Expression, biosynthesis and release of preadipocyte factor-1/ delta-like protein/fetal antigen-1 in pancreatic beta-cells: possible physiological implications

Abstract

Preadipocyte factor-1 (Pref-1)/delta-like protein/fetal antigen-1 (FA1) is a member of the epidermal growth factor-like family. It is widely expressed in embryonic tissues, whereas in adults it is confined to the adrenal gland, the anterior pituitary, the endocrine pancreas, the testis and the ovaries. We have previously cloned Pref-1 from neonatal rat islets stimulated by GH. The aim of the present study was to elucidate the biosynthesis and release of Pref-1/FA1 in -cells and to determine if Pref-1/FA1 is mediating the mitogenic effect of GH in insulinproducing cells. First we studied the biosynthesis and processing of Pref-1 to the soluble form, FA1, in pancreatic islets and insulinoma cells transfected with Pref-1 cDNA. We measured the release of FA1 by ELISA and the possible effect of FA1 in GH-stimulated -cell proliferation by incorporation of bromodeoxyuridine (BrdU) in insulin-positive islet cells. We found that Pref-1 was synthesized in normal islets and in RINm5F insulinoma cells and released into the medium in two forms, of which one corresponded to FA1. Both the expression of the mRNA for Pref-1 and the release of the soluble form(s) were stimulated by GH and prolactin (PRL). Whereas 2 h exposure to high glucose or 3-isobutyl-1-methylxanthine stimulated insulin release, only a small change was seen in FA1 release, suggesting that the FA1 is released by a different pathway than insulin. However, long-term exposure (48 h) to high glucose increased FA1 secretion, indicating that FA1 is regulated by glucose. Neither FA1 nor conditioned medium from GH-stimulated islets depleted for GH was able to increase -cell replication and overexpression of Pref-1 resulted in attenuated proliferation of the RINm5F cells. By immunocytochemistry of GH-stimulated islet cells no correlation between high Pref-1 expression and BrdU incorporation was observed and there was an inverse relationship between the levels of insulin and Pref-1. These results indicate that Pref-1/FA1 is not mediating the mitogenic effect of GH and PRL. Therefore the function of Pref-1 in the -cell remains unknown.

Human serum levels of fetal antigen 1 (FA1/Dlk1) increase with obesity, are negatively associated with insulin sensitivity and modulate inflammation in vitro

“…Long-term exposure of rat islets to high glucose increases FA1 and insulin secretion, indicating that FA1 is regulated by glucose. 28 In our study on the obesity cohort, FA1 was inversely associated with S i and was a predictor of S i index, thus suggesting the involvement of this protein in the peripheral action of insulin. It is difficult to confer a pathogenic role for FA1 in the mechanisms related to insulin-resistance because of the transversal design of our obesity cohort; however, the experimental data in the transgenic mice model for dlk1 showed impaired insulinresistance, 16 which is in accordance with the above-mentioned clinical observations, at least in male subjects.…”

DLK1 Regulates Whole-Body Glucose Metabolism: A Negative Feedback Regulation of the Osteocalcin-Insulin Loop

“…We previously demonstrated that Dlk1 expression and secretion in b-cells are stimulated in vitro and in vivo by recombinant Glu-OCN, an inducer of insulin expression by b-cells (34). Considering that DLK1 is colocalized with insulin in adult b-cells (37,38) and that the secretion of DLK1 and insulin have been reported to be stimulated by the same hormones (including growth hormone and prolactin) (37,39), it is plausible that OCN uses a similar mechanism to stimulate the secretion of sDLK1 and insulin. In this context, it is important to note that OCN favors the receptor Gprc6a in insulin secretion in b-cells (31).…”

Regulation of growth hormone expression by Delta-like protein 1 (Dlk1)

“…Dexamethasone increases Dlk1 expression in neuroblastoma cells [34] and downregulates Dlk1 expression in preadipocytes [35]. In pancreatic islets, both GH and prolactin induce Dlk1 expression [9,36], and GH induces Dlk1 expression in preadipocytes [3]. Additionally, Andersen et al have determined that either a pharmacological increase or decrease in GH levels results in a corresponding change in the concentration of circulating Dlk1 [37].…”

“…Dlk1, also known as pref-1 [4], pG2 [5] and zona glomerulosa specific mRNA [6], contains 6 EGF-like repeat domains in the extracellular domain, a transmembrane domain and a short cytoplasmic tail. It is cleaved by a yet unidentified protease to release a soluble circulating peptide containing the EGF-like repeats [7][8][9][10]; but its function remains elusive. This circulating peptide was originally identified as fetal antigen 1 [7].…”