Non-invasive Biomarkers for Liver Fibrosis and Its Clinical Applications

Introduction

The liver is one of the most essential organs of the human body. Various functions like metabolisms, fibrotic changes, and bile acid production. But abnormal changes in the liver cells can disrupt these functions. One of the common liver disorders is liver fibrosis. All over the world, around 1.5 billion people are suffering from liver fibrosis. That is why diagnosing liver fibrosis is important.

What Is Liver Fibrosis?

Liver fibrosis is a condition when healthy liver tissue is replaced by scar tissue. Chronic liver diseases usually cause this. Accumulation of the extracellular matrix in the hepatic cells is the reason behind such pathology.

A. Factors Responsible for Hepatic Fibrosis

The risk factors for liver fibrosis are:

• Chronic alcoholism.

• Hepatitis B and Hepatitis C infection.

• Conditions responsible for non-alcoholic fatty liver. These conditions are obesity, excessive intake of proteins, hypertension, and diabetes.

• Smoking.

• Autoimmune hepatitis.

• Genetic conditions like hemochromatosis (a genetic disorder caused by excessive iron buildup in the body) and Wilson’s disease (a genetic disorder caused by excessive buildup of copper in the body).

B. Pathogenesis of Liver Fibrosis

The wound-healing process causes hepatic fibrosis due to repeated liver cell injury. These injuries can be of two types hepatotoxic injuries and cholestatic injuries. Factors like chronic alcoholism, viral infections, and non-alcoholic fatty liver cause hepatotoxic injuries. On the other hand, cholestatic injuries can be caused by factors like obstruction in the bile flow, primary sclerosing cholangitis (narrowing of the bile duct caused by inflammation), and biliary atresia (blockage of the bile duct in infants due to the presence of scar tissue).

In acute liver injury, the parenchymal cells of the liver are replaced with necrotic cells. This process is associated with limited production of extracellular matrix and inflammatory mediators. But, during chronic liver injury, this regeneration process does not work. Excessive production of inflammatory mediators destroys the liver cells, and excessive amounts of extracellular matrix are produced. These factors cause the deposition of a large number of fibrillar collagen.

In normal liver cells, hepatic stellate cells are located in a particular location of the liver. But during chronic injury of the liver, the cells are transported to various locations. There these cells are differentiated into fibroblasts-like cells with proinflammatory and fibrogenic properties. Fibroblasts derived from other sites like blood vessels and hepatic cells also participated in this process. The deposition of fibers is aided by the secretion of different cytokines. These cytokines are tumor growth factors- beta, platelet-derived growth factors, interleukin ten, and interferon-gamma. Cytokines with vasoconstrictive properties play a crucial role in the fibrogenesis process. Substances like adipokines (cytokines secreted from adipose tissues) and angiotensin II are the key factors in the process of fibrillogenesis.

The collagen fiber deposited in this process is mainly the type I and type III processes. This fibrosis deposition is mainly seen around the portal tract, pericentral and perisinusoidal areas. Gradually fiber deposition forms fibrous bands. Progressive deposition of fiber from nodules. In this condition, liver function is compromised, and normal function of the liver is lost.

C. Staging of Liver Fibrosis:

Different staging systems can be used for the diagnosis of liver fibrosis. Metavir scoring system can be used to determine the progression of liver fibrosis. The staging is as followers;

1. A0 - No fibrous activity.

2. A1 - Mild fibrous activity.

3. A2 - Moderate fibrous activity.

4. A3 - Severe fibrous activity.

This system also evaluates the level of fibrosis.

1. F0 - Nonfibrosis is present.

2. F1 - Portan fibrosis is resent without no septa.

3. F2 - Involvement of several septa with no cirrhosis.

4. F3 - Presence of liver cirrhosis.

Another classification system that is used for liver fibrosis is the Ishak fibrosis scoring system. The scale is as follows:

1. 0 - Non-fibrosis is present.

2. 1 - Some portal areas are involved with short and fibrous septa.

3. 2 - Most of the portal areas are involved with short and fibrous septa.

4. 3 - Expansion into most portal areas with portal-to-portal bridging.

5. 4 - Significant amount of fibrosis is present with a significant amount of portal-to-portal and portal-to-central bridging is present.

6. 5 - Sporadic nodules are present.

7. 6 - Cirrhosis is present.

All this staging is done on the basis of several diagnostic criteria. One of the gold standard techniques for liver fibrosis is liver biopsy. But this is an invasive procedure that can lead to lots of complications like hospitalization, pain, and high cost. For this purpose, assessment of various non-invasive biomarkers are essential.

What Are the Non-invasive Biomarkers?

Non-invasive biomarkers are those indicators that can be assessed easily without surgical interventions. These biomarkers are:

1. Class I (Direct) Biomarkers: These are known as direct biomarkers. These biomarkers are produced by the hepatic stellate cells during the process of production of the extracellular matrix. These markers can be enzymatic markers, collagen markers, and glycoprotein markers. Collagen biomarkers are most commonly assessed. Assessment factors like procollagen type I carboxy-terminal peptide (PICP), and procollagen type III amino-terminal peptide (PIIINP) are done. These factors are secreted by the hepatic stellate cells to form a basement membrane. The level of proteolytic enzymes like metalloproteinases-2 (MMP-2) also remains high in liver fibrosis cases. Other direct biomarkers that can be assessed are:

• Paraoxonase 1 (PON-1): This is an enzyme that is linked with the death of hepatic cells.

• Hyaluronic Acid (HA): It is a glycosaminoglycan that is part of the extracellular matrix.

• Laminin: This is a non-collagenous glycoprotein that is secreted by the hepatic stellate cells.

2. Class II (Indirect) Biomarkers: These are the indirect biomarkers and depict fibrosis via a change in the liver functions. The liver function in a broad range of activity like the metabolism of various substances, iron, and hemoglobin metabolism, immune balance, and production of bile acids. Biological markers associated with these functions can reflect changes in liver activity. Such markers are:

• AST/ALT Ratio: Aspartate aminotransferase (AST) and Alanine aminotransferase (ALT) are two important hepatic enzymes. In normal conditions, the ratio is less than one, but in liver fibrosis cases, an increased ratio can be seen.

• PGA Index: This is a combination of three measurements prothrombin time, gamma-glutamyl transpeptidase activity, and serum apolipoprotein A1 concentration. The PGA index varies from 0 to 12. An increased PGA index signifies greater chances of liver fibrosis.

• Forns Index: In this method, four different factors like age, platelet count, cholesterol levels, and g glutamyl transferase are evaluated, and the range of fibrosis is determined.

3. Imaging Assessment: Imaging techniques evaluate the biological properties of the hepatic tissues. These techniques are:

• Transient Ultrasound Elastography (FibroScan): This is an ultrasound-based technique that uses both high and low-frequency waves to access the stiffness of the hepatic tissues.

• Acoustic Radiation Force Impulse (ARFI): This is also an ultrasound-based technique in which the stiffness of liver tissues is assessed. In this technique, short-duration acoustic radiation forces are used for localized displacement of liver tissues.

• Magnetic Resonance Imaging (MRI): This can be used for the measurement of the stiffness of liver tissue and water-diffusion abnormalities. Magnetic Resonance Elastography is one of the most commonly used techniques for this purpose.

Conclusion:

Fatty liver is a common health problem. Factors like alcoholism, smoking, and viral infections are responsible for this. Diagnosing the extent of liver fibrosis is necessary for evaluating the extent of the liver damage. Non-invasive assessment of liver biomarkers can be useful in determining the extent of fibrosis.