Cell membrane receptors can be divided into two broad categories:

• ionotropic receptors (ion channels that open directly upon ligan binding) and
• metabotropic receptors (these receptors utilize second messengers to indirectly influence their corresponding ion channels)

Steroid receptors are located in the cell cytoplasm or nucleus, while others are membrane bound

Growth Factors can stimulate cell proliferation by altering the expression of certain genes in the nucleus. After a growth factor binds to its cell membrane receptor, signal transduction systems transfer the signal to the nucleus. Examples of signal transduction systems include:

1. MAP-kinase pathway
2. JAK/STAT pathway
3. PI3K/Akt/mTOR pathway
4. Inositol phospholipid pathway
5. cAMP pathway

PI3K/Akt/mTOR pathway is important for anti-apoptosis, cellular proliferation, and angiogenesis. Mutations in growth factor receptors, Akt, mTOR, or PTEN that enhance the activity of this pathway contribute to cancer pathogenesis.

There are four main types of membrane receptor:

1. ligand-gated ion channels,
2. tyrosine kinase receptors (enzyme-linked),
3. guanylate cyclase receptors and
4. G protein-coupled receptors

Ligand-gated ion channel receptors

• [ ] generally mediate fast responses
• [ ] e.g. nicotinic acetylcholine, GABA-A & GABA-C, glutamate receptors