VHL E3 Ligase: Its Function and Role in Disease

The von Hippel-Lindau (VHL) E3 ligase is a cellular machine that plays a part in how cells respond to their environment. It is a component of the cell’s protein regulation system, responsible for marking specific proteins for disposal. This function is necessary for maintaining normal cellular operations. When the VHL E3 ligase does not work correctly, it can lead to the development of various diseases, making its operations a subject of intense scientific study.

Understanding the VHL Protein Complex

The VHL E3 ligase is a key part of the ubiquitin-proteasome system, which removes unwanted or damaged proteins by tagging them for disposal. The VHL protein itself is part of an E3 ubiquitin ligase complex that places a “tag,” a small protein called ubiquitin, onto specific targets. The functional VHL E3 ligase is not a single protein but a multi-subunit complex.

The VHL protein acts as the substrate recognition subunit, identifying the target destined for degradation. To perform its function, the VHL protein assembles with Elongin B, Elongin C, Cullin-2 (Cul2), and RBX1. Elongins B and C form a unit that connects the VHL protein to the rest of the machinery. This assembly links to Cul2, a scaffold protein providing structural support, which in turn binds to RBX1, a component that recruits the enzyme responsible for attaching the ubiquitin tag.

VHL E3 Ligase in Cellular Oxygen Sensing

One of the most understood functions of the VHL E3 ligase is its role in how cells sense and adapt to oxygen availability. This process centers on Hypoxia-Inducible Factors (HIFs), specifically the HIF-alpha subunit. HIFs are transcription factors that, in low-oxygen conditions, activate genes to help cells survive by promoting new blood vessel growth.

When oxygen levels are normal, enzymes called prolyl hydroxylases (PHDs) add hydroxyl groups to the HIF-alpha subunit. This hydroxylation acts as a recognition signal for the VHL protein complex. Once HIF-alpha is hydroxylated, the VHL E3 ligase binds to it, triggering its ubiquitination. This chain of ubiquitin molecules marks HIF-alpha for destruction by the proteasome, preventing it from accumulating when oxygen is plentiful.

In low-oxygen (hypoxic) conditions, the PHD enzymes are inactive, so HIF-alpha is not hydroxylated. It therefore escapes VHL-mediated degradation, allowing it to build up and initiate the adaptive response.

The Link Between VHL Gene Defects and Von Hippel-Lindau Disease

The VHL protein is produced from instructions encoded in the VHL gene, and inherited mutations cause Von Hippel-Lindau (VHL) disease. This condition is inherited in an autosomal dominant pattern, meaning inheriting one faulty copy of the gene increases the risk of developing tumors. VHL disease predisposes individuals to the growth of tumors and cysts in various parts of the body.

A primary sign of the disease is the development of hemangioblastomas, which are benign tumors made of a dense network of blood vessels. These commonly occur in the brain, spinal cord, and retina and can cause significant problems depending on their location, leading to symptoms like headaches or vision loss. Individuals with VHL disease are also at a higher risk for certain cancerous tumors, including:

• Clear cell renal cell carcinoma (ccRCC), a type of kidney cancer.
• Pheochromocytomas, tumors of the adrenal glands that can cause high blood pressure.
• Cysts and tumors in the pancreas.
• Noncancerous endolymphatic sac tumors in the inner ear, which can lead to hearing loss.

VHL’s Role as a Tumor Suppressor

The VHL gene is classified as a tumor suppressor, meaning its protein product helps prevent cells from growing uncontrollably. Its function extends beyond inherited VHL disease, as inactivation of the VHL gene is a frequent event in sporadic (non-hereditary) cancers. This is most evident in clear cell renal cell carcinoma (ccRCC), where the VHL gene is inactivated in up to 90% of cases through mutation or epigenetic silencing.

The loss of functional VHL protein disrupts the oxygen-sensing pathway. Without VHL to mark it for destruction, the HIF-alpha protein accumulates inside the cancer cell, even when oxygen is abundant. This constant HIF activation tricks the cell into behaving as if it were in a low-oxygen environment. Consequently, HIF drives the expression of genes that promote tumor growth by stimulating new blood vessel formation (angiogenesis), altering glucose metabolism, and helping cancer cells survive.

Harnessing VHL E3 Ligase for Medical Treatments

The understanding of the VHL E3 ligase has opened new avenues for medical therapies. For cancers driven by VHL loss, treatments have been developed to target the downstream consequences. Belzutifan, for example, is a drug that inhibits HIF-2alpha, one of the factors that accumulates when VHL is inactive, and is approved for treating ccRCC and other tumors associated with VHL disease.

A more recent strategy involves “hijacking” the VHL E3 ligase using a technology called Proteolysis Targeting Chimeras (PROTACs). PROTACs are small molecules with two ends: one binds to the VHL E3 ligase, and the other binds to a specific disease-causing protein. This molecule acts as a bridge, bringing the target protein into close proximity with the VHL E3 ligase.

The VHL complex is then tricked into tagging this new protein with ubiquitin, marking it for destruction by the proteasome. This approach co-opts the cell’s own disposal machinery to eliminate proteins that drive diseases like cancer, representing a way to target proteins previously considered “undruggable.”