Recombinant human insulin-like growth factor I induces its own specific carrier protein in hypophysectomized and diabetic rats.

Abstract

The physiology of the specific serum binding proteins which constitute the main storage pool for insulin-like growth factors (IGFs) in mammals is still incompletely understood. We have, therefore, investigated the regulation of these proteins in (i) hypophysectomized (hypox) rats infused with recombinant human growth hormone (rhGH) or recombinant human IGF I (rhIGF I) and (ii) streptozotocin-diabetic rats infused with insulin or rhIGF I. The main carrier protein, a GH-dependent complex of apparent molecular mass 200 kDa, contains N-glycosylated IGF-binding subunits (42, 45, and 49 kDa) that differ in their glycosyl but not in their protein moiety. These subunits are lacking in hypox and diabetic rats. They are induced by GH and insulin, respectively, and appear in the 200-kDa complex. Infusion of rhIGF I induces the subunits in both states; however, only in diabetic, not in hypox, rats do they form the 200-kDa complex. Glycosylated carrier protein subunits do not appear before 8 hr of rhIGF I infusion. During that period, hypox rats may become severely hypoglycemic. After 16 hr, glycosylated subunits are clearly induced, and blood sugar values are normal. We conclude: (i) The Nglycosylated subunits of the 200-kDa complex reflect the IGF I status.(ii) IGF I may mediate the induction of these subunits by GH. (iii) Significant association to the 200-kDa complex occurs only in the presence of GH. It is likely that GH, but not IGF I, induces a component, which itself does not bind IGF, but associates with the glycosylated IGF-binding subunits. (iv) The glycosylated subunits protect against IGF-induced hypoglycemia and may be involved in tissue-specific targeting of IGFs.In mammalian blood, insulin-like growth factors (IGFs) circulate in tight association with specific high-affinity carrier proteins (1-3). Although they constitute the main reservoir of IGFs in the organism, the significance of this storage pool is still under debate. Experimental evidence suggests three possible functions of IGF carrier proteins: (i) protection of the organism against acute insulin-like effects of the large quantities of IGFs in blood by decreased availability to tissue receptors (4-8), (ii) prolongation of the half-life of IGFs in the circulation (6,9), and (iii) potentiation of the growthpromoting effects of IGFs (10).Native serum from normal rats contains at least two IGF carrier protein complexes: upon neutral gel filtration on Sephadex G-200 one of them elutes with an apparent molecular mass of 150-200 kDa, the other with 40-50 kDa [in this paper termed 200-and 40-kDa complexes, respectively, according to our Sephadex G-200 elution profiles (see Fig. 1)]. The 200-kDa complex carries most of the endogenous rat IGF (11) and has been shown to be growth hormone (GH)-dependent (12-15): hypophysectomized (hypox) and diabetic rats, both of which are GH deficient and have low IGF I serum levels, lack the 200-kDa complex. It reappears after GH or insulin treatment, respectively, together with the rise of endogenous IGF. On the ...

Production of carrier proteins for insulin‐like growth factors (IGFs) by rat osteoblastic cells Regulation by IGF I and cortisol

“…this glycosylated species (figs 1,2) corresponds 'to the subunits of the growth hormone-dependent large molecular mass carrier complex, the most abundant IGF-binding protein in rat serum [3]. Similarly to the situation in vivo [3], this carrier protein species is found in increased amounts in conditioned medium of cells exposed to IGF I (figs 2,3) or growth hormone ( fig.1).…”

“…The major species produced by the cells was undistinguishable from the subunits of the large molecular mass carrier protein complex in rat serum [3] ( fig.1): iz51-IGF II detects mainly bands of apparent molecular masses 49/45/42 kDa ('triplet'). The carrier protein species appearing as triplet is more abundant in medium from cells which had been exposed to growth hormone ( fig.1).…”

“…Upon prolonged incubation times (4 days), this non- life in vitro. A similar carrier protein was transiently present in serum from IGF I-treated hypophysectomized rats [3]. It remains to be seen whether this carrier protein species is related to the 'fetal' carrier protein [4].…”

“…irIGF I was determined [3] using a rabbit antiserum against human IGF I with human IGF I as a standard. Sample aliquots were acid-treated and run on Sep-Pak Cts cartridges.…”

“…In serum and other extracellular spaces of the mammalian organism, IGFs are bound to specific carrier proteins [2,3]. The predominant species of carrier protein found in native rat serum is a complex of 150-200 kDa (by neutral gel filtration on Sephadex G-200) consisting of glycosylated subunits with apparent molecular masses of 42-49 kDa (by gel electrophoresis) 131.…”