The novel HSP90 inhibitor STA-1474 exhibits biologic activity against osteosarcoma cell lines.

Osteosarcoma (OSA), the most common malignant bone tumor in dogs and children, exhibits a similar clinical presentation and molecular biology in both species. Unfortunately, 30-40% of children and 90% of dogs still die of disease despite aggressive therapy. The purpose of this study was to test the biologic activity of a novel heat shock protein 90 (HSP90) inhibitor, STA-1474, against OSA. Canine and human OSA cell lines and normal canine osteoblasts were treated with STA-1474 and evaluated for effects on proliferation (CyQuant), apoptosis (Annexin V, PARP cleavage, caspase 3/7 activation) and known HSP90 client proteins. HSP90 was immunoprecipitated from normal and malignant osteoblasts and Western blotting for co-chaperones was performed. Mice bearing canine OSA xenografts were treated with STA-1474, and tumors samples were evaluated for caspase-3 activation and loss of p-Akt/Akt. Treatment with STA-1474 promoted loss of cell viability, inhibition of cell proliferation and induction of apoptosis in OSA cell lines. STA-1474 and its active metabolite STA-9090 also demonstrated increased potency compared to 17-AAG. STA-1474 exhibited selectivity for OSA cells versus normal canine osteoblasts, and HSP90 co-precipitated with co-chaperones p23 and Hop in canine OSA cells but not in normal canine osteoblasts. Furthermore, STA-1474 downregulated the expression of p-Met/Met, p-Akt/Akt and p-STAT3. Finally, STA-1474 induced tumor regression, caspase-3 activation and downregulation of p-Met/Met and p-Akt/Akt in OSA xenografts. Together, these data suggest that HSP90 represents a relevant target for therapeutic intervention in OSA.

Reduced inhibition and sensitivity to neurosteroids in hippocampus of mice lacking the GABA(A) receptor delta subunit.

The delta subunit of the gamma-aminobutyric acid (A) receptor (GABA(A)R) is expressed postnatally mostly in the cerebellum, thalamus, and dentate gyrus. Previous studies in mice with a targeted disruption of the delta subunit revealed a considerable attenuation of behavioral responses to neuroactive steroids but not to other neuromodulatory drugs. Here we show that delta subunit loss leads to a concomitant reduction in hippocampal alpha4 subunit levels. These changes were accompanied by faster decay of evoked inhibitory postsynaptic potentials (IPSPs) in dentate granule neurons of -/- mutants (decay tau = 25 ms) compared with +/+ controls (tau = 50 ms). Furthermore, the GABA(A)R-mediated miniature inhibitory postsynaptic currents (mIPSCs) also decayed faster in delta-mutants (tau = 6.3 ms) than controls (tau = 7.2 ms) and had decreased frequency (controls, 10.5 Hz; mutants, 6.6 Hz). Prolongation of mIPSCs by the neuroactive steroid anesthetic, alphaxalone (1-10 microM), was smaller in delta-mutants (at 10 microM, 65% increase) compared with +/+ littermates (308% increase). In competition binding experiments, alphaxalone (0.03-1 microM) modulation of [35S]t-butylbicyclophosphorothionate binding was reduced in delta-mutant brain homogenates, indicating that the decreased alphaxalone effects on mIPSCs were due to changes in the GABA(A)R protein. Faster decay of evoked IPSPs and mIPSCs in delta-mutants suggests presence of the delta subunit at both synaptic and extrasynaptic GABA(A)Rs. Decreased synaptic and extrasynaptic inhibition likely contributes to the pro-epileptic phenotype of delta-mutants. Reduced neurosteroid sensitivity might also contribute to seizure susceptibility. While the simplest explanation is that delta subunit-containing GABA(A)Rs represent the actual target of neurosteroids, it is possible that the behavioral and physiological sensitivity to neuroactive steroids is indirectly altered in the delta -/- mice.

Intact synaptic GABAergic inhibition and altered neurosteroid modulation of thalamic relay neurons in mice lacking delta subunit.

