BEIJING -- A study aboard China's space station Tiangong is expected to identify the regulatory mechanisms of a type of liver cell's metabolism and provide a new approach to treating fatty liver disease.

This seven-day on-orbit experiment, concluded last week, was designed to elucidate the effects of space-associated biological phase separation on lipid metabolism under microgravity conditions.

Space microgravity constitutes a unique mechanical environment, involving factors that strongly regulate the metabolic functions of hepatocytes — a type of liver cell — said Li Ning, an associate researcher at the Institute of Mechanics of Chinese Academy of Sciences.

The liver is an organ with a highly-complex mechanical micro-environment. "During the progression of diseases such as liver fibrosis or fatty liver disease, changes in the mechanical micro-environment occur, including increased stiffness and altered blood flow," said Li.

Based on extensive experimental findings, scientists believe that the most direct mechanical factor leading to abnormal lipid metabolism in the liver is fluid shear stress.

Under normal conditions, blood flow creates interstitial shear stress as it passes through the walls of hepatic blood vessels, "washing over" the hepatocytes and thereby maintaining metabolic homeostasis.

In a microgravity environment, however, the cephalad redistribution of body fluids leads to increased blood flow above the heart and reduced blood flow below the heart, resulting in a significant decrease in portal venous blood flow to the liver.

At the same time, the disappearance of hydrostatic pressure reduces the pressure load on the liver. "The mechanical effect of blood flow on the liver is reduced in space," said Li.

Consequently, the regulatory role of blood flow diminishes, and fat accumulates more.

To study how blood flow shear stress regulates liver metabolic functions, the experiments conducted during the Shenzhou-16 space flight mission have shown that the microgravity environment activated the SREBP protein, leading to an increase in intracellular lipid droplets. Blood flow, however, inhibits the SREBP and exerts a protective effect, reducing lipid droplets.

Given that previous studies have confirmed the protective effect of the blood flow environment on cells, the new experiment specifically designed a corresponding intervention group to enhance cellular responses to mechanical stimulation from blood flow.

The in-orbit experiment selected hepatocytes as the research subject and designed three experimental conditions that included static culture, simulated blood flow environment and "blood flow environment with drug stimulation," totaling six cell samples.

The entire experiment was initiated by ground-based researchers through remote commands.

"During the in-orbit culture period, we performed microscopic imaging of the cells everyday to observe their growth status in real time," Li said, noting that on the seventh day, the system automatically injected a fixative solution to preserve the cellular state. Afterwards, the samples were placed into a freezer at minus 80 degrees Celsius for storage.

The samples are scheduled to be brought back to Earth in the second half of this year. The collection and in-depth analysis of the vast majority of core data will officially begin afterwards.