Metabolic dysfunction: The liver and beyond

The liver is the centre of our metabolism, with metabolic dysfunction playing a pivotal role in liver diseases and type 2 diabetes. Despite extensive study, the mechanisms underlying these pathologies remain largely unexplained. Professor Philipp Kaldis of Lund University, Sweden, has collaborated with fellow researchers to investigate the role of metabolites during metabolic dysfunction in these diseases. Motivated by the goal of helping patients, they hope their continued research on the underlying mechanisms of metabolic dysfunction in the liver as well as other organs will reveal novel therapeutic targets.

The liver is a principal organ for all metabolic needs, including glucose and lipid metabolism. Professor Philipp Kaldis of Lund University, Sweden has collaborated with a team of researchers to understand metabolism and its links to diseases, such as liver disease and type 2 diabetes (T2D). These diseases have both genetic and environmental causes (behavioural causes or alterations in metabolism due to the environment). The liver is central to this pathology, but other tissues and organs are believed to be involved as well.

Grasping the ‘crosstalk’ or signalling between organs could be pivotal in deciphering the pathophysiology of metabolic diseases. Advances in laboratory technologies, such as mass spectrometry, provide a tool to identify metabolites – biproducts yielded from cell metabolism – and biomarkers implicated in such diseases. Yet, scientists suspect there are still many metabolites and biomarkers not yet identified and still have not fully elucidated the underlying mechanisms involving these molecules in metabolic diseases.

Unravelling the complexity of metabolic dysfunction is a tall order. However, by identifying the mechanisms underlying their observations in animal studies, Kaldis and colleagues hope to translate their research into therapies that will one day benefit patients.

Liver cell regeneration

Metabolism refers to the substantial number of biochemical reactions in our cells that break down food into energy enabling cell, organ, and whole-body functions. It is a complex process involving multiple organs, tissues, and a plethora of metabolites, enzymes, and biomarkers. As the centre of metabolism, the liver consistently suffers a barrage of insults from toxins causing liver cell (hepatocyte) damage. However, hepatocytes have the remarkable ability to renew and regenerate following an injury. Initial stages of regeneration involve hypertrophy (increase in size) of hepatocytes, whereas in the latter stages, cell division dominates. This regeneration requires cell proliferation and regulation of metabolism, but little is known about the metabolic requirements during liver regeneration.

In their 2018 paper, ‘Metabolic remodeling during liver regeneration’, the researchers explored the metabolic requirements using a novel approach, combining functional MRI imaging, metabolomics (the study of metabolites in cells), transcriptomics (study of RNA transcripts), and cell biology. They emphasised the metabolic changes at various times during regeneration and addressed hepatocyte metabolic needs along with the mechanisms and pathways involved.

The researchers’ experiments demonstrated that stopping hepatocytes from dividing results in mitochondrial dysfunction and downregulation of oxidative pathways. Consequently, alanine transaminase (ALT) was increased, and metabolic remodeling ensued. This signalled liver damage, but importantly also regulated metabolism by reducing oxidation, impairing mitochondrial functions, and increasing amino acid metabolism to aid regeneration. This prior work by the team provided a deeper understanding of metabolic events that occur during liver regeneration, acting as a starting point in the development of liver regeneration biomarkers.

The importance of cell division in liver diseases

Recently there has been a major increase in patients suffering from non-alcoholic fatty liver disease (NAFLD), making it the most prevalent liver disease worldwide. Pathological features include fat build-up in hepatocytes, abnormal metabolism, and cell division. Recognising the contribution of metabolic dysfunction within the disease, the new term – metabolic dysfunction-associated fatty liver disease (MAFLD) – is increasingly used. Marked by release of metabolites and signalling molecules, such as organokines (proteins secreted by organs that regulate metabolism) and hormones, this metabolic dysfunction is a driving cause of fat build-up in the liver. These molecules enable communication between metabolic organs to coordinate crosstalk between the liver and other tissues.

Liver disease can be progressive, worsening from MAFLD/NAFLD to non-alcoholic steatohepatitis (NASH) where fatty liver co-exists with inflammation. These diseases can eventually lead to hepatocellular carcinoma (HCC), the most common type of liver cancer. The inability of hepatocyte to divide is a hallmark of such liver diseases, prompting investigation into both the underlying causes and consequences of this inability. The protein family of cyclin-dependent kinases orchestrates cell cycle progression, therefore acting as a key factor in understanding diseases marked by abnormal cell division. Cyclin-dependent kinase 1 (CDK1) is one such protein and previous studies have shown that when CDK1 is deleted, hepatocytes become senescent and are unable to undergo cell division. Instead, they become hypertrophic, causing changes in glucose metabolism.

