Horylated (Fig. 3C and D). Taken together, these results demonstrate a selective transcriptional stimulatory effect of Stat5b on 2 of 6 Stat5b-responsive enhancers in the absence of GH-induced activation, implying that individual Igf1 locus Stat5b-regulated responsive elements have different functional properties.DNA Binding Strength and Transcriptional FunctionQuantitative in vitro DNA-protein binding experiments [31] revealed a ,15-fold difference in affinities of GH-activated wildtype Stat5b for the 3 different Stat5 sites studied with this method: R58, R34, and R35 (Fig. 4, Kd values from 1.3 to 17 nM). Similar results were observed with cells expressing Stat5bCA, although the range of affinities was narrower (Kds from 3.2 to 8.5 nM, data not shown). Analyses using semi-quantitative dose-response competition studies against the labeled R34 probe allowed us to calculate relative affinities of GH-activated wild-type Stat5b for each of the 14 Stat5 sites in the 6 elements. Results show a wide range of binding strengths, as illustrated by measured IC50’s of 1.4 to 158 nM (Fig. 5). Of note, R13.5, which resembled an optimal Stat6 site [15], competed very poorly for binding of Stat5b to the labeled R34 probe (Fig. 5A, C), thus indicating that despite its clear contribution to Stat5b-mediated transcription (Fig. 2), at best it could bind Stat5b very weakly. Also of note are close correlations between binding affinities determined directly with wild-type Stat5 and those measured by competition studies (R58, Kd of 1.3 nM, IC50 of 2.2 nM; R34, Kd of 6.1 nM, IC50 of 2.7 nM; R35, Kd of 17 nM, IC50 of 35 nM). We next examined the direct impact of substituting one Stat5 site for another on both DNA binding affinity and transcriptional activity. Changing low affinity R35 to either higher affinity R34 or R60 within the 18 base pair R35 oligonucleotide probe caused a substantial increase in binding strength of Stat5bCA as revealed by gel-mobility shift experiments (Fig. 6A), along with a commensurate rise in GH-activated and Stat5b mediated transcription of Igf1 promoter 2 fused to modified R34?5 (Fig. 6B, although this did not quite reach statistical significance for R60). Of note, promoter activity also was increased by nearly 4-fold in the absence of GH with the R35 to R34 substituted DNA (Fig. 6B). In CUDC-427 contrast, substitution with R43, which does not bind Stat5b in vitro or in vivo [34], eliminated binding by qualitative EMSA (Fig. 6A) and reduced GH-activated transcriptional activity, with the latter being equivalent to knockout of the R35 site (Fig. 6B). Taken together, the data in Fig. 6 show that 1 or 2 nucleotide modifications within a single Stat5b site that result in a change in binding affinity can alter the functional properties of the entire enhancer element.Figure 6. DNA modifications in the core Stat5b recognition sequence alter binding and Stat5b-dependent transcriptional activity. A. Top: Nucleotide sequence of top strand of DNA probe of R35 used in gel-mobility shift experiments. The 9-base pair central Stat5b recognition site is in bold CY5-SE web script. Nucleotide substitutions to create modified DNA probes are listed below. Bottom: Results of gelmobility shift assays with IR-labeled double-stranded oligonucleotide probes and 2 mg of nuclear protein from Cos-7 cells transfected with empty vector (2) or with expression plasmid for rat Stat5bCA (+). 12926553 The probes are labeled above each pair of lanes. The arrow indicates protein-DNA complexes. FP.Horylated (Fig. 3C and D). Taken together, these results demonstrate a selective transcriptional stimulatory effect of Stat5b on 2 of 6 Stat5b-responsive enhancers in the absence of GH-induced activation, implying that individual Igf1 locus Stat5b-regulated responsive elements have different functional properties.DNA Binding Strength and Transcriptional FunctionQuantitative in vitro DNA-protein binding experiments [31] revealed a ,15-fold difference in affinities of GH-activated wildtype Stat5b for the 3 different Stat5 sites studied with this method: R58, R34, and R35 (Fig. 4, Kd values from 1.3 to 17 nM). Similar results were observed with cells expressing Stat5bCA, although the range of affinities was narrower (Kds from 3.2 to 8.5 nM, data not shown). Analyses using semi-quantitative dose-response competition studies against the labeled R34 probe allowed us to calculate relative affinities of GH-activated wild-type Stat5b for each of the 14 Stat5 sites in the 6 elements. Results show a wide range of binding strengths, as illustrated by measured IC50’s of 1.4 to 158 nM (Fig. 5). Of note, R13.5, which resembled an optimal Stat6 site [15], competed very poorly for binding of Stat5b to the labeled R34 probe (Fig. 5A, C), thus indicating that despite its clear contribution to Stat5b-mediated transcription (Fig. 2), at best it could bind Stat5b very weakly. Also of note are close correlations between binding affinities determined directly with wild-type Stat5 and those measured by competition studies (R58, Kd of 1.3 nM, IC50 of 2.2 nM; R34, Kd of 6.1 nM, IC50 of 2.7 nM; R35, Kd of 17 nM, IC50 of 35 nM). We next examined the direct impact of substituting one Stat5 site for another on both DNA binding affinity and transcriptional activity. Changing low affinity R35 to either higher affinity R34 or R60 within the 18 base pair R35 oligonucleotide probe caused a substantial increase in binding strength of Stat5bCA as revealed by gel-mobility shift experiments (Fig. 6A), along with a commensurate rise in GH-activated and Stat5b mediated transcription of Igf1 promoter 2 fused to modified R34?5 (Fig. 6B, although this did not quite reach statistical significance for R60). Of note, promoter activity also was increased by nearly 4-fold in the absence of GH with the R35 to R34 substituted DNA (Fig. 6B). In contrast, substitution with R43, which does not bind Stat5b in vitro or in vivo [34], eliminated binding by qualitative EMSA (Fig. 6A) and reduced GH-activated transcriptional activity, with the latter being equivalent to knockout of the R35 site (Fig. 6B). Taken together, the data in Fig. 6 show that 1 or 2 nucleotide modifications within a single Stat5b site that result in a change in binding affinity can alter the functional properties of the entire enhancer element.Figure 6. DNA modifications in the core Stat5b recognition sequence alter binding and Stat5b-dependent transcriptional activity. A. Top: Nucleotide sequence of top strand of DNA probe of R35 used in gel-mobility shift experiments. The 9-base pair central Stat5b recognition site is in bold script. Nucleotide substitutions to create modified DNA probes are listed below. Bottom: Results of gelmobility shift assays with IR-labeled double-stranded oligonucleotide probes and 2 mg of nuclear protein from Cos-7 cells transfected with empty vector (2) or with expression plasmid for rat Stat5bCA (+). 12926553 The probes are labeled above each pair of lanes. The arrow indicates protein-DNA complexes. FP.
