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 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 as 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 subject over time will help normalizes the IGFBP-4 variability between subjects and also increase the detection rate 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.

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IGFBP-4 (Total) ELISA AL-126
Anastasilakis AD, Koulaxis D, Upadhyay J, Pagkalidou E, Kefala N, Perakakis N, Polyzos SA, Economou F, Mantzoros CS. Free IGF-1, Intact IGFBP-4, and PicoPAPP-A are Altered in Acute Myocardial Infarction Compared to Stable Coronary Artery Disease and Healthy Controls. Horm Metab Res. 2019 Feb;51(2):112-119. doi: 10.1055/a-0794-6163. Epub 2018 Nov 29. PMID: 30497090.

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Babiker A, Al Noaim K, Al Swaid A, et al. Short stature with low insulin-like growth factor 1 availability due to pregnancy-associated plasma protein A2 deficiency in a Saudi family. Clin Genet. 2021;100(5):601-606. doi: 10.1111/cge.14030.

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Becker M, Haluska P, Bale L, Oxvig C, Conover C. A Novel Neutralizing Antibody Targeting Pregnancy-Associated Plasma Protein-A Inhibits Ovarian Cancer Growth and Ascites Accumulation in Patient Mouse Tumorgrafts. Mol Cancer Ther. 2015 Apr;14(4):973-81. doi: 10.1158/1535-7163.MCT-14-0880. Epub 2015 Feb 18. PMID: 25695953; PMCID: PMC4394033.

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Bøtkjær JA, Pors SE, Petersen TS, Kristensen SG, Jeppesen JV, Oxvig C, Andersen CY. Transcription profile of the insulin-like growth factor signaling pathway during human ovarian follicular development. Assist Reprod Genet
. 2019 May;36(5):889-903. doi: 10.1007/s10815-019-01432-x. Epub 2019 Mar 15.

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Donegan D, Bale LK, Conover CA. PAPP-A in normal human mesangial cells: effect of inflammation and factors related to diabetic nephropathy. J Endocrinol. 2016 Oct;231(1):71-80. doi: 10.1530/JOE-16-0205. Epub 2016 Aug 12. PMID: 27519211.

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Kumar A, Kalra B, Kommareddy V, Chowdavarapu K, Mistry S, Savjani G, Oxvig C. Development of Well Characterized ELISAs for Bound and Unbound Insulin-Like Growth Factors and their Binding Proteins. Poster presented at 98th Annual Endocrine Society Meeting; 2016 Apr 1-3; Boston, MA.

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Martín-Rivada Á, Guerra Cantera S, Campillo-Calatayud A, Andrés-Esteban EM, Sánchez Holgado M, Martos-Moreno GÁ, Pozo J, Güemes M, Soriano-Guillén L, Pellicer A, Oxvig C, Frystyk J, Chowen JA, Barrios V, Argente J. Pappalysins and Stanniocalcin and Their Relationship With the Peripheral IGF Axis in Newborns and During Development. J Clin Endocrinol Metab. 2022 Sep 28;107(10):2912-2924. doi: 10.1210/clinem/dgac453. PMID: 35902207.

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Nur SI, Ozturk A, Kavas M, Bulut I, Alparslan S, Aydogan ES, Atinkaya BC, Kolay M, Coskun A. IGFBP-4: A promising biomarker for lung cancer. J Med Biochem. 2021 Jun 5;40(3):237-244. doi: 10.5937/jomb0-25629. PMID: 34177367; PMCID: PMC8199439.

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Panagiotou G, Papakonstantinou E, Vagionas A, Polyzos SA, Mantzoros CS. Serum Levels of Activins, Follistatins, and Growth Factors in Neoplasms of the Breast: A Case-Control Study. J Clin Endocrinol Metab. 2019 Feb 1;104(2):349-358. doi: 10.1210/jc.2018-01581. PMID: 30388235.

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Parry S, McCarthy C, Devine A, Leite R. Markers of placental dysfunction and spontaneous preterm birth. Amer Jour Obst Gyne, January 2020, Volume 222, Issue 1, Supplement S1-S760 (SMFM 40th Annual Meeting–The Pregnancy Meeting)

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Polyzos SA, Perakakis N, Boutari C, Kountouras J, Ghaly W, Anastasilakis AD, Karagiannis A, Mantzoros CS. Targeted Analysis of Three Hormonal Systems Identifies Molecules Associated with the Presence and Severity of NAFLD. J Clin Endocrinol Metab. 2020 Mar 1;105(3):e390–400. doi: 10.1210/clinem/dgz172. PMID: 31690932; PMCID: PMC7112980.

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Vicente Barrios, Álvaro Martín-Rivada, Gabriel Á Martos-Moreno, Sandra Canelles, Francisca Moreno-Macián, Carmen De Mingo-Alemany, Maurizio Delvecchio, Roberta Pajno, Danilo Fintini, Julie A Chowen, Jesús Argente, Increased IGFBP Proteolysis, IGF-I Bioavailability, and Pappalysin Levels in Children With Prader-Willi Syndrome, The Journal of Clinical Endocrinology & Metabolism, Volume 109, Issue 9, September 2024, Pages e1776–e1786, https://doi.org/10.1210/clinem/dgad754

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