IGFBPs can inhibit IGF actions, enhance IGF actions, or function independently as cell
regulatory factors. IGFBPs act as transporters of IGF, prolonging the life of IGF within
the body. Insulin-like growth factor (IGF) binding proteins (IGFBPs) play a pivotal role
in the regulation of IGF activity and availability in the circulation and extracellular
environment.
The IGFBP family has at least 6 members, which serve as transporter proteins and as
storage pools for IGF-1. All IGFBP family members are cysteine-rich proteins with high
binding affinity for IGF-I and IGF-II. IGFBP-3, the major IGF binding protein in plasma,
exists in a ternary complex with IGF-I or IGF-II and an acid-labile subunit (ALS). The
expressions of IGFBPs are tissue- and developmental stage–specific, and the
concentrations of IGFBPs in different body compartments are different.
The functions of IGFBPs are regulated by phosphorylation, proteolysis, polymerization, and cell or matrix
association of the IGFBP. Whereas the IGFBPs consistently have extremely high affinity
for IGF-1, the N-terminally truncated des-IGF-1 and a variety of IGF-1 analogues have
markedly reduced affinity for IGFBPs but retain normal affinity for IGF-1R.1 Moreover,
IGFBP-1 may play regulatory role inside of cells. It was reported that a portion of
intracellular IGFBP1 protein localizes to mitochondria where it binds to the
proapoptotic protein BAK and hinders BAK activation and apoptosis induction.
Interestingly, in many cells and tissues p53 also has a direct apoptotic function at
mitochondria that includes BAK binding and activation. When IGFBP1 is in a complex with
BAK, formations of a proapoptotic p53/BAK complex and apoptosis induction are impaired,
both in cultured cells and in liver. All 6 IGFBPs have been shown to inhibit IGF-1
action, but IGFBP-1, -3, and -5 are also shown to stimulate IGF-1 action. Some of IGFBPs
’ effects might be IGF-1 independent.
Specification: 96Test
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