Phospholipids form the plasma membrane of new daughter cells. This implies that lipid metabolism is essential for cell division. But how does the cell cycle and metabolism, specifically lipid metabolism, interlink in MAFLD/NAFLD during regeneration after both acute and chronic damage? Can pathways be better understood for the development of targeted therapeutics? Aiming to comprehend the molecular regulation of the cell cycle, the researchers reviewed the process in detail, emphasising the importance of cell cycle regulation in MAFLD/NAFLD.

By using a mouse model with loss of the cell cycle regulatory gene CDK1 in hepatocytes, the team was able to study how the impairment of hepatocyte cell division alters lipid metabolism. In their paper, ‘Remodeling of whole-body lipid metabolism and a diabetic-like phenotype caused by loss of CDK1 and hepatocyte division’, the team proposes that the loss of this cell cycle regulator could be both a result of liver disease and a contributor to it. Their in-depth analysis revealed that a consequence of CDK1 activity loss is dysfunctional fatty acid oxidation, resulting in increased free fatty acids in the blood and subsequently, elevated blood insulin and insulin resistance. Increased blood glucose levels conclude in a diabetic-like phenotype along with liver disease.

Another consequence of CDK1 activity loss is increased storage of free fatty acids in adipose tissue, leading to additional fat mass in the mice. CDK1 knockout mice also suffer hepatic steatosis and fibrosis, distinct features of NASH. The team proposes the loss of hepatocyte CDK1 activity as both a cause and an outcome of liver disease. They also suggest that therapeutics targeting senescent cells could help in the treatment of metabolic diseases of the liver.

The team acknowledge there is no known mutation of CDK1 in liver disease and T2D. However, other studies indicated increased abundance of senescent cells and reduced CDK1 activity in NAFLD, possibly due to elevated levels of the cyclin-dependent kinase inhibitor – p21 (CDKN1A). Combining the results of the studies, the researchers concluded that CDK1 has a role in the disease pathology. These results illustrate the knock-on effects of CDK1 loss in hepatocytes, helping to further unravel the interplay between the cell cycle and lipid metabolism in liver disease and T2D. Since the loss of CDK1 occurs in various pathologies, this research exposes the intricacies of this pathology, possibly paving way for wide-ranging clinical implications.

Interorgan crosstalk

Although a major focus for the team has been researching metabolites in the liver specifically, they also advocate for the importance of taking a holistic view to study the communication or crosstalk with other organs to piece together the complex pathology of metabolic diseases. Common comorbidities associated with fatty liver disease are T2D, obesity, increased blood pressure and blood lipid levels, and the metabolic syndrome. Such comorbidities suggest that the pathology of fatty liver disease is not limited to the liver, but involves other organs and tissues, including pancreas, muscle, fat, heart, and brain.

Involvement of other organs and tissues requires communication or signalling between tissues through molecules in the blood. This communication is called interorgan crosstalk. For the liver, the signalling molecules are commonly organokines, hormones, microRNAs, and extracellular vesicles. Termed ’the master regulator of energy homeostasis’ because of its involvement in glucose synthesis, glycogen storage, and bile acid synthesis, the liver has been extensively studied for its role in T2D pathology. In addition to elevated blood glucose in T2D, other metabolic changes occur, and numerous metabolites from other tissues are implicated.

The researchers draw particular attention to the role of the liver and adipose tissue in producing metabolites and organokines that affect the insulin producing islets in the pancreas. In a recently published review, they advocate for the importance of interorgan crosstalk in T2D. They suggest that far-reaching effects of liver metabolites and hepatokines (proteins secreted by the liver that regulate metabolic pathways) on other tissues could stimulate inflammation and oxidative stress in T2D pathology. Additionally, inflammatory and obesity effects induced by metabolites, released from adipose tissue, may contribute to the diabetic phenotype.

Kaldis and colleagues explain that further exploration of crosstalk in the next few years could be key in understanding fatty liver disease and other diseases with metabolic origin. This may elucidate disease mechanisms and reveal therapeutic targets. However, there are many challenges involved in the study of organ crosstalk, not just technically, but also in proving that just because metabolites associate with a disease, does not mean they cause it. Further, the complexity of the ‘signalling traffic’ between organs and tissues makes it difficult to determine the intended target of each metabolite.

The recent spike in fatty liver disease and T2D may be testament to the change in our eating habits due to a surge in the consumption of a high sugar and high fat diet, tipping metabolic homeostasis into dysfunction. With the ultimate goal of helping patients, the team have dedicated years to elucidate the mechanisms of these diseases, unveil metabolites at the centre of this pathology, and unpick the complexities of organ crosstalk. What we know about metabolites in metabolic dysfunction is just the tip of the iceberg, leaving a void that opens the door for future mechanistic studies and more research in this area.