growth factor
Meaning of growth factor: Cells do not decide alone when to multiply; these protein messages from neighboring tissue grant or withhold the permission.
Definition of growth factor
A growth factor is a naturally occurring protein that carries instructions between cells, telling a target cell to divide, to mature into a specialized type, to move, or simply to stay alive. Each one works by locking onto a matching receptor on the cell surface, which starts a chain of signals inside the cell. Dozens are known, and each has its own set of target tissues.
How does a growth factor deliver its message?
The molecule itself never enters the cell it acts on. It binds to a receptor that sits in the cell membrane with part of itself outside and part inside. Binding changes the shape of the inner part, which switches on the first of a series of relay proteins, and the message is passed inward step by step until it reaches the nucleus and alters which genes are read. Most growth factors travel only a short distance, acting on the cell that made them or on the cells immediately around it.
How is it different from a hormone?
The two overlap, and a few molecules are counted as both. The usual distinction is one of range and of the tissue that produces the signal.
| Feature | Growth factor | Hormone |
|---|---|---|
| Where it acts | Nearby cells, often in the same tissue | Distant organs, reached through the blood |
| Made by | Many ordinary cell types | Usually a dedicated gland |
| Typical effect | Division, repair, survival, specialization | Metabolism, growth, reproduction, salt and water balance |
Growth hormone sits awkwardly across the line, since a gland releases it into the blood yet much of its effect comes from local growth factors it triggers in the tissues it reaches.
Which ones are given as medicines?
Several are manufactured and used routinely rather than experimentally. A hematopoietic growth factor pushes the marrow to make more blood cells, which shortens the period of low white cell counts after chemotherapy and reduces the need for transfusion in some forms of kidney failure. Others speed the healing of wounds and of the mouth lining damaged by treatment. The catch is that a signal telling cells to multiply is not always welcome, so the tissue targeted and the timing are chosen with care.
Why do cancers involve them?
A tumor grows when the instruction to divide arrives constantly instead of occasionally. That happens either because too much of the signal is produced, or because the receptor is stuck in the on position and no longer waits to be told. The receptor HER2 is the best known example of the second pattern, and drugs that block such receptors form a large part of targeted cancer treatment.
Used in a sentence
The tumor was tested for overexpression of a growth factor receptor before treatment was planned.
Good to know
The same protein signals that drive normal repair also drive tumor growth when their receptors are locked on, which is why several cancer drugs block growth factor receptors rather than the growth factors themselves.