Limits...
Role of SLC5A8, a plasma membrane transporter and a tumor suppressor, in the antitumor activity of dichloroacetate.

Babu E, Ramachandran S, CoothanKandaswamy V, Elangovan S, Prasad PD, Ganapathy V, Thangaraju M - Oncogene (2011)

Bottom Line: Credible evidence exists for the antitumor activity of this compound, but high concentrations are needed for significant therapeutic effect.Here we show that SLC5A8 transports DCA very effectively with high affinity.The mechanism of the compound's antitumor activity still remains its ability to inhibit pyruvate dehydrogenase kinase and force mitochondrial oxidation of pyruvate.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology, Medical College of Georgia, Georgia Health Sciences University, Augusta, GA 30912, USA.

ABSTRACT
There has been growing interest among the public and scientists in dichloroacetate (DCA) as a potential anticancer drug. Credible evidence exists for the antitumor activity of this compound, but high concentrations are needed for significant therapeutic effect. Unfortunately, these high concentrations produce detrimental side effects involving the nervous system, thereby precluding its use for cancer treatment. The mechanistic basis of the compound's antitumor activity is its ability to activate the pyruvate dehydrogenase complex through inhibition of pyruvate dehydrogenase kinase. As the compound inhibits the kinase at micromolar concentrations, it is not known why therapeutically prohibitive high doses are needed for suppression of tumor growth. We hypothesized that lack of effective mechanisms for the entry of DCA into tumor cells may underlie this phenomenon. Here we show that SLC5A8 transports DCA very effectively with high affinity. This transporter is expressed in normal cells, but expression is silenced in tumor cells by epigenetic mechanisms. The lack of the transporter makes tumor cells resistant to the antitumor activity of DCA. However, if the transporter is expressed in tumor cells ectopically, the cells become sensitive to the drug at low concentrations. This is evident in breast cancer cells, colon cancer cells and prostate cancer cells. Normal cells, which constitutively express the transporter, are however not affected by the compound, indicating tumor cell-selective therapeutic activity. The mechanism of the compound's antitumor activity still remains its ability to inhibit pyruvate dehydrogenase kinase and force mitochondrial oxidation of pyruvate. As silencing of SLC5A8 in tumors involves DNA methylation and its expression can be induced by treatment with DNA methylation inhibitors, our findings suggest that combining DCA with a DNA methylation inhibitor would offer a means to reduce the doses of DCA to avoid detrimental effects associated with high doses but without compromising antitumor activity.

Show MeSH

Related in: MedlinePlus

Lack of inhibitory effect on histone deacetylase (HDAC) activity by dichloroacetate (DCA)(A) SW620 cell lysates were used as the source of HDAC activity. Assays were done in the absence or presence of acetate, monochloroacetate (MCA), dichloroacetate (DCA), or butyrate at indicated concentrations. (B-E) Inhibition of human recombinant HDAC isoforms by butyrate, dichloroacetate, acetate, and monochloroacetate. Concentration of the fatty acids was 1 mM. Data are presented as percent of control activity measured in the absence of the fatty acids. *, p < 0.05; **, p < 0.01; ***, p < 0.001.
© Copyright Policy
Related In: Results  -  Collection

License
getmorefigures.php?uid=PMC3140604&req=5

Figure 7: Lack of inhibitory effect on histone deacetylase (HDAC) activity by dichloroacetate (DCA)(A) SW620 cell lysates were used as the source of HDAC activity. Assays were done in the absence or presence of acetate, monochloroacetate (MCA), dichloroacetate (DCA), or butyrate at indicated concentrations. (B-E) Inhibition of human recombinant HDAC isoforms by butyrate, dichloroacetate, acetate, and monochloroacetate. Concentration of the fatty acids was 1 mM. Data are presented as percent of control activity measured in the absence of the fatty acids. *, p < 0.05; **, p < 0.01; ***, p < 0.001.

Mentions: Until now the only mechanism by which SLC5A8 functions as a tumor suppressor was through its ability to concentrate inside cells butyrate and pyruvate, which are inhibitors of histone deacetylases. We have shown in the present study that dichloroacetate also induces apoptosis in cancer cells and that the effect is dependent on the expression of SLC5A8. These findings are similar to those with butyrate and pyruvate. Therefore, we wondered whether there is any involvement of inhibition of histone deacetylases in dichloroacetate-induced apoptosis in cancer cells. This seemed unlikely however because acetate is not an inhibitor of histone deacetylases (Thangaraju et al., 2006). However, we could not rule out the possibility that dichloroacetate may function as an inhibitor of histone deacetylases. Therefore, we examined the influence of acetate, monochloroacetate, dichloroacetate, and butyrate on the activity of histone deacetylases. First, we used lysates of SW620 cells as the source of histone deacetylases (Fig. 7A). Butyrate was used as a positive control in these experiments. As expected, butyrate inhibited the activity of histone deacetylases. Under identical conditions, dichloroacetate was at least 100-fold less potent compared to butyrate in inhibiting histone deacetylases. We then used recombinant human histone deacetylase isoforms to determine if specific isoforms of the enzyme could be inhibited by dichloroacetate. Our previous studies have shown that butyrate is a specific inhibitor of the histone deacetylase isoforms HDAC1 and HDAC3 (Thangaraju et al., 2009b). In the present study, we found the same with butyrate (Fig. 7B). Dichloroacetate also inhibited the isoforms HDAC1, HDAC2, and HDAC5 to a significant extent, but the potency was very low (~20% inhibition at a concentration of 1 mM) (Fig. 7C). Acetate (Fig. 7D) and monochloroacetate (Fig. 7E) did not have any effect on any of the isoforms examined in the study.


