Hint2, a mitochondrial apoptotic sensitizer down-regulated in hepatocellular carcinoma

Abstract

Background & aims: Hints, histidine triad nucleotide-binding proteins, are adenosine monophosphate-lysine hydrolases of uncertain biological function. Here we report the characterization of human Hint2.

Methods: Tissue distribution was determined by real-time quantitative polymerase chain reaction and immunoblotting, cellular localization by immunocytochemistry, and transfection with green fluorescent protein constructs. Enzymatic activities for protein kinase C and adenosine phosphoramidase in the presence of Hint2 were measured. HepG2 cell lines with Hint2 overexpressed or knocked down were established. Apoptosis was assessed by immunoblotting for caspases and by flow cytometry. Tumor growth was measured in SCID mice. Expression in human tumors was investigated by microarrays.

Results: Hint2 was predominantly expressed in liver and pancreas. Hint2 was localized in mitochondria. Hint2 hydrolyzed adenosine monophosphate linked to an amino group (AMP-pNA; k(cat):0.0223 s(-1); Km:128 micromol/L). Exposed to apoptotic stress, fewer HepG2 cells overexpressing Hint2 remained viable (32.2 +/- 0.6% vs 57.7 +/- 4.6%), and more cells displayed changes of the mitochondrial membrane potential (87.8 +/- 2.35 vs 49.7 +/- 1.6%) with more cleaved caspases than control cells. The opposite was observed in HepG2 cells with knockdown expression of Hint2. Subcutaneous injection of HepG2 cells overexpressing Hint2 in SCID mice resulted in smaller tumors (0.32 +/- 0.13 g vs 0.85 +/- 0.35 g). Microarray analyses revealed that HINT2 messenger RNA is downregulated in hepatocellular carcinomas (-0.42 +/- 0.58 log2 vs -0.11 +/- 0.28 log2). Low abundance of HINT2 messenger RNA was associated with poor survival.

Conclusion: Hint2 defines a novel class of mitochondrial apoptotic sensitizers down-regulated in hepatocellular carcinoma.

Figure 1

(A) Alignment of amino acid sequences of the human Hint1 and Hint2. Amino acids conserved between both sequences are indicated in blue, and the 3 histidines of the HIT motif are indicated in red. (B) Phylogenic tree established by comparison of interspecies sequences of Hint1, Hint2, and Hint3 proteins using ClustalW software (http://www.ebi.ac.uk/clustalw/).

Figure 2

(A) Tissue distribution of HINT2 mRNA assessed by real-time quantitative PCR using a human cDNA panel (Clontech). Results were normalized to GAPDH expression and presented as fold variations comparatively with skeletal muscle. (B) Expression of Hint2 protein in different human tissues assessed by immunoblot analysis with anantibody against Hint2. (C) Immunocytochemistry for Hint2 in Huh-7 cells. Hint2 (in green, left panel, bar = 10μm) colocalized with the mitochondrial marker Mito Tracker (in red, middle panel) as shown in the overlay (in yellow, right panel). (D) Overlay confocal microscopy pictures of HEK-293 cells incubated with Mito Tracker to mark the mitochondria in red and expressing a chimeric protein made of Hint2 and GFP. When the GFP was at the C-terminus of Hint2, Hint2 localized to the mitochondria (left panel). When the GFP protein was at the N-terminus of Hint2, Hint2 remained in the cytoplasm (right panel; original magnification 40×).

Figure 3

(A) The phosphorylation of histones (330 ng) by PKC was not affected by increasing amounts of Hint2 protein or by increasing amounts of bovine serum albumin (control, not shown). (B) Activated PKC phosphorylated histones but not Hint2 (upper panel). The presence of Hint2 in the reaction was confirmed by immunoblot analysis (lower panel).

Figure 4

(A) HepG2 cells were stably transfected with HINT2cDNA, vector (pControl), and HINT2-H149A cDNA. mRNA abundance was assessed by real-time quantitative PCR (upper panel) and protein expression by immunoblot analysis (lower panel). (B) HepG2 cells were stably transfected to silence HINT2 mRNA ex-pression as assessed by real-time quantitative PCR (upper panel) and by immunoblot analysis (lower panel).

