Slc5a8, A Novel Transporter With Tumor Suppressor Function, Mediates Its Effect Through Survivin Depletion

Proceedings: AACR 102nd Annual Meeting 2011‐‐ Apr 2‐6, 2011; Orlando, FL Introduction: SLC5A8 is a Na+-coupled transporter for lactate, pyruvate, nicotinate and ketone bodies and is down-regulated in a wide variety of cancers. It is expressed in normal cells whereas tumor cells suppress its expression by methylation of CpG islands in its promoter. Studies from our laboratory have shown that (i) pyruvate (but not lactate) is a tumor suppressor through its ability to inhibit HDAC1/3, and that ectopic expression of SLC5A8 in breast cancer cells suppresses expression of the anti-apoptotic protein survivin. Apart from its role in protecting tumor cells from apoptosis, survivin also serves as an important determinant of chemoresistance. Based on these findings, we hypothesized that forced expression of SLC5A8 in breast cancer cells will deplete survivin and consequently enhance chemosensitivity. Methods: The estrogen receptor-positive human breast cancer cell line MCF7 does not express SLC5A8. These cells were engineered to express SLC5A8 by a doxycycline-inducible promoter. The expression of the transporter was monitored by Na+-coupled nicotinate uptake and by immunocytochemistry. The interaction between SLC5A8 and survivin was investigated by immunocytochemical analysis for co-localization. The chemosensitivity was studied by monitoring the cytotoxic effects of docetaxel in SLC5A8-negative control cells and SLC5A8-expressing cells. Results: MCF7 cells transfected with vector alone did not show Na+-coupled nicotinate uptake, indicating the absence of functional expression of SLC5A8. In contrast, MCF7 cells transfected with the SLC5A8 construct under a doxycycline-inducible promoter exhibited marked activity for Na+-coupled nicotinate uptake but only when exposed to doxycycline. This was complemented with immunocytochemical evidence of SLC5A8 protein expression in the plasma membrane. In SLC5A8-expressing control MCF7 cells, survivin was expressed robustly and the expression was restricted predominantly to the nucleus. In contrast, in SLC5A8-expressing MCF7 cells, the cellular levels of survivin were depleted. Furthermore, the location of survivin was shifted from the nucleus to the plasma membrane. Co-localization studies revealed merging of immuno-positive signals for SLC5A8 and survivin at the plasma membrane. Chemosensitivity studies showed that SLC5A8-expressing MCF7 cells were more susceptible to docetaxel than SLC5A8-negative control MCF7 cells. Conclusions: MCF7 cells are SLC5A8-negative, but express survivin with its localization found predominantly in the nucleus. Ectopic expression of SLC5A8 in these cells depletes survivin as well as changes the localization of survivin from the nucleus to the plasma membrane through interaction with SLC5A8. This is accompanied with a marked increase in chemosensitivity. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 2194. doi:10.1158/1538-7445.AM2011-2194