Role of SLC5A8, a plasma membrane transporter and a tumor suppressor, in the antitumor activity of dichloroacetate.

Babu E, Ramachandran S, CoothanKandaswamy V, Elangovan S, Prasad PD, Ganapathy V, Thangaraju M - Oncogene (2011)

Lack of inhibitory effect on histone deacetylase (HDAC) activity by dichloroacetate (DCA)(A) SW620 cell lysates were used as the source of HDAC activity. Assays were done in the absence or presence of acetate, monochloroacetate (MCA), dichloroacetate (DCA), or butyrate at indicated concentrations. (B-E) Inhibition of human recombinant HDAC isoforms by butyrate, dichloroacetate, acetate, and monochloroacetate. Concentration of the fatty acids was 1 mM. Data are presented as percent of control activity measured in the absence of the fatty acids. *, p < 0.05; **, p < 0.01; ***, p < 0.001.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3140604&req=5

Figure 7: Lack of inhibitory effect on histone deacetylase (HDAC) activity by dichloroacetate (DCA)(A) SW620 cell lysates were used as the source of HDAC activity. Assays were done in the absence or presence of acetate, monochloroacetate (MCA), dichloroacetate (DCA), or butyrate at indicated concentrations. (B-E) Inhibition of human recombinant HDAC isoforms by butyrate, dichloroacetate, acetate, and monochloroacetate. Concentration of the fatty acids was 1 mM. Data are presented as percent of control activity measured in the absence of the fatty acids. *, p < 0.05; **, p < 0.01; ***, p < 0.001.
Mentions: Until now the only mechanism by which SLC5A8 functions as a tumor suppressor was through its ability to concentrate inside cells butyrate and pyruvate, which are inhibitors of histone deacetylases. We have shown in the present study that dichloroacetate also induces apoptosis in cancer cells and that the effect is dependent on the expression of SLC5A8. These findings are similar to those with butyrate and pyruvate. Therefore, we wondered whether there is any involvement of inhibition of histone deacetylases in dichloroacetate-induced apoptosis in cancer cells. This seemed unlikely however because acetate is not an inhibitor of histone deacetylases (Thangaraju et al., 2006). However, we could not rule out the possibility that dichloroacetate may function as an inhibitor of histone deacetylases. Therefore, we examined the influence of acetate, monochloroacetate, dichloroacetate, and butyrate on the activity of histone deacetylases. First, we used lysates of SW620 cells as the source of histone deacetylases (Fig. 7A). Butyrate was used as a positive control in these experiments. As expected, butyrate inhibited the activity of histone deacetylases. Under identical conditions, dichloroacetate was at least 100-fold less potent compared to butyrate in inhibiting histone deacetylases. We then used recombinant human histone deacetylase isoforms to determine if specific isoforms of the enzyme could be inhibited by dichloroacetate. Our previous studies have shown that butyrate is a specific inhibitor of the histone deacetylase isoforms HDAC1 and HDAC3 (Thangaraju et al., 2009b). In the present study, we found the same with butyrate (Fig. 7B). Dichloroacetate also inhibited the isoforms HDAC1, HDAC2, and HDAC5 to a significant extent, but the potency was very low (~20% inhibition at a concentration of 1 mM) (Fig. 7C). Acetate (Fig. 7D) and monochloroacetate (Fig. 7E) did not have any effect on any of the isoforms examined in the study.

Bottom Line: Credible evidence exists for the antitumor activity of this compound, but high concentrations are needed for significant therapeutic effect.Here we show that SLC5A8 transports DCA very effectively with high affinity.The mechanism of the compound's antitumor activity still remains its ability to inhibit pyruvate dehydrogenase kinase and force mitochondrial oxidation of pyruvate.

View Article: PubMed Central - PubMed

Affiliation: Department of Biochemistry and Molecular Biology, Medical College of Georgia, Georgia Health Sciences University, Augusta, GA 30912, USA.

ABSTRACT
There has been growing interest among the public and scientists in dichloroacetate (DCA) as a potential anticancer drug. Credible evidence exists for the antitumor activity of this compound, but high concentrations are needed for significant therapeutic effect. Unfortunately, these high concentrations produce detrimental side effects involving the nervous system, thereby precluding its use for cancer treatment. The mechanistic basis of the compound's antitumor activity is its ability to activate the pyruvate dehydrogenase complex through inhibition of pyruvate dehydrogenase kinase. As the compound inhibits the kinase at micromolar concentrations, it is not known why therapeutically prohibitive high doses are needed for suppression of tumor growth. We hypothesized that lack of effective mechanisms for the entry of DCA into tumor cells may underlie this phenomenon. Here we show that SLC5A8 transports DCA very effectively with high affinity. This transporter is expressed in normal cells, but expression is silenced in tumor cells by epigenetic mechanisms. The lack of the transporter makes tumor cells resistant to the antitumor activity of DCA. However, if the transporter is expressed in tumor cells ectopically, the cells become sensitive to the drug at low concentrations. This is evident in breast cancer cells, colon cancer cells and prostate cancer cells. Normal cells, which constitutively express the transporter, are however not affected by the compound, indicating tumor cell-selective therapeutic activity. The mechanism of the compound's antitumor activity still remains its ability to inhibit pyruvate dehydrogenase kinase and force mitochondrial oxidation of pyruvate. As silencing of SLC5A8 in tumors involves DNA methylation and its expression can be induced by treatment with DNA methylation inhibitors, our findings suggest that combining DCA with a DNA methylation inhibitor would offer a means to reduce the doses of DCA to avoid detrimental effects associated with high doses but without compromising antitumor activity.

Show MeSH
Related in: MedlinePlus