Margaret A. Shupnik

Platelet-derived growth factor increases prostaglandin production and decreases epidermal growth factor receptors in human osteosarcoma cells.

Human platelet-derived growth factor (PDGF) stimulated the production of prostaglandin E2 (PGE2) by G-292 cells, a clonal line of human osteosarcoma cells. Half-maximal stimulation occurred with 9 ng/ml PDGF and maximal stimulation, 3-fold above control values, occurred with 40 ng/ml of the protein. Treatment of G-292 cells with 40 ng/ml PDGF also reduced the binding of iodinated epidermal growth factor (EGF) to the EGF receptor on G-292 cells. The effect was time-dependent, and EGF binding was reduced to 60% of control by 24-48 h. PDGF did not, however, compete directly for binding to the EGF receptor. The effects of PDGF and EGF on increased PGE2 production appeared to be additive at all concentrations tested, indicating that they may act through a common pathway, but not via the same membrane receptors.

Pure α-subunit producing tumor derived from a thyrotropic tumor: Impaired regulation of a-subunit and its mRNA by thyroid hormone

Abstract We have recently described a mouse pituitary tumor line which produces only the α-subunit of the glycoprotein hormones. This tumor line may be a useful animal model to study autonomous pituitary tumors which secrete only α-subunit. Our pure α-subunit producing tumor was derived from a thyrotropic tumor which secreted intact TSH as well as free α-subunit. Our current studies compare the regulation of α-subunit biosynthesis in a conventional thyrotropic tumor and the α-subunit producing tumor. Thyroxine or triiodothyronine administration to mice bearing the a-subunit producing tumor resulted in no change in plasma α-subunit concentration, and a 10–19% reduction in tumor α-subunit mRNA concentration that was not statistically significant. In contrast, thyroxine administration to mice bearing the thyrotropic tumor resulted in an 81% reduction in plasma α-subunit concentration, and a 75% reduction in tumor α-subunit mRNA concentration (P

A non-responsive α-secreting thyrotropic tumor contains T3 receptors and a TSHβ gene

We have recently described a mouse pituitary tumor line, MGH 101A which is derived from a TSH-producing thyrotropic tumor line and now produces only the α-subunit of the glycoprotein hormones. In these studies, we have investigated the mechanism for the lack of TSHβ subunit expression in MGH 101A, as well as the failure of triiodothyronine (T3) to regulate α-subunit. Southern blot analysis of restriction endonuclease-digested DNA from MGH 101A tumors indicates the presence of a TSHβ gene and an α-subunit gene indistinguishable from those in a TSH-producing tumor (TtT 97). In MGH 101A tumors, however, TSHβ gene transcription was minimal (4 ± 2 ppm) relative to α-subunit (283 ± 29 ppm) and there was no significant difference in transcription after T3 treatment. In contrast, TtT 97 tumors had nearly equal rates of α-subunit (375 ± 25 ppm) and TSHβ (386 ± 31 ppm) gene transcription, and T3 suppressed the transcription of α-subunit and TSHβ genes by 76% and 87%, respectively. The MGH 101A tumor contained T3 receptors with a binding affinity (1.54 × 10−10 M) similar to receptors on TtT 97 tumors (1.78 × 10−10 M), but at a lower concentration (2800 vs. 4000 sites/cell). We conclude that the absence of TSHβ production in MGH 101A tumors is not due to the absence of the TSHβ gene, but perhaps to some other modification of the gene structure. This could also explain the failure of MGH 101A tumors to respond to T3, since they do contain T3 receptors of normal affinity.

Divergent dopaminergic regulation of TSH, free α-subunit, and TSH-β in pituitary cell culture☆☆☆

TSH is a glycoprotein hormone composed of two nonidentical, noncovalently associated subunits, alpha and beta. We have previously shown in vivo that intrapituitary free α-subunit and intact TSH have divergent responses to hypothyroidism and thyroxine treatment, suggesting fundamental differences in their regulation. To explore this further, we exposed anterior pituitary cell cultures from rats previously rendered hypothyroid to thyrotropin releasing hormone (TRH), dopamine (DA), or TRH and DA and determined TSH, free α-subunit and TSH-β responses. While positive or negative trends were noted at four hours, the most significant changes were observed at 24 and 48 hours. TRH increased media TSH at 24 hours to 180% of its basal value (P < 0.01), with a comparable response at 48 hours. TRH also increased free α-subunit to 155% of the basal value (P < 0.01) and TSH-β to 145% of the basal value (P < 0.01) at 24 hours. In contrast, DA produced concordant inhibition of TSH to 85% (P < 0.05), free a-subunit to 42% (P < 0.01), and TSH-β to 53% (P < 0.01) of the basal values at 24 hours. However, coincubation with both TRH and DA produced discordant responses: TSH was stimulated to 126% of the basal value at 24 hours (P < 0.01), while both free α-subunit and TSH-β fell significantly below the basal values (81% and 65% respectively, P < 0.01). These data suggest this divergent response is due, not to opposite effects of TRH and DA on TSH and its subunits, but rather to a difference in sensitivity to dopaminergic stimulation, or fundamental differences in hypothalamic regulation of TSH, free α-subunit, and TSH-β.