Sodium-dependent Vitamin C Transporter 2 Research Articles

Requirement of calcium and phosphate ions in expression of sodium-dependent vitamin C transporter 2 and osteopontin in MC3T3-E1 osteoblastic cells

Osteoclasts dissolve mineralized bone matrix at bone resorption sites and release large amounts of calcium (Ca 2+) and phosphate (PO 4 3−) ions into the extracellular fluid. However, the exact nature of Ca 2+ and PO 4 3− on osteoblasts remains unclear. We proposed that Ca 2+ and PO 4 3− ions are required for the expression of sodium-dependent vitamin C transporter (SVCT) 2 and a differentiation marker, osteopontin (OPN), in osteoblasts as a response to the osteoclastic degradation. Results from Northern blotting indicated that a deficiency of Ca 2+ or PO 4 3− inhibited both SVCT2 and OPN expression in a time-dependent manner, whereas elevated Ca 2+ (1 to 4 mM) or PO 4 3− (1 to 4 mM) dose-dependently induced SVCT2, OPN expression and OPN promoter activity. In addition, the L-type calcium channel blocker, nifedipine (5 to 20 μM) and the phosphate transporter inhibitor, foscarnet (0.15 to 0.6 mM), dose-dependently abolished Ca 2+- and PO 4 3−-induced SVCT2, OPN expression and OPN promoter activity. Furthermore, the results from l-ascorbic acid uptake assay and Western blotting indicated that the stimulatory effect of Ca 2+ and PO 4 3− on functional SVCT2 protein expression. These findings suggested that Ca 2+ and PO 4 3− regulate osteoblastic phenotype by entering into cells to stimulate SVCT2 and OPN expression.

Age-related decline of sodium-dependent ascorbic acid transport in isolated rat hepatocytes

This study investigated whether the age-related decline in hepatic ascorbic acid (AA) levels in rats was due to altered AA uptake. AA concentrations were 68% lower in freshly isolated hepatocytes from old (24–26 months) versus young (3–5 months; p<0.0005) Fischer 344 rats. When incubated with 100 μM AA, cells from old as compared to young rats showed a 66% decline in both the rate of AA transport and the steady state intracellular levels. Sodium-free media significantly reduced AA uptake, suggesting that the sodium-dependent vitamin C transporter (SVCT) was largely responsible for declines in AA transport. Analysis of SVCT messenger RNA (mRNA) levels shows that one isoform of this transport protein, SVCT1, declines 45% with age, with no significant changes in SVCT2 mRNA levels.These results show for the first time that sodium-dependent AA transport declines during the aging process, which may account for much of the loss in tissue AA content.

The effects of ascorbic acid on cartilage metabolism in guinea pig articular cartilage explants.

Ascorbic acid has been associated with the slowing of osteoarthritis progression in guinea pig and man. The goal of this study was to evaluate transcriptional and translational regulation of cartilage matrix components by ascorbic acid. Guinea pig articular cartilage explants were grown in the presence of L-ascorbic acid (L-Asc), D-isoascorbic acid (D-Asc), sodium L-ascorbate (Na L-Asc), sodium D-isoascorbate (Na D-Asc), or ascorbyl-2-phosphate (A2P) to isolate and analyze the acidic and nutrient effects of ascorbic acid. Transcription of type II collagen, prolyl 4-hydroxylase (alpha subunit), and aggrecan increased in response to the antiscorbutic forms of ascorbic acid (L-Asc, Na L-Asc, and A2P) and was stereospecific to the L-forms. Collagen and aggrecan synthesis also increased in response to the antiscorbutic forms but only in the absence of acidity. All ascorbic acid forms tended to increase oxidative damage over control. This was especially true for the non-nutrient D-forms and the high dose L-Asc. Finally, we investigated the ability of chondrocytes to express the newly described sodium-dependent vitamin C transporters (SVCTs). We identified transcripts for SVCT2 but not SVCT1 in guinea pig cartilage explants. This represents the first characterization of SVCTs in chondrocytes. This study confirms that ascorbic acid stimulates collagen synthesis and in addition modestly stimulates aggrecan synthesis. These effects are exerted at both transcriptional and post-transcriptional levels. The stereospecificity of these effects is consistent with chondrocyte expression of SVCT2, shown previously to transport L-Asc more efficiently than D-Asc. Therefore, this transporter may be the primary mechanism by which the L-forms of ascorbic acid enter the chondrocyte to control matrix gene activity.