Table 1.
Isoforms of major components of the IGF1-Akt pathway
| Component | Isoforms | References |
|---|---|---|
| IGF11 | Two (mouse: 1A, 1B) or three (human: 1A, 1B & 1C) isoforms, differing in the C terminal peptide (E peptide); another two (mouse: 2A, 2B) or three (human: 2A, 2B & 2C) isoforms differing in signal peptide because of utilization of exon 2 instead of exon 1 | [49] |
| IGF1 receptor | Heterotetramer made of two α-subunit (IGF-binding) and two β-subunit (tyrosine kinase). No isoforms, but hybrids with insulin receptor are present in skeletal muscle | [112] |
| IRS2 | IRS1 and IRS2 | [65,113] |
| PDK13 | No isoforms | [114] |
| PI3K4 (class I) | Heterodimer of p85 regulatory and p110 catalytic subunits. p85: three isoforms (p85α, p55α, p50α) encoded by a single gene; two other isoforms (p85β, p55γ) coded by other genes. p55γ is not expressed in muscle. p110: three isoforms (p110α, p110β, p110δ). p110δ is not expressed in muscle | [115] |
| Akt/PKB5 | Akt1/PKBα, Akt2/PKBβ & Akt3/PKBγ. Akt3 is not expressed in muscle | [4,116] |
| mTOR6 | No isoforms, but mTOR can interact with different partners: Raptor in the rapamycin-sensitive complex mTORC1, or Rictor in the rapamycin-insensitive complex mTORC2 | [5] |
| S6K7 | S6K1 and S6K2 | [117-119] |
| 4EBP8 | 4EBP1, 4EBP2, 4EBP3. 4EBP3 is not expressed in muscle | [120] |
| FoxO | 4 isoforms: FoxO1, FoxO3, FoxO4 and FoxO6. FoxO6 is not expressed in muscle | [121] |
1Insulin-like growth factor.
2Insulin receptor substrate.
33-Phosphoinositide-dependent kinase 1, coded by the PDPK1 gene (unfortunately the same abbreviation PDK1 is also used to indicate the pyruvate dehydrogenase kinase isoform 1, coded by the PDK1 gene).
4Phosphoinositide-3 kinase.
5Protein kinase B.
6Mammalian target of rapamycin.
7S6 kinase.
8Eukaryotic initiation factor 4E binding protein.