Technical Discussion

Insulin-like growth factors or IGFs are peptide hormones secreted from many different cells. Their designation as "insulin-like" originated from experiments in which treatment of serum with antibodies to insulin failed to eliminate all insulin activity; the remaining activity was ultimately ascribed to the IGFs. Due to their growth promoting activity, they were formerly called somatomedins.

Lay Interpretation

IGFs are Insulin like Growth Factors.

IGFs, IGF Receptors and IGF-Binding Proteins

There are two principle IGFs referred to as IGF-I and IGF-II. Each of these has a number of variant forms, resulting from use of alternative gene promoters and alternative splicing. Structurally, both IGFs resemble insulin in having two chains (A and B) connected by disulfide bonds. Human IGF-I and IGF-II are, respectively, 70 and 67 amino acids in length.

In addition to two IGFs, there are three receptors that bind IGFs with differing affinities:

• Type 1 IGF receptor: binds both IGF-I and IGF-II with high affinity. This receptor has been identified in essentially all tissues except liver, and virtually all of the biological activities of the IGFs result from binding to the type 1 receptor.
• Type 2 IGF receptor: binds IGF-II with high affinity and IGF-I with low affinity. It appears primarily to be involved in clearance and degradation of IGF-II, although it may also elicit some distinct signalling. This is also the cation-independent mannose-6-phosphate receptor used for targeting mannosylated enzymes to lysosomes.
• Insulin receptor: binds IGF-I with roughly 100-fold lower affinity than insulin. High concentrations of IGF may stimulate insulin signalling through this receptor.

Interestingly, the type 1 IGF and insulin receptors are very similar structurally. In fact, in cells that express both receptors, hybrid receptors appear to readily form, although the biological consequences of these hybrids is not clear.

IGFs come in two flavours,
IGF-1 and IGF-2. In humans, these peptides have 70 and 67 amino acids per molecule respectively.

A type of IGF binding protein will often attach to IGFs, extending its half life markedly.  At least six different variants of these have been identified.

The technical information on these pages is the work of Professor Bowen et al, Colorado State University and are reproduced without endorsement of any kind.  The "lay" interpretations are the work of this site and do not necessarily reflect Professor Bowen's opinions.