Abstract
Breast cancer is the second most common cancer among women in the US. The active form of vitamin D, 1,25-dihydroxyvitamin D (1,25(OH)2D), is proposed to inhibit cellular processes and to prevent breast cancer. The current studies investigated the effect of 1,25(OH)2D on glutamine metabolism during cancer progression employing Harvey-ras oncogene transformed MCF10A human breast epithelial cells (MCF10A-ras). Treatment with 1,25(OH)2D significantly reduced intracellular glutamine and glutamate levels measured by nuclear magnetic resonance (NMR) by 23±2% each. Further, 1,25(OH)2D treatment reduced glutamine and glutamate flux, determined by [U-(13)C5] glutamine tracer kinetics, into the TCA cycle by 31±0.2% and 17±0.4%, respectively. The relative levels of mRNA and protein abundance of the major glutamine transporter, solute linked carrier family 1 member A5 (SLC1A5), was significantly decreased by 1,25(OH)2D treatment in both MCF10A-ras cells and MCF10A which overexpress ErbB2 (HER-2/neu). Consistent with these results, glutamine uptake was reduced by 1,25(OH)2D treatment and the impact was eliminated with the SLC1A5 inhibitor L-γ-Glutamyl-p-nitroanilide (GPNA). A consensus sequence to the vitamin D responsive element (VDRE) was identified in silico in the SLC1A5 gene promoter, and site-directed mutagenesis analyses with reporter gene studies demonstrate a functional negative VDRE in the promoter of the SLC1A5 gene. siRNA-SLC1A5 transfection in MCF10A-ras cells significantly reduced SLC1A5 mRNA expression as well as decreased viable cell number similar to 1,25(OH)2D treatment. SLC1A5 knockdown also induced an increase in apoptotic cells in MCF10A-ras cells. These results suggest 1,25(OH)2D alters glutamine metabolism in MCF10A-ras cells by inhibiting glutamine uptake and utilization, in part through down-regulation of SLC1A5 transcript abundance. Thus, 1,25(OH)2D down-regulation of the glutamine transporter, SLC1A5, may facilitate vitamin D prevention of breast cancer.
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