, A, C, E) and the bile canaliculi marker ABCC2 (a.k.a. MRP2) (Fig. 3, B, D, F). The presence of ACE2 within bile canaliculi is consistent with the facts that the extracellular part of ACE2 can be cleaved off the cell surface by ADAM17, thus shedding functional ACE2 fragments32;and that ACE2 is an abundant component of the normal human bile proteome33. Also, ACE2 was co-detected with CD34 in capillary vessels of the periportal vascular plexus (Fig. 3G). By contrast, ACE2 was not detected in CD68-positive sinusoidal cells (Kupffer cells, Fig. 3H) or ACTA2-positive vascular smooth muscle cells or myofibroblasts (Fig. 3I,J).
ACE2 and TMPRSS2 co-expression networkshighlight metabolic functions typical of normal hepatocytes.
By Weighted Gene Correlation Network Analysis34comparing periportal-type (n= 342) and perivenous-type (n=225) HCCs within the Désert’s dataset (n= 1133), wefound thatACE2 and TMPRSS2were related through interacting metabolic functions (Fig. 4A), including detoxification, aminoacid catabolism, lipid metabolism, fatty acid oxidation and catabolism of organic compounds, which is probably related to the carboxypeptidase functions of ACE2, involved in the degradation of inflammatory polypeptides35,36(Fig. 4B). These findings are in line with the above-described expression of ACE2 and TMPRSS2 in well-to-moderately differentiated HCCs preserving the metabolic programs of adult hepatocytes.Next, we specifically investigated the ACE2 co-expression network in HCCs and analyzed associated gene functions. Not surprisingly, ACE2was associated with hepatocyte-specific functions such as aminoacid metabolism, xenobiotic detoxification, fatty acid uptake and oxidation. High ACE2 expression was associated with low expression levels of oxidative stress markers and cytokine-mediated inflammatory signals (Supplementary Fig. 7A,B). These findings raise the hypothesis of a link between inflammation, ACE2 dysfunction and metabolic breakdown that may be relevant to the pathogenesis of COVID-19.