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Glucose phosphorylation by hexokinases (HKs) traps glucose in cells and facilitates its usage in metabolic processes dependent on cellular needs. HK domain-containing protein-1 (HKDC1) is a recently discovered protein with wide expression containing HK activity, first noted through a genome-wide association study (GWAS) to be linked with gestational glucose homeostasis during pregnancy. Since then, HKDC1 has been observed to be expressed in many human tissues. Moreover, studies have shown that HKDC1 plays a role in glucose homeostasis by which it may affect the progression of many pathophysiological conditions such as gestational diabetes mellitus (GDM), nonalcoholic steatohepatitis (NASH), and cancer. Here, we review the key studies contributing to our current understanding of the roles of HKDC1 in human pathophysiological conditions and potential therapeutic interventions.

The product of hexokinase (HK) enzymes, glucose-6-phosphate, can be metabolized through glycolysis or directed to alternative pathways, such as the pentose phosphate pathway (PPP) to generate anabolic intermediates. However, it is not known what determines the fate of G6P. HK1 contains an N-terminal mitochondrial–binding domain, but its physiologic significance remains unclear. We overexpressed full-length and truncated HK1 in tissue culture and generated mice lacking the HK1 mitochondrial-binding domain (ΔE1HK1). Although ΔE1HK1 mice displayed no overt phenotype, HK1 dislocation from the mitochondria increased glucose flux through the PPP, decreased flux below the level of GAPDH, and induced a hyper-inflammatory response to lipopolysaccharide. The mechanism for the increased PPP flux is through glycolytic block at GAPDH, which is mediated by binding of cytosolic HK1 with S100A8/A9 and increased GAPDH nitrosylation through iNOS. Additionally, human and mouse macrophages from conditions of low-grade inflammation, such as aging and diabetes, displayed an increase in cytosolic HK1 and cytokine production, along with reduced GAPDH activity. Our data indicate that HK1 mitochondrial-binding alters glucose metabolism through regulation of GAPDH.

In a recent genome-wide association study, hexokinase domain-containing protein 1, or HKDC1, was found to be associated with gestational glucose levels during 2-hour glucose tolerance tests at 28 weeks of pregnancy. Because our understanding of the mediators of gestational glucose homeostasis is incomplete, we have generated the first transgenic mouse model to begin to understand the role of HKDC1 in whole-body glucose homeostasis. Interestingly, deletion of both HKDC1 alleles results in in utero embryonic lethality. Thus, in this study, we report the in vivo role of HKDC1 in whole-body glucose homeostasis using a heterozygous-deleted HKDC1 mouse model (HKDC1+/−) as compared with matched wild-type mice. First, we observed no weight, fasting or random glucose, or fasting insulin abnormalities with aging in male and female HKDC1+/− mice. However, during glucose tolerance tests, glucose levels were impaired in both female and male HKDC1+/− mice at 15, 30, and 120 minutes at a later age (28 wk of age). These glucose tolerance differences also existed in the female HKDC1+/− mice at earlier ages but only during pregnancy. And finally, the impaired glucose tolerance in HKDC1+/− mice was likely due to diminished whole-body glucose use, as indicated by the decreased hepatic energy storage and reduced peripheral tissue uptake of glucose in HKDC1+/− mice. Collectively, these data highlight that HKDC1 is needed to maintain whole-body glucose homeostasis during pregnancy but also with aging, possibly through its role in glucose use.

Glucokinase (GCK) is the principal hexokinase (HK) in the liver, operating as a glucose sensor to regulate glucose metabolism and lipid homeostasis. Recently, we proposed HK domain-containing 1 (HKDC1) to be a fifth HK with expression in the liver. Here, we reveal HKDC1 to have low glucose-phosphorylating ability and demonstrate its association with the mitochondria in hepatocytes. As we have shown previously that genetic deletion of HKDC1 leads to altered hepatic triglyceride levels, we also explored the influence of overexpression of HKDC1 in hepatocytes on cellular metabolism, observing reduced glycolytic capacity and maximal mitochondrial respiration with concurrent reductions in glucose oxidation and mitochondrial membrane potential. Furthermore, we found that acute in vivo overexpression of HKDC1 in the liver induced substantial changes in mitochondrial dynamics. Altogether, these findings suggest that overexpression of HKDC1 causes mitochondrial dysfunction in hepatocytes. However, its overexpression was not enough to alter energy storage in the liver but led to mild improvement in glucose tolerance. We next investigated the conditions necessary to induce HKDC1 expression, observing HKDC1 expression to be elevated in human patients whose livers were at more advanced stages of nonalcoholic fatty liver disease (NAFLD) and similarly, found high liver expression in mice on diets causing high levels of liver inflammation and fibrosis. Overall, our data suggest that HKDC1 expression in hepatocytes results in defective mitochondrial function and altered hepatocellular metabolism and speculate that its expression in the liver may play a role in the development of NAFLD.

For mammals, acetylcholine (ACh) promotes endothelium-dependent vasodilation primarily through nitric oxide (NO) and prostaglandin-mediated pathways, with varying reliance on endothelial-derived hyperpolarizing factors. Currently, no studies have been conducted on small systemic arteries from wild birds. We hypothesized that ACh-mediated vasodilation of isolated small arteries from mourning doves (Zenaida macroura) would likewise depend on endothelial-derived factors. Small resistance mesenteric and cranial tibial (c. tibial) arteries (80-150 μm, inner diameter) were cannulated and pre-constricted to 50 % of resting inner diameter with phenylephrine then exposed to increasing concentrations of ACh (10(-9)-10(-5) M) or the NO donor, sodium nitroprusside (SNP; 10(-12)-10(-3) M). For mesenteric arteries, ACh-mediated vasodilation was significantly blunted with the potassium channel antagonist tetraethylammonium chloride (TEA, 10 mM); whereas responses were only moderately impaired with endothelial disruption or inhibition of prostaglandins (indomethacin, 10 μM). In contrast, endothelial disruption as well as exposure to TEA largely abolished vasodilatory responses to ACh in c. tibial arteries while no effect of prostaglandin inhibition was observed. For both vascular beds, responses to ACh were moderately dependent on the NO signaling pathway. Inhibition of NO synthase had no impact, despite complete reversal of phenylephrine-mediated tone with SNP, whereas inhibition of soluble guanylate cyclase (sGC) caused minor impairments. Endothelium-independent vasodilation also relied on potassium channels. In summary, ACh-mediated vasodilation of mesenteric and c. tibial arteries occurs through the activation of potassium channels to induce hyperpolarization with moderate reliance on sGC. Prostaglandins likewise play a small role in the vasodilatory response to ACh in mesenteric arteries.