Robust GABA-mediated inhibitory postsynaptic currents (IPSCs) in neurons of the thalamic relay (TC) nuclei are important in sustaining oscillatory activity within thalamic and thalamocortical circuits. The biophysical properties and pharmacological sensitivities of these IPSCs both depend on the subunit combination of postsynaptic gamma-aminobutyric acid-A (GABA(A)) receptors. Recombinant GABA(A) receptors containing the delta subunit (heavily expressed in TC nuclei) have been shown to exhibit slowed desensitization rates and high affinity for GABA in heterologous expression systems. We tested whether the GABA(A)-mediated synaptic inhibition in TC neurons would be affected by loss of the delta subunit. Spontaneous and evoked IPSCs were recorded from neurons in the ventral basal complex (VB) of the thalamus from brain slices of wild-type (delta(+/+)) and homozygous delta subunit deficient mice (delta(-/-)). Spontaneous IPSCs (sIPSCs) from delta(-/-) mice had no significant differences in amplitude, duration, or frequency compared with their delta(+/+) counterparts. However, baseline noise (63% of control) and the relative contribution of the slow component to overall decay (79% of control) were significantly lower in delta(-/-) VB recordings. Evoked IPSCs (eIPSCs) in delta(-/-) neurons showed no difference in peak amplitude, but had an accelerated slow decay component (40- vs. 55-ms time constant). We further tested whether neurosteroid modulation of GABA(A) receptors was dependent on the presence of the delta subunit, as previously reported in recombinant systems. Pregnenolone sulfate (PS) significantly reduced eIPSC peak amplitude (-30%) and increased duration in delta(-/-), but not in delta(+/+) mice. sIPSCs were not affected in any neurons, delta(-/-) or delta(+/+). In contrast, 3-alpha,5-alpha-tetrahydrodeoxycorticosterone (THDOC) increased the durations of eIPSCs and sIPSCs in both delta(-/-) and delta(+/+) VB neurons. Our findings show that although the delta subunit confers a striking PS insensitivity to eIPSCs in VB neurons, it plays only a minor role in the synaptic inhibition of VB neurons. This suggests delta subunit containing GABA(A) receptors may be functionally limited to an extrasynaptic locus in VB neurons.

Behavior and physiology of mice lacking the GABAA-receptor delta subunit.

PURPOSE: Mice with a targeted disruption of the gamma-aminobutyric acid (A) receptor (GABA(A)R) delta subunit exhibited spontaneous seizures and a strikingly selective attenuation of responses to neuroactive steroids, but not to other neuromodulators. This study further characterized the behavior and physiology of the delta mutants. METHODS: Seizure susceptibility to pentylenetetrazol (PTZ) injection, intracellular recordings, and whole-cell patch recordings in dentate granule neurons was determined. RESULTS: Susceptibility to PTZ-induced convulsive seizures was significantly higher in -/- versus +/+ mice. In intracellular recordings, the evoked inhibitory postsynaptic potentials (IPSPs) had much faster decay tau in -/- mutants (25 +/- 3.5 ms) compared with +/+ controls (51 +/- 5.2 ms). In whole-cell patch recordings, the decay tau of pharmacologically isolated miniature spontaneous GABA(A)R-mediated synaptic currents (mIPSCs) from -/- mice was only slightly, but significantly faster (5.7 +/- 0.13 ms; n = 18) than that of +/+ controls (6.8 +/- 0.32 ms; n = 20). Furthermore, mIPSC prolongation by bath application of alphaxalone (10 microM), was much smaller in -/- mice (52 +/- 12%; n = 3) compared with +/+ littermates (192 +/- 14%; n = 3). CONCLUSIONS: Faster decay of evoked IPSPs and mIPSCs is consistent with altered GABA(A)R subunit composition in the delta mutants, but suggests predominant extrasynaptic delta subunit location in the dentate gyrus of normal mice. Faster-decaying IPSPs likely contribute to the proepileptic phenotype of the delta mutants. Reduced neurosteroid sensitivity might also contribute to seizure susceptibility. It remains to be determined whether delta subunit-containing GABA(A)Rs represent the actual target of neurosteroids or whether the behavioral and physiologic sensitivity to neurosteroids is indirectly altered in the delta-/- mice.

GABA(A) receptor changes in delta subunit-deficient mice: altered expression of alpha4 and gamma2 subunits in the forebrain.

The delta subunit is a novel subunit of the pentameric gamma-aminobutyric acid (GABA)(A) receptor that conveys special pharmacological and functional properties to recombinant receptors and may be particularly important in mediating tonic inhibition. Mice that lack the delta subunit have been produced by gene-targeting technology, and these mice were studied with immunohistochemical and immunoblot methods to determine whether changes in GABA(A) receptors were limited to deletion of the delta subunit or whether alterations in other GABA(A) receptor subunits were also present in the delta subunit knockout (delta-/-) mice. Immunohistochemical studies of wild-type mice confirmed the restricted distribution of the delta subunit in the forebrain. Regions with moderate to high levels of delta subunit expression included thalamic relay nuclei, caudate-putamen, molecular layer of the dentate gyrus, and outer layers of the cerebral cortex. Virtually no delta subunit labeling was evident in adjacent regions, such as the thalamic reticular nucleus, hypothalamus, and globus pallidus. Comparisons of the expression of other subunits in delta-/- and wild-type mice demonstrated substantial changes in the alpha4 and gamma2 subunits of the GABA(A) receptor in the delta-/- mice. gamma2 Subunit expression was increased, whereas alpha4 subunit expression was decreased in delta-/- mice. Importantly, alterations of both the alpha4 and the gamma2 subunits were confined primarily to brain regions that normally expressed the delta subunit. This suggests that the additional subunit changes are directly linked to loss of the delta subunit and could reflect local changes in subunit composition and function of GABA(A) receptors in delta-/- mice.