Skip to main page content
U.S. flag

An official website of the United States government

Dot gov

The .gov means it’s official.
Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

Https

The site is secure.
The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
Review
. 2006 Aug 7;25(34):4777-86.
doi: 10.1038/sj.onc.1209603.

Hexokinase II: cancer's double-edged sword acting as both facilitator and gatekeeper of malignancy when bound to mitochondria

Affiliations
Review

Hexokinase II: cancer's double-edged sword acting as both facilitator and gatekeeper of malignancy when bound to mitochondria

S P Mathupala et al. Oncogene. .

Abstract

A key hallmark of many cancers, particularly the most aggressive, is the capacity to metabolize glucose at an elevated rate, a phenotype detected clinically using positron emission tomography (PET). This phenotype provides cancer cells, including those that participate in metastasis, a distinct competitive edge over normal cells. Specifically, after rapid entry of glucose into cancer cells on the glucose transporter, the highly glycolytic phenotype is supported by hexokinase (primarily HK II) that is overexpressed and bound to the outer mitochondrial membrane via the porin-like protein voltage-dependent anion channel (VDAC). This protein and the adenine nucleotide transporter move ATP, newly synthesized by the inner membrane located ATP synthase, to active sites on HK II. The abundant amounts of HK II bind both the ATP and the incoming glucose producing the product glucose-6-phosphate, also at an elevated rate. This critical metabolite then serves both as a biosynthetic precursor to support cell proliferation and as a precursor for lactic acid, the latter exiting cancer cells causing an unfavorable environment for normal cells. Although helping facilitate this chemical warfare, HK II via its mitochondrial location also suppresses the death of cancer cells, thus increasing their possibility for metastasis and the ultimate death of the human host. For these reasons, targeting this key enzyme is currently being investigated in several laboratories in a strategy to develop novel therapies that may turn the tide on the continuing struggle to find effective cures for cancer. One such candidate is 3-bromopyruvate that has been shown recently to eradicate advanced stage, PET positive hepatocellular carcinomas in an animal model without apparent harm to the animals.

PubMed Disclaimer

Figures

Figure 1
Figure 1
Delivery of glucose and ATP to hexokinase (HK) II bound to the outer mitochondrial membrane within a malignant cell and metabolic fates of the glucose-6-phosphate (G-6-P) formed. Glucose brought across the plasma membrane by glucose transporters (1) is rapidly phosphorylated by HK II (5) bound to voltage-dependent anion channel (VDAC) (4) located on the outer mitochondrial membrane. The VDAC allows direct access to HK II of ATP generated by the ATP synthase (2) and transported across the inner-mitochondrial membrane by the adenine nucleotide translocator (3). To maintain the highly glycolytic metabolic flux of such malignant cells, the product G-6-P is rapidly distributed across key metabolic routes. The primary routes are (a) direct entry of the G-6-P into the pentose-phosphate shunt for biosynthesis of nucleic-acid precursors and (b) conversion of the G-6-P via the glycolytic pathway to pyruvate and lactic acid. Here, whereas the lactic acid is transported out on lactate transporters (6) to provide an unfavorable environment for surrounding normal cells, some pyruvate is directed to mitochondria via the pyruvate transporter (7), to provide substrates for the tri-carboxylic acid (TCA) cycle. Citrate produced by this cycle then exits the mitochondria on the citrate transporter (8) to help synthesize membrane components (phospholipids and cholesterol) that are essential for tumor proliferation.
Figure 2
Figure 2
Tumors harness a multitude of genetic, epigenetic, transcriptional and post-translational strategies for enhanced expression and function of hexokinase (HK) II. During tumorigenesis of tissues where HK II is absent, the gene may be first brought out of its hibernation by demethylation, and then amplified 5–10-fold. Subsequently, the highly promiscuous promoter of the gene, which is activated by HIF-1, p53, glucose, and by both insulin and glucagon, further facilitates the tumor’s requirements regardless of the nutritional status of the tumor-bearing host, and fuels the enhanced and continued synthesis of the gene product. In contrast to other hexokinase isoforms, HK II harbors two active sites per enzyme moiety. As much as a 100-fold amplification of the enzyme may be observed in malignant tumors owing to these different processes. The mitochondrial voltage-dependent anion channel (VDAC)-bound localization of HK II further facilitates enzyme activity by allowing HK II to escape product inhibition, likely via conformational constraints.
Figure 3
Figure 3
Mitochondrial-bound hexokinase (HK) II plays a major role in preventing tumor apoptosis. Right: Without control mechanisms in place to prevent it, cell death would be highly likely within the unfavorable conditions that exist in a tumor microenvironment. Thus, caspase-mediated induction of apoptosis would be facilitated first by activation of the mitochondrial permeability transition pore complex (MPTP), indicated on the right by a question mark (?), that in turn would facilitate the release to the cytoplasm of the caspase activator cytochrome c (located within the inter-membrane space). Bcl-2-related proteins (Bax and Bad) would likely overcome effects of the MPTP inhibiting protein Bcl-XL and help facilitate release of cytochrome c. Left: By populating mitochondrial voltage-dependent anion channels (VDACs) with HK II and by persistent channeling of adenine nucleotides, opening of the MPTP is inhibited. This in turn inhibits access of VDACs to Bax and Bad, and most likely maintains cytochrome c in a state favorable for its mitochondrial retention in the inter-membrane space. Thus, HK II helps assure a highly malignant tumor’s proliferation, and its escape from cell death, under conditions that would otherwise favor this process. (The authors recognize that some aspects of this figure remain open to discussion and will necessitate additional studies to verify, modify or negate.)

References

    1. Ardehali H, Yano Y, Printz RL, Koch S, Whitesell RR, May JM, et al. J Biol Chem. 1996;271:1849–1852. - PubMed
    1. Arora KK, Parry DM, Pedersen PL. J Bioenerg Biomembr. 1992;24:47–53. - PubMed
    1. Arora KK, Pedersen PL. J Biol Chem. 1988;263:17422–17428. - PubMed
    1. Azoulay-Zohar H, Israelson A, Abu-Hamad S, Shoshan-Barmatz V. Biochem J. 2004;377:347–355. - PMC - PubMed
    1. Bay DC, Court DA. Biochem Cell Biol. 2002;80:551–562. - PubMed

Publication types