A new study published in the journal Nature reports the discovery of a new mechanism of gene regulation mediated by lactate, a metabolic product accumulated in cancer cells. The researchers show that histones are lactylated in the presence of lactate.

Cancer cells are known to use a lot of glucose to produce energy. Glucose break down to molecules called pyruvate by a process called glycolysis. In normal cells pyruvate enters mitochondria, gets oxidized and generate more energy. This energy is produced in a storage form called ATP. The oxidation of pyruvate and production of energy in mitochondria require oxygen.

However in the cancer cell, though oxygen is available, pyruvate fails to go to mitochondria, instead it gets converted to a molecule called lactate. It was about 100 years ago that Otto Warburg described this metabolic process in the cancer cell. Because of the cells inability to oxidize pyruvate even in the presence of oxygen he called the process aerobic glycolysis.

When glycolyis takes place under anerobic conditions, the end effect is the same as in aerobic glycolysis, but the process is called anaerobic glycolysis. In both aerobic glycolysis and anerobic glycolysis, pyruvate is converted to lactate and the lactate accumulates in the cell.

When lactate accumulates inside the cells, it can move across cell membrane barriers and enter circulation. Accumulated lactate gets converted back to pyruvate in the liver to make new glucose. Though scientists knew for the last 100 years that lactate accumulates inside the cell under certain pathological conditions, no one looked at the fate of lactate, other than its metabolic fate, as a gene expression modulator through epigenetics.

In the article published in the journal Nature, the scientists show that lactate plays an important role in epigenetic modification regulating biological functions. Using a series of well controlled and sophisticated experiments scientists establish that intracellular lactate can be added to the amino group of lysine residues of nuclear proteins called histones. This protein modification is called lactylation. Histones are essential proteins in the formation of structures called nucleosomes, which are clusters of histones with DNA segments wound around it. Epigenetic modification of histones influence gene expression by providing access for other proteins to read and transcribe the proximal genes. Epigenetic modifications have been known to be caused by chemical groups such as acetyl, methyl and phosphate groups.

To confirm that lactylation of histones takes place, they used a technique called HPLC-tandem mass spectrometry. Using mass spectrometry proteins can be ionized and precise mass of each fragments can be determined. They purified histones from a cell line called MCF-7 and after digestion they were tested using HPLC-mass spectrometry. They showed that by increasing intracellular lactate or exogenouse sources of lactate, such as glucose, lactylation of histones can be increased.

They further found that hypoxia and altered level of glucose can modulate lactylation of histones. Hypoxia increases intracellular lactate. The functional significance of histone lactylation was shown by demonstrating lactylation in a type of white blood cells called macrophages that fight infection. On infection and injury these macrophages differentiate to a killer type of macrophage called M1 macrophage. They found that these M1 macrophages start accumulating lactate after few hours and histone lactylation follows. The histone lactylation was accompanied by the expression of a number of genes that oppose the M1 differentiation. They speculate that late lactylation in M1 macrophages tames the killer phenotype and this inherent defensive mechanism helps macrophage to restore homeostasis.

One may wonder whether lactylation of histones is a non-enzymatic process, however the authors show that a protein called p300 can act as a biological catalyst (enzyme) for lactylation of lysines on histones. p300 is known to have acetyl transferase activity and this report shows that p300 may also catalyze lactylation of histones.

The discovery that lysine amino acid units in histones can be lactylated is a major advance as it is important in a number of health and disease conditions, including cancer. The biological significance of protein lactylation is just beginning to be unraveled. The functional distinction between histone acetylation and lactylation is also an important aspect to be investigated further.

The study opens up a number of questions. The first and foremost is the identity of the enzyme that catalyses lactylation. The authors show p300 can do this function, however it is not clear whether p300 is the enzyme that performs this function in vivo. Lactate is first accumulated in the cytosol, so cytosol proteins are more exposed to increased lactate. Are they also lactylated? What are the functional alterations following protein lactylation? Do lactylated proteins get delactylated? If that happens what is the nature of the catalytic enzyme involved in delactylation? In spite of these unanswered questions, the discovery reported in the paper is a paradigm shifting finding and one may wonder why nobody attempted to demonstrate lactylation since Warburg postulated aerobic glycolysis one hundred years ago.