Regulated catalysis of extracellular nucleotides by vascular CD39/ENTPD1 is required for liver regeneration

Abstract

Background & aims: Little is known about how endothelial cells respond to injury, regulate hepatocyte turnover and reconstitute the hepatic vasculature. We aimed to determine the effects of the vascular ectonucleotidase CD39 on sinusoidal endothelial cell responses following partial hepatectomy and to dissect purinergic and growth factor interactions in this model.

Methods: Parameters of liver injury and regeneration, as well as the kinetics of hepatocellular and sinusoidal endothelial cell proliferation, were assessed following partial hepatectomy in mice that do not express CD39, that do not express ATP/UTP receptor P2Y2, and in controls. The effects of extracellular ATP on vascular endothelial growth factor (VEGF), hepatocyte growth factor (HGF), and interleukin-6 responses were determined in vivo and in vitro. Phosphorylation of the endothelial VEGF receptor in response to extracellular nucleotides and growth factors was assessed in vitro.

Results: After partial hepatectomy, expression of the vascular ectonucleotidase CD39 increased on sinusoidal endothelial cells. Targeted disruption of CD39 impaired hepatocellular regeneration, reduced angiogenesis, and increased hepatic injury, resulting in pronounced vascular endothelial apoptosis, and decreased survival. Decreased HGF release by sinusoidal endothelial cells, despite high levels of VEGF, reduced paracrine stimulation of hepatocytes. Failure of VEGF receptor-2/KDR transactivation by extracellular nucleotides on CD39-null endothelial cells was associated with P2Y2 receptor desensitization.

Conclusions: Regulated phosphohydrolysis of extracellular nucleotides by CD39 coordinates both hepatocyte and endothelial cell proliferation following partial hepatectomy. Lack of CD39 activity is associated with decreased hepatic regeneration and failure of vascular reconstitution.

Figure 1

Expression of CD39 in normal and regenerating liver postpartial hepatectomy. (A) In untreated mice, CD39 is expressed on Kupffer cells and chiefly the endothelium of muscularized vessels. (B) After partial hepatectomy, heightened expression on LSEC is found at day 2 (not shown) with further increases to day 5. (C) CD39-null mice exhibit acute and substantial defects in liver regeneration. Survival of mice null for CD39 is significantly decreased compared with wild-type mice (30/32 [94%] vs 21/32 [66%], respectively, P = .003). (D) ALT measurements of postpartial hepatectomy revealing significantly increased liver injury in mice null for CD39 at postoperative day 1. (E) Representative pictures of the liver 4 hours post-partial hepatectomy. Vascular injury is seen in CD39 null but not in wild-type mice. (F) Ratios of dry liver weight to total body weight at day 2 postpartial hepatectomy show significant decreases in mice null for CD39. Values are means ± standard deviation of at least 4 animals per time point. Levels of significance were assessed by unpaired t tests. P values are as indicated.

Figure 2

CD39-null mice show abnormalities in hepatocyte proliferation with altered kinetics of LSEC proliferation in vivo and in vitro. (A and B) Representative immunohistochemistry images showing hepatocyte BrdU incorporation at day 2 postpartial hepatectomy in (A) wild-type and mice null for (B) CD39. (C) BrdU uptake of hepatocytes at various time points postpartial hepatectomy. At day 2 postpartial hepatectomy, numbers of BrdU-positive hepatocytes are significantly lower in mice null for CD39 (P = .02). (D) Proliferation of LSEC (defined by morphology and location) in vivo was significantly decreased in mutant mice compared with wild-type mice at day 3 and day 5 postpartial hepatectomy as measured by BrdU uptake. (E) After partial hepatectomy, expression of active Ras is relatively decreased in mice null for CD39, at days 1 and 2 post-surgery. Values are means ± standard deviation of at least 4 animals per time point. Levels of significance were assessed by unpaired t tests. P values are as indicated.

Figure 3

Regulation of endothelial cell apoptosis by CD39. (A and B) Heightened levels of apoptosis of CD39-null LSEC relative to wild-type are noted at day 7, postpartial hepatectomy. (C) Significant increases of numbers of apoptotic LSEC (TUNEL-positive cells with distinct morphology within the vascular sinusoids) occur at day 5 and day 7 post-partial hepatectomy. (D) Colocalization of TUNEL staining and CD31 expression in hepatic sinusoids (arrowheads) by fluorescence immunohistochemistry confirms the specific apoptosis induction impacting LSEC: DAPI staining nuclei in blue, CD31 as a marker of endothelial cells in green, and TUNEL in red. (E) In vitro apoptosis is induced after 48 hours of serum starvation with 1% FBS. Sequential administration of cycloheximide (2 μg/mL) and TNF-α (400 U/mL) resulted in significantly increased levels of apoptosis in LSEC null for CD39 (P = .01). Further administration of ATPγS (a nonhydrolysable ATP analog) results in increased apoptosis in wild-type and mutant LSEC. Values are expressed as means ± standard deviation. Levels of significance were assessed by unpaired t tests. P values are as indicated.

