A protooncogene is a gene that is involved in signal transduction and execution of mitogenic signals, usually through its protein product. Upon activation, it (or its product) becomes a tumor inducing agent, an oncogene.
The protooncogene can become an oncogene by a relatively small modification of its original function. There are two basic activation types:
- A mutation within a protooncogene can cause a change in the protein structure, caused by
- An increase in protein concentration, caused by
- an increase of protein expression (through misregulation)
- an increase of protein stability, prolonging its existence and thus its activity in the cell
- a gene duplication, resulting in a doubled amount of protein in the cell
[Growth factor]]s are usually secreted by few special cells to induce cell proliferation in other cells. If a cell that usually does not produce growth factors suddenly starts to do so (because it developed an oncogene), it will thereby induce its own uncontrolled proliferation (autocrine loop), as well as the proliferation of neighbouring cells.
There are six known classes of tyrosine kinases that can go onkogene:
- Receptor tyrosine kinases that become constitutive (permanently) active.
- Cytoplasmic tyrosine kinases, often products of viral oncogenes.
- Regulatory GTPases, for example, the Ras kinase.
- Cytoplasmic Serine/Threonine kinases and their regulatory subunits, for example, the Raf kinase, and cyclines (through overexpression).
- Adaptor proteins in signal transduction.
- Transcription factors.