Hedgehog influences the accumulation of fat in the liver

The signaling pathway regulates the amount of lipids adult liver cells store

Molecular biologist, Dr. Madlen Matz-Soja, LiSyM Junior Group Leader at the Rudolf-Schönheimer Institute of Biochemistry at Leipzig University, could demonstrate that the Hedgehog signaling pathway has a major influence on whether fat accumulates in liver cells or not. This occurs in the case of non-alcoholic fatty liver disease (NAFLD), a common condition. Now the molecular biologist is keen to find out if the signaling pathway plays an additional role in the adult liver. Hedgehog may well be partially responsible for the gender differences apparent in liver cirrhosis and liver cancer. Matz-Soja’s long-term goal is to improve therapies based on Hedgehog.

Hedgehog signaling pathway (Hh) – through it, cells respond to external signals. It leads from the cell membrane through several stations into the cell nucleus. Hh mainly regulates embryonic cell development.
Smoothened (smo) – a protein, and intracellular molecule in the Hh signal chain.
Gli factors – three transcription factors. The quantity and relative amounts of each change in response to Hh signals. For example, Gli1, Gli2 and Gli3 regulate, and hence change, the activity of the target genes.
Gli code – signature of the relative amount of Gli factors.
Hepatocytes – liver cells, which make up approx. 80 percent of the liver volume. They are responsible for most of the organ’s metabolic processes.

“It was surprising to find that the Hedgehog signaling pathway is still active in adult hepatocytes,” says Matz-Soja. In mammals, the morphogenetic signaling pathway regulates how embryonic cells differentiate, i.e. specialize, to form different organs and tissue. In most fully-grown, adult cells Hh is barely active or not at all, Matz-Soja explains: “Until only a few years ago, it was thought that Hedgehog was physiologically insignificant in adult liver cells.”

However, during her doctoral research under Prof. Rolf Gebhardt at the Faculty of Medicine, Leipzig University, Matz-Soja discovered that Hh has a significant influence in adult hepatocytes. The idea of shutting down the signal chain emerged. Matz-Soja began rearing transgene mice, which would allow her to deactivate a specific molecule in their hepatocytes – the protein smoothened. The result: adult mice whose Hh has been deactivated develop a fatty liver. Translated into human patients, doctors would diagnose NAFLD in this case. Many people suffer from this disease, especially from industrialized nations. Some of them run the risk of developing liver damage – partly because too little is known about the onset of NAFLD.

Non-alcoholic Fatty Liver Disease, NAFLD

Experts diagnose NAFLD when more than five percent of hepatocytes consist have stored fat and alcohol does not play a role. Estimates indicate between 20 to 30 percent of the population in European is affected. The major causes are an unfavorable genetic disposition and an unhealthy lifestyle with poor diet and lack of exercise. Weight loss can reverse NAFLD in the early stage. Failing that, it will progress gradually. In up to 20 percent of those affected, the non-inflammatory form (simple or bland fatty liver) will graduate to an inflammatory form (NASH: non-alcoholic steatohepatitis). By this stage the liver cell function is in part already damaged. Between 10 to 20 percent of these patients develop severe fibrosis, some a pronounced cirrhosis.

A metabolism network changes its activity

“The accumulation of fat in the adult liver is directly related to Hedgehog,” Matz-Soja emphasizes. By deactivating the signaling pathway, the amount of Gli1 and Gli3 decreases. These two transcription factors occur at the end of the Hh signal chain. This reduction causes changes in the activities throughout the whole network of proteins involved in metabolic processes. Matz-Soja could demonstrate this with DNA chips, inhibitory RNAs and other molecular biological methods.

Her next step was to interfere directly with the Gli code, i.e. with the relative proportions of the Gli transcription factors. She developed hepatocytes that produced excessive amounts of Gli1, Gli2 or Gli3. Simply put, this is the opposite of what happens when Hh no longer functions. In fact, hepatocytes with excess Gli, be it in mice or humans, store much less fat than normal liver cells. “The experimental results correspond,” Matz-Soja states and concludes: “A healthy metabolism needs a well-balanced Gli code.”

An inactive, or barely active Hh signaling pathway promotes the accumulation of fat in adult hepatocytes. The changes in the relative amounts of Gli transcription factors has a major impact on the lipid metabolism in these cells.

Matz-Soja is currently working with various members of the LiSyM network on a computer model. The team utilizes neural networks and Deep Learning to simulate a comprehensive Hh signaling pathway of adult hepatocytes from mice with all known components. “We provide the results from our in vitro and in vivo experiments,” she says, explaining the work of her six-member research group. With the model she hopes to understand fully the signaling pathway and learn more about the onset of NAFLD.

Sex Hormones in liver cells become unbalanced

The model could also shed light on how Hh functions in relation to gender. When the signaling pathway is deactivated via smoothened, hepatocytes in adult mice – in contrast to the normal situation – produce testosterone. Thus the sex hormones become unbalanced. Matz-Soja believes this “could play a role in gender-specific diseases.” Males are three times more likely to develop liver cancer and three times more likely to die from it than females. Similarly, cirrhosis of the liver is more common among males. “These differences cannot be explained by lifestyle choices alone,” says the 38-year-old scientist. Other, unknown factors must also play a role: possibly within the Hh signaling pathway? She explains that pharmaceuticals can influence this pathway. “Patients with certain types of cancer are at times given Hedgehog inhibitors.” Although the drugs are effective against tumors, they also affect the liver. Consequently, many patients discontinue the therapy on account of the strong side effects. Is it possible perhaps to improve the selectivity of Hh inhibitors?

“I sincerely hope to see my research results lead to therapies,” says Matz-Soja who initially studied ecotrophology (branch of nutritional science) at the Anhalt University of Applied Sciences in Bernburg before transferring to the Institute of Biochemistry at Leipzig University’s medical faculty. Here she received her doctorate and now runs a LiSyM Junior Group. “I am striving to show the potential of science,” says Madlen Matz-Soja. “It would be wonderful if my findings inspired other scientists to simply try something new.”

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