E. Rudolf Froesch

Release of insulin-like growth factor carrier proteins by osteoblasts: Stimulation by estradiol and growth hormone

Osteoblast-like rat calvaria cells release specific insulin-like growth factor (IGF) carrier proteins (CPs). As analyzed by SDS-PAGE under nonreducing conditions, Western blotting and detection by 125I-IGFs, CPs migrating with the IGF-binding subunits of the major CP species of rat serum (42/45/49 kDa) accumulate in cell culture medium. Treatment of the cells with growth hormone and estradiol increases the abundance of this glycosylated CP species. Since the two hormones were previously found to stimulate osteoblast replication via an IGF I-dependent mechanism, the data indicate that hormones may control local IGF action not only by regulating synthesis of IGFs and their receptors but also their presentation by CPs.

Isolation and NH2-terminal amino acid sequences of rat serum carrier proteins for insulin-like growth factors.

Three N-glycosylated carrier proteins (CP) for insulin-like growth factors (apparent molecular weights 30-32, 42 and 45 kDa) were isolated from adult rat serum. They share the same amino terminus (up to amino acid 31) and are constituents of the growth hormone-dependent native 150-200 kDa IGF carrier complex. Residues 12-31 display 60 and 50% sequence homology, respectively, to residues 2-21 of fetal rat and to residues 4-22 of a human amniotic fluid IGF carrier protein. No homology exists with the type I or II IGF receptors. Adult rat serum also contains a fourth IGF CP (24 kDa) whose 9 NH2-terminal amino acids are identical to those of the fetal form. Our findings suggest that the three N-glycosylated components originate from the same IGF carrier protein (adult form) and that the 24 kDa protein is a separate (fetal) species.

Phenol red mimics biological actions of estradiol: enhancement of osteoblast proliferation in vitro and of type I collagen gene expression in bone and uterus of rats in vivo.

Since osteoblasts are direct targets for estradiol in vitro, and Phenol Red has been reported to bear estrogen-like bioactivity, we investigated whether the pH indicator also mimicked the biological effects of estradiol on bone cells in vitro. We then asked whether estrogenic effects of Phenol Red could be observed in vivo, firstly on the uterus, and if so, whether Phenol Red could also effect bone in vivo. The proliferation of calvarial osteoblasts was stimulated by commercially available preparations of Phenol Red in a dose-dependent manner at 1.5-50 microM. This effect was not abolished in the presence of an antibody against insulin-like growth factor I. In addition, Phenol Red increased alpha 1 (I) collagen mRNA levels of osteoblasts in vitro. 17 beta-estradiol (1.5 micrograms) or Phenol Red (10 mg) administration to immature female rats (45-50 g) resulted in a weight gain of the uterus, and alpha 1(I) procollagen transcripts were more abundant in RNA prepared from uterus of drug-treated rats than observed in the control rats. Similarly, higher procollagen mRNA steady-state levels were observed in RNA prepared from parietal bones of Phenol Red or estradiol-treated rats compared to RNA from control rats. The data extend previous findings in vitro by demonstrating that Phenol Red also exerts estrogen-like effects in vivo. Moreover, we show that Phenol Red stimulates bone cells and, therefore, is likely to interfere at least in experiments designed to investigate estrogen effects on osteoblasts.

Therapeutic potential of insulinlike growth factor i

Human recombinant insulinlike growth factor I is a promising therapeutic agent for diseases characterized by relative insulin resistance, e.g., diabetes mellitus, obesity, and hypertriglyceridemia, since it suppresses growth hormone, insulin, C-peptide, and triglyceride levels and lowers the total cholesterol to high-density lipoprotein-cholesterol ratio. Moreover, insulinlike growth factor administration increases kidney function in healthy subjects (glomerular filtration rate, renal plasma flow) and may prove useful in the treatment of degenerative disorders of cartilage and bone (arthrosis, osteoporosis) as well as in catabolic states.

Growth hormone, but not insulin-like growth factor I, induces a serum protease activity for insulin-like growth factor binding protein-3 in hypophysectomized rats in vivo

Insulin‐like growth factor binding proteins (IGFBPs) modulate IGF action. Proteolytic cleavage of IGFBPs yields lower molecular forms with reduced ability to bind IGFs, thereby increasing IGF bioavailability. In serum from normal adult rats, we found a proteolytic activity for IGFBP‐3, presumably a cation‐dependent serine protease. It is lacking in serum from hypophysectomized rats and restored by infusion of growth hormone (GH), but not IGF I. Thus, IGF I does not appear to mediate the GH effect on IGFBP‐3 proteolysis. Rather, GH seems to modulate IGF action indirectly via alteration of IGFBP‐3 structure.

Recombinant Human Insulin-Like Growth Factor I: Effects in Normal Subjects and Implications for Use in Patients

Two major forms of insulin-like growth factors (IGF-I and IGF-II) and their specific binding proteins have been detected in body fluids, including serum, cerebrospinal fluid, breast milk, lymph, and saliva. Extracts of almost any tissue reveal the presence of IGFs. Many cells in culture secret IGFs and various forms of their carrier proteins into the medium (1–3).

EFFECT OF PROLONGED OPTIMAL BLOOD SUGAR CONTROL BY I.V. INSULIN ADMINISTRATION ON PLASMA FREE FATTY ACID AND GLYCEROL CONCENTRATIONS OF INSULIN-DEPENDENT DIABETICS DURING EXERCISE

SummaryFour type I diabetics were kept close to normoglycemic during a period of 1 month by means of a an open loop i.v. insulin infusion system (Siemens). During submaximal physical exercise their metabolic response was nearly normal. No hypoglycemic episodes during exercise were registered despite the fact that the regular basal infusion rate was not changed. The level of free fatty acids rose approximately to the same extent as in normal subjects. A slight inhibition of lipolysis during i.v. insulin therapy was apparent from the less marked rise of the glycerol levels. The feasibility of a simple open loop i.v. insulin regimen over a prolonged period of time is underscored by these results which show that the basal insulin infusion rate can be kept constant during exercise.