Figure 5

(A) Effect of Hint2 overexpression on cell viability. Cells were incubated for 16 hours with anti-Fas antibody (100 ng/mL) and actinomycin D (50ng/mL), and viability was assessed by the means of 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyltetrazolium bromide assay. Viability was significantly lower in cells overexpressing Hint2 (32.2% ± 0.6% vs 57.7% ± 4.6%; *P < .02). (B) Cells were labeled withJC-1, a mitochondrial membrane potential marker, and fluorescent properties were analyzed by flowcytometry after 0-, 3-, 6-, 10-, 16-, and 24-hour exposure to anti-Fas antibody (100 ng/mL) and actinomycin D (50ng/mL). The percentage of cells with a dissipated mitochondrial potential was significantly higher in the case of Hint2 overexpression (87.8% ± 2.3% vs 49.7% ± 1.6% at 16 hours and 92.3% ± 2.6% vs 68.3% ± 1.8% at 24 hours; *P < .05). (C) The expression of phosphorylated Blc-2, Bcl-xL, Bak, Bid, and cytochrome c was not affected by overexpression of Hint2. More cleaved caspase-9, caspase-3, and caspase-7 and PARP were found in HepG2 cells overexpressing Hint2 in response to incubation with anti-Fas antibody (100 ng/mL) and actinomycin D (50 ng/mL) for 16hours. The figure shows immunoblots representative of 3 independent experiments. (D) Incubation with the caspase inhibitors zVAD-fmk (50 mol/L) or zIETD-fmk (10 mol/L) prevented the anti-Fas antibody–and actinomycin D–induced cleavage of PARP as assessed by immunoblot analysis.

Figure 6

Effect of incubation with ethanol. The expression of phosphorylated Bcl-xL, Bak, Bid, and cytochrome c was not affected by overexpression of Hint2. More cleaved caspase-9, caspase-3, and caspase-7 and PARP were found in HepG2 cells overexpressing Hint2 in response to incubation with ethanol (400 mmol/L) for 24hours. The figure shows immunoblots representative of 3 independent experiments.

Figure 7

(A) Cells were labeled with JC-1, a mitochondrial membrane potential marker, and fluorescent properties were analyzed by flow cytometry after 0, 3, 6, 10, 16, and 24 hours of exposure to anti-Fas antibody (100 ng/mL) and actinomycin D (50 ng/mL). The percentage of apoptotic cells was significantly lower in the case of Hint2-silenced expression (42.4% ± 4.2% vs 60.9% ± 3.9% at 16 hours and 62.5% ± 8.6% vs 85.8% ± 3.7% at 24 hours; *P < .05). (B) Less cleaved caspase-3 was observed in HepG2 cells with silenced expression of Hint2 in response to anti-Fas antibody (100 ng/mL) and actinomycin D (50 ng/mL) for 16 hours compared with the other cell lines.

Figure 8

Hint2 protein expression and tumor burden. (A) Effect of Hint2 overexpression on tumor size in SCID mice after subcutaneous injection of 3 × 10⁶HepG2 cells stably transfected with pControl vector or HINT2cDNA (0.32 ± 0.13 g vs 0.85 ± 0.35g; *P < .05; n = 5 in each group). (B) Expression of cleaved caspase-9, PARP, and Hint2 was assessed by immunoblot analysis in the tumors. The level of expression of these proteins was significantly different between the Hint2 tumors and the pControl tumors (1.79 ± 0.07 vs 0.65 ± 0.06, 0.84 ± 0.09 vs 1.38 ± 1.1, and 3.62 ± 0.08 vs 0.54 ± 0.5, respectively; *P < .05).

Figure 9

(A) Expression of HINT1, HINT2, and HINT3 in 91 hepatocellular carcinomas and their surrounding tissues analyzed by microchip array (upper panel) and in the 91 hepatocellular carcinoma samples divided according to prognosis (lower panel). HINT2 mRNA was less abundant in hepatocellular carcinoma than in adjacent tissue (−0.424 ± 0.579 vs −0.109 ± 0.277 log2; P < .0002) and less abundant in tumors with a poor prognosis than in those with a better prognosis (−0.728 ± 0.522 vs −0.187 ± 0.506 log2; P < 1.0 × 10⁻⁵). Such changes were not observed for HINT1 or HINT3. (B) Survival difference of patients with hepatocellular carcinoma based on the expression patterns of HINT2. Patients were ranked on the basis of HINT2 mRNA levels in tumoral tissue using normal liver as reference and separated into 2 groups as indicated (upper panel). Kaplan–Meier plot of overall survival of patients with hepatocellular carcinoma grouped on the basis of HINT2 mRNA expression level (lower panel). The difference between groups was significant (P = .01, log-rank test).