Radiosensitization of tumour cell lines by the polyphenol Gossypol results from depressed double-strand break repair and not from enhanced apoptosis

PURPOSE

New drugs are needed to increase the efficiency of radiotherapy in order to improve the therapeutic outcome of tumour patients. In this respect, the polyphenol Gossypol might be of interest, because of its effect on apoptosis and DNA repair, which is either mediated directly or indirectly via the inositol phosphate metabolism. It was investigated, whether these effects result in enhanced radiosensitivity of tumour cells.

MATERIAL AND METHODS

Tumour cell lines investigated: A549, FaDu, H1299, MCF7 and Du145. Cell cycle distribution was determined by FACS analysis, apoptosis was measured by DAPI staining and caspase3/7 activity. Double-strand breaks (DSB) were investigated via gammaH2AX-foci and cell survival by colony formation assay. The level of inositol phosphates was determined by HPLC, protein expression by Western blot.

RESULTS

In A549 cells, Gossypol at concentrations 1microM strongly affects proliferation with only a modest arrest in the G1-phase, but with no increase in the fraction of apoptotic cells or the number of additional DSB. Additional DSB were only seen in FaDu cells, where Gossypol (2microM) was extremely toxic with a plating efficiency <0.002. When combined with irradiation, incubation with Gossypol (1-2microM) was found to result in an enhanced radiosensitivity with, however, a substantial variation. While there was a strong radiosensitization for FaDu and Du145 cells, there was an intermediate response for A549 cells, but almost no effect for H1299 and MCF7 cells. This sensitization was not caused from an elevated rate of apoptosis, but primarily resulted from reduced DSB repair capacity. The reduction in DSB repair could be ascribed neither to changes in the level of repair proteins relevant for non-homologous end-joining (Ku70, Ku80, DNA-PKcs) nor to changes in the level of higher phosphorylated inositols, whereby the latter were even found to be enhanced by Gossypol.

CONCLUSIONS

For some tumour cell lines treatment with low concentrations of Gossypol can be used to inhibit DSB repair capacity and with that to increase the cellular radiosensitivity.

Modulation of Gq-protein-coupled inositol trisphosphate and Ca2+ signaling by the membrane potential

Gq-protein-coupled receptors (GqPCRs) are widely distributed in the CNS and play fundamental roles in a variety of neuronal processes. Their activation results in phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis and Ca2+ release from intracellular stores via the phospholipase C (PLC)-inositol 1,4,5-trisphosphate (IP3) signaling pathway. Because early GqPCR signaling events occur at the plasma membrane of neurons, they might be influenced by changes in membrane potential. In this study, we use combined patch-clamp and imaging methods to investigate whether membrane potential changes can modulate GqPCR signaling in neurons. Our results demonstrate that GqPCR signaling in the human neuronal cell line SH-SY5Y and in rat cerebellar granule neurons is directly sensitive to changes in membrane potential, even in the absence of extracellular Ca2+. Depolarization has a bidirectional effect on GqPCR signaling, potentiating thapsigargin-sensitive Ca2+ responses to muscarinic receptor activation but attenuating those mediated by bradykinin receptors. The depolarization-evoked potentiation of the muscarinic signaling is graded, bipolar, non-inactivating, and with no apparent upper limit, ruling out traditional voltage-gated ion channels as the primary voltage sensors. Flash photolysis of caged IP3/GPIP2 (glycerophosphoryl-myo-inositol 4,5-bisphosphate) places the voltage sensor before the level of the Ca2+ store, and measurements using the fluorescent bioprobe eGFP-PH(PLCdelta) (enhanced green fluorescent protein-pleckstrin homology domain-PLCdelta) directly demonstrate that voltage affects muscarinic signaling at the level of the IP3 production pathway. The sensitivity of GqPCR IP3 signaling in neurons to voltage itself may represent a fundamental mechanism by which ionotropic signals can shape metabotropic receptor activity in neurons and influence processes such as synaptic plasticity in which the detection of coincident signals is crucial.

Photolabile precursors of inositol phosphates. Preparation and properties of 1-(2-nitrophenyl)ethyl esters of myo-inositol 1,4,5-trisphosphate

1-(2-Nitrophenyl)ethyl esters of D-myo-inositol 1,4,5-trisphosphate (InsP3) have been synthesized and shown to have suitable properties for use as photolabile precursors of InsP3. Synthesis was accomplished by treatment of InsP3 with 1-(2-nitrophenyl)diazoethane in a CHCl3/water mixture. This resulted in esterification of each of the three phosphate residues in InsP3, the 1-phosphate being more reactive than the 4- or 5-phosphate. Singly esterified P-1, P-4, and P-5 esters, termed P-1, P-4, and P-5 caged InsP3, were isolated from the reaction mixture by anion-exchange HPLC and characterized by 500-MHz 1H NMR spectroscopy. Each of these caged InsP3 esters exists as a pair of diastereoisomers and was identified by examining the effects of pH and nitrophenyl ring current shielding on the chemical shifts of nonexchangeable inositol protons. 1H NMR spectra of InsP3 were analyzed for comparison. On photolysis the compounds released InsP3 with rate constants of 175 (P-1), 225 (P-4), and 280 s-1 (P-5) as determined by monitoring the aci-nitro decay reaction at pH 7.1, 0.2 M ionic strength, 21 degrees C. Quantum yields determined by steady-state near-UV photolysis were 0.65 +/- 0.08 for each compound. P-4 and P-5 caged InsP3 were the most promising biologically inactive InsP3 precursors since at concentrations up to 50 microM they did not release Ca2+ from smooth muscle sarcoplasmic reticulum (SR) and were not metabolized by vascular smooth muscle InsP3 5-phosphatase or bovine brain InsP3 3-kinase.(ABSTRACT TRUNCATED AT 250 WORDS)