Figure 4

Deficiency of CD39 results in secondary VEGF resistance. (A) Serum levels of VEGF are increased in mice null for CD39 within first 48 hours post-partial hepatectomy. (B) Serum IL-6 levels in vivo are not significantly different between the 2 groups. (C) HGF levels in vivo are significantly decreased in mice null for CD39 at 4 and 24 hours post-partial hepatectomy. (D) In vitro, HGF levels are measured in the supernatants of cultured LSEC. Wild-type LSEC release HGF in response to stimulation with VEGF but not after stimulation with adenosine (Ado) alone. LSEC null for CD39 do not show these increases in HGF secretion in response to stimulation with these agonists. (E) Failure of P2Y(2)-mediated phosphorylation of VEGFR2. LSEC of wild-type and CD39-null mice were stimulated with UTP (100 μmol/L, 10 minutes) or VEGF (30 ng/mL, 3 minutes) in vitro. Stimulation with VEGF alone does not result in any significant differences. Stimulation with UTP results in heightened phosphorylation of the VEGFR2 in wild-type LSEC but not in CD39-null LSEC. The enhanced effect seen with VEGF and UTP co-stimulation is lost in CD39-null cells. Values are given as mean ± standard deviation. Level of significance was assessed by unpaired t tests. P values are as indicated.

Figure 5

P2Y2R is required for vascular coordination of liver regeneration. (A) Ki67 staining of hepatocytes is shown at various time points postpartial hepatectomy. At day 2 postpartial hepatectomy, the number of replicating hepatocytes is significantly lower in mice null for P2Y2R compared with wild-type (P = .003). (B) Proliferative responses ([³H]-thymidine) of hepatocytes to combinations of ATP (100 μmol/L) and/or a submitogenic dose of HGF (20 ng/mL) were studied. Synergistic effects of ATP and HGF in the boosting of proliferation of P2Y2R null hepatocytes are attenuated in a statistically significant manner when compared with wild-type hepatocytes. (C) Phosphorylation of VEGFR2 in LSEC was studied following P2Y2R costimulation with VEGF. In mice null for P2Y2R, phospho-VEGFR2 levels do not increase in response to UTP or to low concentrations of VEGF. Phosphorylation of VEGFR2 is substantively attenuated after optimal VEGF and UTP stimulation.(D)Hepatocellular proliferation postpartial hepatectomy in wild-type mice is increased by continuous infusion of apyrase (8.3 U/kg/h) (*P = .027). ATP (0.8 μmol/kg/h) and the A2A agonist ATL146e (600 ng/kg/h) had no significant effects on hepatocyte proliferation. (E) Images of Ki67 staining in wild-type livers postapyrase supplementation. Values are given as mean ± standard error. Levels of significance were assessed by unpaired t tests and ANOVA. P values are as indicated.

Figure 6

Impact of purinergic signaling upon interactions between LSEC and hepatocytes during early liver regeneration. (1) Absence of CD39 leads to elevated nucleotide levels. These changes in fluxes of extracellular nucleotides (ATP, UTP, and ADP) impact P2 receptor signaling in paracrine (and autocrine) manner(s) for hepatocytes (left) and LSEC (right). (2) Continuous stimulation of LSEC P2Y receptors results in preferential desensitization responses, here specifically of P2Y2R. P2Y2R and VEGFR2 colocalize on vascular cell membranes, and activation of the P2Y2R induces rapid tyrosine phosphorylation of VEGFR2 in endothelial cells. Inhibition of P2Y2R function is associated with failure of VEGFR2 transactivation and VEGF resistance (see text for details). (3) Failure of VEGFR2 signaling in turn results in decreased secretion of HGF and decreased hepatocyte proliferation. IL-6 pathways appear minimally impacted (Figure 4B, and Supplementary Figure 3). Unopposed P2X7R activation in this setting because of resistance to desensitization results in heightened LSEC apoptosis and failure of late angiogenesis (not shown here; see Figure 3).