Insulin‐like growth factor‐binding protein‐4 is a member of the insulin‐like growth factor binding protein (IGFBP) family and encodes a protein with an IGFBP domain and a thyroglobulin type‐I domain. The cDNA for human IGFBP‐4 encodes a 258‐residue protein that is processed, by removal of the signal sequence, to a mature protein of 237 residues (25.6 kDa) with a single asparagine‐linked glycosylation site(1). Although various cell types when in culture secrete both glycosylated (28‐29 kDa) and nonglycosylated (24‐25 kDa) forms of IGFBP‐4, the nonglycosylated is typically the most abundant in normal human blood (2, 3).

IGFBP‐4 is unique among the six IGFBPs in having two extra cysteine residues in the variable L‐domain and may be responsible for the distinctive biological functions of IGFBP‐4 (4). Although the exact functional role for serum IGFBP‐4 is not absolutely clear, in vitro studies have shown that IGFBP‐4 inhibits IGF activity in bone cells and other cell types. IGFBP‐4 has been reported to inhibit IGF‐I‐ and IGF‐II‐induced cell proliferation of embryonic chick calvaria cells and MC3T3‐E1 mouse osteoblasts (5, 6), IGF‐I‐ and IGF‐II stimulated DNA synthesis in a variety of cell types.(3)

Proteolysis is a major regulatory mechanism of IGFBP‐4 functions. An IGF dependent IGFBP‐4‐specific protease was first reported in the media conditioned by both human and sheep dermal fibroblasts. This protease was later identified as pregnancy‐associated plasma protein‐A (PAPP‐A). It was shown that recombinant PAPP‐A is an active protease able to cleave IGFBP‐4 at a single site, between M135/K136. IGFBP‐4 cleavage by PAPP‐A is possible only in case when IGFBP is complexed with IGF. PAPP‐A also cleaves IGFBP‐5 between S143/K144, but in this case the presence of IGF is not required.

Several studies have shown that concentration of PAPP‐A in blood of patients with acute coronary syndrome (ACS) is higher than in blood of patients with stable coronary artery disease or control subjects. PAPP‐A has been suggested as a marker of cardiovascular diseases associated with coronary artery blood clotting, such as unstable angina and myocardial infarction (MI) (7‐14). It was hypothesized that in atherosclerotic plaques PAPP‐A expressed by activated smooth muscles cells could function as an active enzyme cleaving IGFBP‐4 complexed with IGF, thus enhancing IGF bioavailability. The IGF system might contribute to the atherosclerotic plaque development, destabilization, and rupture leading to acute coronary events (15). It was shown that IGFBP‐4 is expressed by different cells of tumor origin, such as lung adenocarcinoma, non‐small‐cell lung cancer, breast cancer, colon carcinoma, follicular thyroid carcinoma, gastric cancer, glioma, hepatoma, myeloma, neuroblastoma, osteosarcoma and prostate cancer. In vitro and in vivo studies suggest that IGFBP‐4 plays an important role in the growth regulation of a variety of tumors, possibly by inhibiting autocrine IGF actions. Regulation of IGF bioavailability may play a crucial role in tumor growth and development (13). The measurements of IGFBP‐4 along with PAPP‐A enzyme activity could be of higher clinical value than just PAPP‐A measurements alone since PAPP‐A concentration in blood is affected by heparin injections. The concentration of PAPP‐A, total IGFBP‐4 and intact IGFBP‐4 in biological fluid can be measured accurately using immunoassay methods (picoPAPP‐A ELISA, AL‐101; Total IGFBP‐4 ELISA, AL‐126; and Intact IGFBP‐4 ELISA, AL‐128 respectively). The ratio of total to Intact IGFBP‐4 concentration measured in individual subjects over time may help normalizes the IGFBP‐4 variability between subjects and also increase the detection of increased PAPP‐A activity in MI subjects. The immunoassay methods designed for the measurement of total and Intact IGFBP‐4 in patient samples could be of practical value for the diagnosis or prediction of various pathologies including ACS and cancer.

References:
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