HINT2

Histidine triad nucleotide binding protein 2 (HINT2) is a mitochondrial protein that in humans is encoded by the HINT2 gene on chromosome 9. This protein is an AMP-lysine hydrolase and phosphoamidase and may contribute to tumor suppression.^{[5][6][7][8][9]}

HINT2
Available structures PDB Ortholog search: PDBe RCSB List of PDB id codes 4INC, 4INI, 4NJX, 4NJY, 4NJZ, 4NK0
Identifiers
Aliases	HINT2, HIT-17, histidine triad nucleotide binding protein 2
External IDs	OMIM: 609997; MGI: 1916167; HomoloGene: 13072; GeneCards: HINT2; OMA:HINT2 - orthologs
Gene location (Human) Chr. Chromosome 9 (human)[1] Band 9p13.3 Start 35,812,960 bp[1] End 35,815,354 bp[1]
Gene location (Mouse) Chr. Chromosome 4 (mouse)[2] Band 4|4 A5 Start 43,654,227 bp[2] End 43,656,466 bp[2]
RNA expression pattern Bgee Human Mouse (ortholog) Top expressed in apex of heart right lobe of liver right adrenal gland right adrenal cortex left adrenal cortex human kidney muscle of thigh left ventricle mucosa of transverse colon olfactory zone of nasal mucosa Top expressed in right kidney choroid plexus of fourth ventricle left lobe of liver Ileal epithelium human kidney proximal tubule interventricular septum right ventricle myocardium of ventricle yolk sac More reference expression data BioGPS n/a
Gene ontology Molecular function nucleotide binding catalytic activity hydrolase activity Cellular component nucleolus mitochondrion Biological process steroid biosynthetic process lipid metabolism apoptotic process negative regulation of peptidyl-lysine acetylation lipid catabolic process Sources:Amigo / QuickGO
Orthologs Species Human Mouse Entrez 84681 68917 Ensembl ENSG00000137133 ENSMUSG00000028470 UniProt Q9BX68 Q9D0S9 RefSeq (mRNA) NM_032593 NM_026871 RefSeq (protein) NP_115982 NP_081147 Location (UCSC) Chr 9: 35.81 – 35.82 Mb Chr 4: 43.65 – 43.66 Mb PubMed search [3] [4]
Wikidata
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Structure

As a member of the histidine triad nucleotide-binding (Hint) protein family, which is a subfamily of the histidine triad (HIT) family, HINT2 contains a conserved histidine and HIT sequence motif (His-X-His-X-His-X-X), and the latter two histidines contribute to a catalytic triad.^[6][8] The 163-amino acid protein encoded by this gene forms a 17-kDa homodimer.^[6][7][8] Compared to other members of the Hint family, HINT2 has a 61% sequence homology to HINT1 and 28% sequence homology to HINT3.^[6] When compared with HINT1, the 35–amino acid extension at the HINT2 N-terminal corresponds to a predicted mitochondria import signal.^[8]

Function

HINT2 is a member of the HIT superfamily and Hint subfamily, which are characterized as nucleotide hydrolases and transferases that act on the alpha-phosphate of ribonucleotides.^[5] The Hint family is the oldest within the HIT superfamily and thus, its members are highly conserved among eukaryotes and archaebacteria.^[6] The Hint proteins function as AMP-lysine hydrolases and phosphoramidases.^[6][7][8][9] In mammals, HINT2 is expressed in the liver, adrenal cortex, and pancreas and localizes to the mitochondria within their cells.^[7][8] Specifically, the protein is located in the inner mitochondrial membrane, facing the mitochondrial matrix. This positioning likely facilitates the transport of cholesterol from the cytosol to the matrix, which is necessary for steroidogenesis, by providing a contact site for the hydrophobic molecule and allowing it to cross the mitochondrial intermembrane space. HINT2 regulates steroidogenesis through calcium-dependent and calcium-independent signalling pathways that may serve to maintain a favorable mitochondrial potential. Its role in calcium homeostasis may also contribute to its proapoptotic function in hepatocytes and other non-steroidogenic cells, though the exact mechanism remains unclear.^[7]

Clinical significance

Hint2, one of the three members of the Hint family of proteins, is localized to mitochondria of various cell types. In human adrenocarcinoma cells, Hint2 modulates Ca2+ handling by mitochondria. In all living organisms, intracellular calcium controls a wide variety of physiological processes. Extracellular stimuli generate temporally organized Ca2+ signals, which most of the time occur as repetitive spikes. The frequency of these oscillations controls the nature and the extent of the cellular response. Ca2+ oscillations originate from the repetitive opening of the inositol 1,4,5-trisphosphate (InsP3) receptors that are Ca2+ channels embedded in the membrane of the endoplasmic reticulum (ER). Opening of these channels is initiated by the stimulus-induced rise in InsP3; because their activity is biphasically regulated by the level of cytoplasmic Ca2+, oscillations can occur. Mitochondria also affect cytoplasmic Ca2+ signals. They can both buffer cytosolic Ca2+ changes (7 and 8) and release Ca2+. At rest, intramitochondrial ([Ca2+]m) and cytosolic Ca2+ concentration ([Ca2+]i) are similar, of the order of 100 nM (9).

