he cell lines. In light of the results obtained, we hypothesized that a check-point mechanism is activated upon total SdhD loss, which must be overcome by a subsequent third hit in order for the tumor transformation to occur. We also discuss the actual role of the Hif1a pathway in this process. Materials and Methods Mouse Strain, Husbandry and Treatment The SDHD-ESR, with a SdhDflox/2 Cre-ERTM genotype, tamoxifen-inducible mouse strain was generated as reported previously. Littermates with SdhDflox/+ and SdhDflox/2 genotypes lacking CRE recombinase are referred to as wild-type homozygous and heterozygous mice, respectively, in this work. When indicated, results from both genotypes were pooled and assigned to a control group as no differences between them was found for the phenotypes tested. Mitochondrial Isolation and Enzymatic Complex Activities Isolation of mitochondria from mouse kidney was performed as reported. Mitochondrial complex I and II activities were determined according to ref. 25 with slight modifications. Briefly, 3050 mg of protein were assayed at 30uC. Samples were diluted 1:4 in the assay reaction buffer and liquid nitrogen frozen-thawed three times before the assay. Rotenone-sensitive NADH-dehydrogenase activity was measured as the decrease in absorbance at 340 nm, referenced to 425 nm, due to oxidation of 130 mM NADH in the presence of 3.6 mM 19094061 PG-490 biological activity antimycin and 130 mM ubiquinone-1. The absorbance was measured for 2 min before and after the addition of 5 mM rotenone to the reaction mixture. Differences between rates were considered when determining activity due to MCI. The succinate-ubiquinoneoxidoreductase activity of MCII was measured for a period of two minutes as the decrease in the absorbance at 600 nm due to the reduction of 50 mM 2,6-dichlorophenolindophenol coupled to the reduction of 130 mM ubiquinone-1. The reaction was carried out in the presence of 3.6 mM antimycin, 5 mM rotenone and 10 mM succinate. Cell Lines Cells were cultured in a humidified atmosphere of 5% CO2 at 37u in DMEM supplemented with 10% fetal bovine serum, MEM non-essential amino acids, 100 U/ ml penicillin, 1 mg/ml streptomycin and 0.29 mg/ml L-glutamine. p21WAF1/Cip1 Overexpression in a SdhD Mouse Mutant 3 p21WAF1/Cip1 Overexpression in a SdhD Mouse Mutant Mouse embryonic fibroblasts were obtained from E13.5E15.5 embryos according to standard procedures. MEFs were immortalized by electroporation with the plasmid pEF321-T containing the SV40 large T antigen encoding gene, and seeding of the cells at clonal density. Stably transformed clones were subcultured, checked for the presence of TAg, and tested for continuous growth ability. Baby mouse kidney cells were obtained from five-days-old mouse litters as reported. For immortalization, BMK cells were electroporated with the plasmids pCMVE1A containing the viral oncogene E1A, and p53DD containing a dominant 
negative mutant allele of p53 according to a previously reported protocol. Four-hydroxy-tamoxifen was added to the 21385929 medium at 66 nM from a concentrated 2 mM stock in ethanol. RNA Analysis Tissues were dissected and stored frozen at 280u until processing. Total RNA was prepared from mouse tissues and cultured cells with TRIzolH reagent, according to the manufacturer’s directions for each type of sample. Total RNA from the adrenal medulla, surgically separated from cortex, was prepared using the RNeasyH microkit. High-throughput Gene Expression Studies Total RNA was prepared from the adrenal medhe cell lines. In light of the results obtained, we hypothesized that a check-point mechanism is activated upon total SdhD loss, which must be overcome by a subsequent third hit in order for the tumor transformation to occur. We also discuss the actual role of the Hif1a pathway in this process. Materials and Methods Mouse Strain, Husbandry and Treatment The SDHD-ESR, with a SdhDflox/2 Cre-ERTM genotype, tamoxifen-inducible mouse strain was generated as reported previously. Littermates with SdhDflox/+ and SdhDflox/2 genotypes lacking CRE recombinase are referred to as wild-type homozygous and heterozygous mice, respectively, in this work. When indicated, results from both genotypes were pooled and assigned to a control group as no differences between them was found for the phenotypes tested. Mitochondrial Isolation and Enzymatic Complex Activities Isolation of mitochondria from mouse kidney was performed as reported. Mitochondrial complex I and II activities were determined according to ref. 25 with slight modifications. Briefly, 3050 mg of protein were assayed at 30uC. Samples were diluted 1:4 in the assay reaction buffer and liquid nitrogen frozen-thawed three times before the assay. Rotenone-sensitive NADH-dehydrogenase activity was measured as the decrease in absorbance at 340 nm, referenced to 425 nm, due to oxidation of 130 mM NADH in the presence of 3.6 mM antimycin and 130 mM ubiquinone-1. The absorbance was measured for 2 min before and after the addition of 5 mM rotenone to the reaction mixture. Differences between rates were considered when determining activity due to MCI. The succinate-ubiquinoneoxidoreductase activity of MCII was measured for a period of two minutes as the decrease in the absorbance at 600 nm due to the reduction of 50 mM 2,6-dichlorophenolindophenol coupled to the reduction of 130 mM ubiquinone-1. The reaction was carried out in the presence of 3.6 mM antimycin, 5 mM rotenone and 10 mM succinate. Cell Lines Cells were cultured in a humidified atmosphere of 5% CO2 at 37u in DMEM supplemented with 10% fetal bovine serum, MEM non-essential amino acids, 100 U/ ml penicillin, 1 mg/ml streptomycin and 0.29 mg/ml L-glutamine. p21WAF1/Cip1 Overexpression in a SdhD Mouse Mutant 3 p21WAF1/Cip1 Overexpression in a 12931192 SdhD Mouse Mutant Mouse embryonic fibroblasts were obtained from E13.5E15.5 embryos according to standard procedures. MEFs were immortalized by electroporation with the plasmid pEF321-T containing the SV40 large T antigen encoding gene, and seeding of the cells at clonal density. Stably transformed clones were subcultured, checked for the presence of TAg, and tested for continuous growth ability. Baby mouse kidney cells were obtained from five-days-old mouse litters as reported. For immortalization, BMK cells were electroporated with the plasmids pCMVE1A containing the viral oncogene E1A, and p53DD containing a dominant negative mutant allele of p53 according to a previously reported protocol. Four-hydroxy-tamoxifen was added to the medium at 66 nM from a concentrated 2 mM stock in ethanol. RNA Analysis Tissues were dissected and stored frozen at 280u until processing. Total RNA was prepared from mouse 21836025 tissues and cultured cells with TRIzolH reagent, according to the manufacturer’s directions for each type of sample. Total RNA from the adrenal medulla, surgically separated from cortex, was prepared using the RNeasyH microkit. High-throughput Gene Expression Studies Total RNA was prepared from the adrenal med