A novel in-frame deletion mutation in a case of lactate dehydrogenase (LD) H subunit deficiency showing an atypical LD isoenzyme pattern in serum and erythrocytes

Targeting Lactate Dehydrogenase-B as a Strategy to Fight Cancer: Identification of Potential Inhibitors by In Silico Analysis and In Vitro Screening

Lactate dehydrogenase (LDH) is an enzyme that catalyzes the reversible conversion of lactate to pyruvate while reducing NAD+ to NADH (or oxidizing NADH to NAD+). Due to its central role in the Warburg effect, LDH-A isoform has been considered a promising target for treating several types of cancer. However, research on inhibitors targeting LDH-B isoform is still limited, despite the enzyme's implication in the development of specific cancer types such as breast and lung cancer. This study aimed to identify small-molecule compounds that specifically inhibit LDH-B. Our in silico analysis identified eight commercially available compounds that may affect LDH-B activity. The best five candidates, namely tucatinib, capmatinib, moxidectin, rifampicin, and acetyldigoxin, were evaluated further in vitro. Our results revealed that two compounds, viz., tucatinib and capmatinib, currently used for treating breast and lung cancer, respectively, could also act as inhibitors of LDH-B. Both compounds inhibited LDH-B activity through an uncompetitive mechanism, as observed in in vitro experiments. Molecular dynamics studies further support these findings. Together, our results suggest that two known drugs currently being used to treat specific cancer types may have a dual effect and target more than one enzyme that facilitates the development of these types of cancers. Furthermore, the results of this study could be used as a new starting point for identifying more potent and specific LDH-B inhibitors.

Tumor lactic acid: a potential target for cancer therapy

Tumor development is influenced by circulating metabolites and most tumors are exposed to substantially elevated levels of lactic acid and low levels of nutrients, such as glucose and glutamine. Tumor-derived lactic acid, the major circulating carbon metabolite, regulates energy metabolism and cancer cell signaling pathways, while also acting as an energy source and signaling molecule. Recent studies have yielded new insights into the pro-tumorigenic action of lactic acid and its metabolism. These insights suggest an anti-tumor therapeutic strategy targeting the oncometabolite lactic acid, with the aim of improving the efficacy and clinical safety of tumor metabolism inhibitors. This review describes the current understanding of the multifunctional roles of tumor lactic acid, as well as therapeutic approaches targeting lactic acid metabolism, including lactate dehydrogenase and monocarboxylate transporters, for anti-cancer therapy.

Identification of the first highly selective inhibitor of human lactate dehydrogenase B

Lactate dehydrogenase (LDH) catalyses the conversion of pyruvate to lactate and NADH to NAD⁺; it has two isoforms, LDHA and LDHB. LDHA is a promising target for cancer therapy, whereas LDHB is necessary for basal autophagy and cancer cell proliferation in oxidative and glycolytic cancer cells. To the best of our knowledge, selective inhibitors for LDHB have not yet been reported. Here, we developed a high-throughput mass spectrometry screening system using an LDHB enzyme assay by detecting NADH and NAD⁺. As a result, we identified a small-molecule LDHB selective inhibitor AXKO-0046, an indole derivative. This compound exhibited uncompetitive LDHB inhibition (EC₅₀ = 42 nM). X-ray crystallography revealed that AXKO-0046 bound to the potential allosteric site away from the LDHB catalytic active site, suggesting that targeting the tetramerisation interface of the two dimers is critical for the enzymatic activity. AXKO-0046 and its derivatives can be used to validate LDHB-associated pathways in cancer metabolism.

Lactate Dehydrogenase B Controls Lysosome Activity and Autophagy in Cancer

Metabolic adaptability is essential for tumor progression and includes cooperation between cancer cells with different metabolic phenotypes. Optimal glucose supply to glycolytic cancer cells occurs when oxidative cancer cells use lactate preferentially to glucose. However, using lactate instead of glucose mimics glucose deprivation, and glucose starvation induces autophagy. We report that lactate sustains autophagy in cancer. In cancer cells preferentially to normal cells, lactate dehydrogenase B (LDHB), catalyzing the conversion of lactate and NAD(+) to pyruvate, NADH and H(+), controls lysosomal acidification, vesicle maturation, and intracellular proteolysis. LDHB activity is necessary for basal autophagy and cancer cell proliferation not only in oxidative cancer cells but also in glycolytic cancer cells.

Regulation of pyruvate metabolism and human disease

Pyruvate is a keystone molecule critical for numerous aspects of eukaryotic and human metabolism. Pyruvate is the end-product of glycolysis, is derived from additional sources in the cellular cytoplasm, and is ultimately destined for transport into mitochondria as a master fuel input undergirding citric acid cycle carbon flux. In mitochondria, pyruvate drives ATP production by oxidative phosphorylation and multiple biosynthetic pathways intersecting the citric acid cycle. Mitochondrial pyruvate metabolism is regulated by many enzymes, including the recently discovered mitochondria pyruvate carrier, pyruvate dehydrogenase, and pyruvate carboxylase, to modulate overall pyruvate carbon flux. Mutations in any of the genes encoding for proteins regulating pyruvate metabolism may lead to disease. Numerous cases have been described. Aberrant pyruvate metabolism plays an especially prominent role in cancer, heart failure, and neurodegeneration. Because most major diseases involve aberrant metabolism, understanding and exploiting pyruvate carbon flux may yield novel treatments that enhance human health.

A novel missense mutation in human lactate dehydrogenase B-subunit gene

Reduced activity of serum lactate dehydrogenase (LDH; EC 1.1.1.27) was found in a male medical student during practical examinations of his own blood. Serum LDH isoenzyme pattern showed reductions in activities of the isoenzymes with lower subunit A/B ratios such as LDH1 and LDH2. These findings were indicative of a partial LDH-B subunit deficiency, which was confirmed in erythrocyte hemolysates by Western blotting. Polymerase chain reaction (PCR)-based DNA sequence analysis of the LDH-B subunit gene revealed a heterozygous nucleotide change: a guanine to adenine substitution in codon 69 (GGG --> GAG) at the third exon of the LDH-B subunit gene that resulted in a glycine to glutamic acid substitution (G69E). The mutation was confirmed by PCR-restriction fragment length polymorphism (RFLP) analysis using a mismatched primer to introduce a new NcoI restriction site. The same heterozygous mutation was found in his mother but not in other family members. This mutation involves a residue belonging to alphaC helix in LDH-B subunit protein molecule that functions as an interface for other subunits.