Introduction: Hematological malignancies like leukemia, lymphoma, and multiple myeloma are the results of the abnormal growth of hematopoietic cells. They are famously recognized for their heterogeneity both clinically and within their biological roots. A fundamental characteristic that is increasingly seen as crucial for cancer growth is the reprogramming of metabolism, whereby there is the reordering of nutrient utilization for higher energy and biosynthetic demands of the cancer cells. A critical switch comes into the limelight via the emergence of carbohydrates as a necessary hallmark, such that there can be brisk growth of the cancer cells within unfavorable or nutrient-deprived situations.
An interesting example of this reprogramming of metabolism is the Warburg effect, where cancer cells favor the use of glycolysis even when oxygen is plentiful, which is in opposition to normal cells, which typically rely on mitochondrial oxidative phosphorylation because it is more effective. Hexokinase 2 (HK2), an isoform of hexokinase that converts glucose into glucose-6-phosphate, the first and rate-controlling step of glycolysis, has been recognized as a key player for this process. HK2 is overexpressed in a variety of cancers, such as hematological malignancies, and suggests that it is responsible for enabling the elevated glycolytic flux upon which tumors rely. Beyond the content of their metabolism, HK2 is also recognized for its interaction with the mitochondrial membranes and its potential ability for the evasion of apoptosis and the maintenance of the stem-like features of cancer cells.
In spite of this increasing comprehension, the exact role of HK2 in cancer metabolism and its promise as a target for chemotherapy continues to be multifaceted. The metabolism of cancer cells is incredibly adaptive, frequently compensating for inhibition of the targeted pathway. Therefore, understanding the ways drugs engage HK2 and the ways these engage normal and cancerous cells is paramount for the development of successful chemotherapy.
Methods: In order to examine the function and druggability of HK2 in blood malignancies, we assembled a comprehensive database of molecular and metabolically oriented studies of carbohydrate metabolism and hexokinase isoform expression within hematopoietic cancers. This database was interrogated via Megagene software, which combines pathway mapping and molecular annotation for the purpose of explaining HK2 expression profiles, regulation, and metabolism throughout leukemia, lymphoma, and multiple myeloma cases.
Accordingly, we initiated molecular docking studies for the simulation of the interaction of HK2 with recognized or proposed small-molecule hexokinase inhibitors. Considered ligands were small molecules such as 3-bromopyruvate and lonidamine, and newly designed experimental inhibitors for the selective binding of HK2 active or allosteric sites. Docking models were implemented on HK2 structures extracted from cancer and normal cell isoforms for the identification of pertinent differences of binding affinity and interaction profiles. This was performed with the objective of unveiling viable selective target mechanisms competent for the inhibition of HK2 activity in cancer versus normal cells with minimal toxicity.
Results: Megagene analysis confirmed substantial upregulation of HK2 in hematological malignancies versus normal hematopoietic tissues. Elevated expression was coupled with elevated glycolytic capacity, indicative of the Warburg phenotype for the facilitation of rapid ATP synthesis and biosynthetic precursor formation for tumor growth. Mitochondrial localization of HK2 was also coupled with suppression of apoptosis of cancer cells and the conservation of stemness, indicative of its multi-faceted role.
Docking analyses identified that some ligands had high binding affinities for HK2 amongst cancer cell models, specifically at locations of enzyme activity and for mitochondrial interaction. As an example, 3-bromopyruvate had high binding affinity for the active site of HK2 within cancer cells and impaired glucose phosphorylation and subsequent glycolytic flux. There were some differences, however, with binding affinities and interaction profiles for the docking done on HK2 isoforms characteristic of normal cells. The differences can be attributed to differences in the structure or the post-translational modifications specific for the cancer-related HK2 and could translate into a potential therapeutic window for selective inhibition.
Lonidamine, another clinical candidate hexokinase inhibitor, exhibited moderate binding affinity but seemed preferential for the disruption of the mitochondrial association of HK2 over the enzyme function itself. Such mechanism could underlie its capacity for the sensitization of the cancer cells to apoptosis without causing significant impairment of normal cell metabolism. More recent experimental inhibitors developed based upon the docking data had favorable prospective selectivity profiles, ostensibly reducing off-target toxicities.
Functionally, HK2 inhibition with these ligands in cancer cells resulted in decreased glycolytic throughput, diminished anabolic intermediate production, and increased chemotherapy and radiation sensitivity. Even the acidic tumor microenvironment produced by HK2-mediated lactic acid production for facilitating immune evasion and tumor growth was abrogated upon drug therapy. Nevertheless, the ability of cancer cells for metabolic compensation continues to be an obstacle; output compensatory mechanisms, such as upregulated glutamine metabolism, can restrict the prolonged effectiveness of HK2-targeting theraplies as single agents.
Conclusion: Hexokinase 2 is pivotal for the reprogramming of metabolism that defines hematological malignancies and for glycolysis-mediated energy production, tumor proliferation, and immunoescape. Our comparative studies of pathways and molecular docking analyses point to HK2 as a promising target and identify the structural and functional differences amenable for the development of selective drugs. Although inhibitors such as 3-bromopyruvate and Lonidamine are promising, the molecular complexity of cancer metabolism implies that HK2-centered drugs will likely require incorporation into the full complement of therapeutic regimens for the multiplicity of metabolites and signal molecules. Future investigations must prioritize the development of selective inhibitors, the definition of hexokinase isoform-specific function, and the addressing of the plasticity of metabolism for the realization of superior therapeutic benefits for hematological malignancies.
Keywords: Hematological malignancies, Hexokinase2, Glycolysis, Warburg, Leukemia, Cancer Metabolism,Docking