Basic fibroblast growth factor induces proteolysis of secreted and cell membrane-associated insulin-like growth factor binding protein-2 in human neuroblastoma cells

Insulin-like growth factor (IGF) action in the brain is modulated by IGF-binding proteins (IGFBPs) whose abundance can be altered by other locally expressed growth factors. However, the mechanisms involved are unclear. We here employed the neuroblastoma cell line SK-N-MC as a model to define the mechanisms involved in modulation of IGFBPs in neuronal cells. Western ligand blotting analysis and immunoprecipitation of conditioned media (CM) from SK-N-MC cells showed that in these cells, as in the brain, the most abundantly expressed IGFBP was IGFBP-2. However, IGFBP-2 was barely detectable in CM from cells treated with basic fibroblast growth factor (bFGF) without a change in IGFBP-2 messenger RNA (mRNA) abundance. These CM contained specific IGFBP-2 proteolytic activity, resulting in two IGFBP-2 fragments of 14 and 22 kDa. The activity was inhibited by EDTA/phenylmethylsulfonyl fluoride or aprotinin. Competitive binding studies indicated that IGFBP-2 fragments had reduced binding affinity for IGF-I. bFGF induced IGFBP-3 mRNA and protein. Affinity cross-linking of [125I]IGF-I to neuroblastoma cell membranes followed by immunoprecipitation revealed a approximately 38 kDa [125I]IGF-I/IGFBP-2 complex. Cell surface-associated IGFBP-2 was also susceptible to bFGF-induced proteolysis, with the appearance of a single cross-linked 21-kDa complex with low affinity for IGF-I. These findings indicate that intact IGFBP-2 and the 14-kDa, but not the 22-kDa fragment, bind to the cell surface. Our data suggest that induction of IGFBP-2 proteolysis on neuronal cell surface is a novel mechanism whereby IGF availability is modulated by the local growth factor bFGF.

Abnormal androgen receptor binding affinity in subjects with Kennedy's disease (spinal and bulbar muscular atrophy)

We have investigated androgen-binding properties of the androgen receptor (AR) in cultured suprapubic skin fibroblasts from six subjects with Kennedy's disease (X-linked spinal and bulbar muscular atrophy). Binding of the synthetic androgen methyltrienolone (R1881) was measured in a monolayer assay, and Scatchard analysis was performed to determine the total number of binding sites and the apparent binding affinity (Kd) of the AR for androgen. Five of the six subjects investigated had an abnormal apparent binding affinity, with Kd values ranging from 0.34-11.7 nmol/L, more than 2 SD from the mean of the normal range (0.19 +/- 0.06 nmol/L). In this group of six patients, there was a significant correlation between the AR Kd and the severity of testicular atrophy and gynecomastia. The number of CAG repeats in the expanded region of exon A of the AR gene was determined in all subjects from whom suprapubic skin fibroblasts were cultured and an additional 12 subjects with Kennedy's disease. In the total group of 18 subjects investigated, there was a trend for an increasing number of CAG repeats associated with decreasing age at onset of different symptoms; however, this correlation was not statistically significant. Thus, we report for the first time a quantitative abnormality of the AR apparent binding affinity in subjects with Kennedy's disease, which appears to be related to the severity of the symptoms of androgen insensitivity.