An RTK, or receptor tyrosine kinase, is a type of protein embedded in the surface of your cells that acts like a molecular antenna. It detects chemical signals outside the cell and relays instructions inward, telling the cell to grow, divide, move, or survive. Humans have 58 known RTKs grouped into 20 families, and because they control such fundamental processes, problems with RTKs are at the center of many cancers and a growing number of targeted therapies.
How RTKs Are Built
Every RTK is a single protein that spans the cell membrane, creating three functional zones. The outer portion, called the extracellular domain, sticks out from the cell surface and is shaped to grab onto specific signaling molecules (called ligands), usually growth factors. A short segment passes through the fatty membrane itself, anchoring the receptor in place. The inner portion, sitting inside the cell, contains the enzyme machinery that kicks off a chain reaction once a signal arrives.
That inner portion is the “tyrosine kinase” the name refers to. A kinase is an enzyme that attaches a small chemical tag (a phosphate group) to other proteins, switching them on or off. This particular type of kinase tags a specific building block of proteins called tyrosine. That tagging step is the first domino in a cascade of signals that ultimately reach the cell’s nucleus and change its behavior.
One notable exception to the standard design is the insulin receptor. Most RTKs start as single chains that pair up only when activated. The insulin receptor, by contrast, is permanently assembled as a two-part structure: two pieces that sit outside the cell linked to two pieces that cross the membrane and contain the kinase machinery. It’s pre-built as a pair, while nearly every other RTK has to find a partner in the moment.
What Happens When a Signal Arrives
In its resting state, an RTK sits in the membrane mostly alone and inactive. Its outer domain is folded in a way that hides the surfaces it would need to link up with a neighboring receptor. Think of it as a closed handshake position.
When a growth factor binds, the receptor’s outer domain shifts into an open shape, exposing a contact surface sometimes called the “dimerization arm.” This lets two receptor molecules lock together side by side, forming a pair (a dimer). Each growth factor molecule is clamped between two lobes of the same receptor, so the final complex looks like two receptors, each holding its own growth factor, pressed back to back.
Pairing is the critical step. Once two receptors are held together, their internal kinase domains are positioned close enough to tag each other’s tyrosine residues, a process called trans-autophosphorylation. Some of those tagged sites keep the kinase itself switched on, while others serve as docking ports. Adaptor proteins inside the cell recognize those ports, latch on, and relay the message deeper into the cell through branching signaling pathways.
The Signaling Pathways RTKs Activate
Despite the wide variety of growth factors and RTKs in the body, they funnel into a surprisingly small number of internal pathways. Two of the most important are commonly known by their abbreviations: the MAPK/ERK pathway and the PI3K/AKT pathway.
The MAPK/ERK pathway is best known for driving cell proliferation. When active, it tells cells to multiply, helps them survive by counteracting self-destruct signals, and guides cell movement and specialization. It’s one of the reasons a wound heals: growth factors released at the injury site activate RTKs, which fire up this pathway and push nearby cells to divide and fill the gap.
The PI3K/AKT pathway overlaps in function but has its own emphasis. It promotes survival by blocking proteins that would otherwise trigger cell death. It also plays a major role in cell migration and growth. This pathway converts one membrane lipid into another, creating a landing pad for a protein called AKT, which then fans out to influence dozens of downstream targets involved in metabolism, growth, and survival.
Other pathways activated by RTKs include the stress-responsive p38 pathway, the JNK pathway (involved in proliferation and programmed cell death), and the ERK5 pathway, which is important for cardiovascular development. There are additional branches involving enzymes and transcription factors, but the core theme is the same: a signal at the surface gets amplified and diversified as it moves inward, eventually altering which genes the cell turns on or off.
Why RTKs Matter in Cancer
Because RTKs sit at the controls of cell growth and survival, defects in these receptors are a common driver of cancer. There are four main ways RTKs go wrong in tumors.
- Gain-of-function mutations: A change in the receptor’s DNA can leave the kinase permanently switched on, even without a growth factor present. In lung cancer, for example, roughly 90% of activating mutations in the EGF receptor are either small deletions in one region of the gene or a single-letter swap in another. Both changes hyperactivate the kinase and its downstream signals.
- Gene amplification: Cancer cells sometimes make extra copies of an RTK gene, flooding the membrane with receptors. The higher concentration makes it easier for receptors to bump into each other and activate spontaneously.
- Chromosomal rearrangements: Parts of an RTK gene can accidentally fuse with another gene during DNA damage, producing a hybrid protein that is active all the time.
- Autocrine activation: Some tumors produce their own growth factors, creating a feedback loop where the cell stimulates its own receptors continuously.
All four mechanisms converge on the same outcome: uncontrolled signaling that pushes cells to divide without the normal checks.
RTK-Targeted Therapies
The central role of RTKs in cancer has made them prime targets for drug development. Two broad strategies exist: small-molecule inhibitors that slip inside the cell and block the kinase directly, and antibody-based drugs that attach to the receptor’s outer surface and prevent growth factors from binding.
Small-molecule tyrosine kinase inhibitors (TKIs) are the largest class of targeted cancer drugs approved by the FDA. Many are designed for specific RTK targets. Drugs targeting the EGF receptor are widely used in non-small cell lung cancer, and at least eight TKIs approved between 2018 and 2023 alone are indicated for that disease. Other recently approved TKIs target receptors involved in bile duct cancers, bladder cancers, and blood cancers. Beyond oncology, a related group of kinase inhibitors targeting a pathway called JAK is now used for autoimmune and inflammatory conditions like rheumatoid arthritis and eczema.
Antibody therapies work differently. Trastuzumab, one of the best-known examples, is an antibody that binds the HER2 receptor (a member of the EGF receptor family) and is a cornerstone treatment for HER2-positive breast cancer. It can be combined with small-molecule inhibitors for added effect.
How Tumors Resist RTK-Targeted Drugs
One of the biggest challenges with these therapies is that tumors often find ways around them. The best-understood escape route is a secondary mutation in the targeted receptor itself, sometimes called a “gatekeeper” mutation, which changes the shape of the drug’s binding pocket so the drug no longer fits. In lung cancers treated with EGF receptor inhibitors, one particular gatekeeper mutation shows up in about half of resistant tumors.
Tumors can also amplify the gene for the targeted receptor, overwhelming the drug with sheer numbers of receptors. Perhaps more troubling is a strategy called bypass signaling: the tumor activates a completely different RTK or parallel pathway that the drug doesn’t touch, restoring growth signals through an alternate route. These resistance mechanisms are a major focus of ongoing drug development, with newer-generation inhibitors designed to remain effective against the most common escape mutations.