The Hint family has been implicated in tumor suppression.^[6] Int2, a member of the superfamily of histidine triad proteins, has been localized exclusively in mitochondria, near the contact sites of the inner membrane. This enzyme is highly expressed in the liver, where it has been shown to stimulate mitochondrial lipid metabolism, respiration, and glucose homeostasis. Hint2 modulates cytoplasmic and mitochondrial Ca2+ dynamics by stimulating the activity of the mitochondrial respiratory chain. It appears that the absence of Hint2 leads to a premature opening of the mitochondrial permeability transition pore (mPTP) in mitochondrial suspensions. As such, HINT2 plays a prominent role in mitochondrial cell death signaling (e.g. apoptosis) and in ischemia-reperfusion injury (for instance during heart attacks) through calcium homeostasis.

In particular, HINT2 is also observed to be upregulated in breast, pancreatic, and colon cancer cells, while it is downregulated in hepatocellular carcinoma and endometrial cancer. Its exact role in tumor suppression remains unknown, though studies suggest it may promote apoptosis in hepatocellular carcinoma and endometrial cancer.^[6][7][8][9] In double knockout Hint2 mice, higher acylation and morphological alterations were observed in the mitochondria, suggesting that Hint2 may regulate glucose and lipid metabolism.^[10]

Interactions

Currently, HINT2 has no known protein-protein interaction partners.^[6]

See also

• Histidine triad nucleotide-binding protein 1 (HINT1)

References

1. GRCh38: Ensembl release 89: ENSG00000137133 – Ensembl, May 2017
2. GRCm38: Ensembl release 89: ENSMUSG00000028470 – Ensembl, May 2017
3. "Human PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
4. "Mouse PubMed Reference:". National Center for Biotechnology Information, U.S. National Library of Medicine.
5. "Entrez Gene: HINT2 histidine triad nucleotide binding protein 2".
6. Maize KM, Wagner CR, Finzel BC (Jul 2013). "Structural characterization of human histidine triad nucleotide-binding protein 2, a member of the histidine triad superfamily". The FEBS Journal. 280 (14): 3389–98. doi:10.1111/febs.12330. PMC 3703500. PMID 23659632.
7. Lenglet S, Antigny F, Vetterli L, Dufour JF, Rossier MF (Nov 2008). "Hint2 is expressed in the mitochondria of H295R cells and is involved in steroidogenesis". Endocrinology. 149 (11): 5461–9. doi:10.1210/en.2008-0400. PMID 18653718.
8. Martin J, Magnino F, Schmidt K, Piguet AC, Lee JS, Semela D, St-Pierre MV, Ziemiecki A, Cassio D, Brenner C, Thorgeirsson SS, Dufour JF (Jun 2006). "Hint2, a mitochondrial apoptotic sensitizer down-regulated in hepatocellular carcinoma". Gastroenterology. 130 (7): 2179–88. doi:10.1053/j.gastro.2006.03.024. PMC 2569837. PMID 16762638.
9. Lee LR, Teng PN, Nguyen H, Hood BL, Kavandi L, Wang G, Turbov JM, Thaete LG, Hamilton CA, Maxwell GL, Rodriguez GC, Conrads TP, Syed V (Jul 2013). "Progesterone enhances calcitriol antitumor activity by upregulating vitamin D receptor expression and promoting apoptosis in endometrial cancer cells". Cancer Prevention Research. 6 (7): 731–43. doi:10.1158/1940-6207.CAPR-12-0493. PMID 23682076.
10. Martin, Juliette; Maurhofer, Olivier; Bellance, Nadège; Benard, Giovanni; Graber, Franziska; Hahn, Dagmar; Galinier, Anne; Hora, Caroline; Gupta, Anirudh (2013-05-01). "Disruption of the histidine triad nucleotide-binding hint2 gene in mice affects glycemic control and mitochondrial function". Hepatology. 57 (5): 2037–2048. doi:10.1002/hep.26060. ISSN 1527-3350. PMID 22961760. S2CID 28755223.