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Intestinal absorption of phosphate is stimulated by 1,25(OH)$_{2}$ vitamin D$_{3.}$ At least two distinct mechanisms underlie phosphate absorption in the gut, an active transcellular transport requiring the Na$^{+}$ /phosphate cotransporter NaPi-IIb/Slc34a2, and a poorly characterized paracellular passive pathway. 1,25(OH)$_{2}$ vitamin D$_{3}$ stimulates NaPi-IIb expression and function and loss of NaPi-IIb reduces intestinal phosphate absorption. However, it is remains unknown whether NaPi-IIb is the only target for hormonal regulation by 1,25(OH)$_{2}$ vitamin D$_{3}$ . Here we compared the effects of intraperitoneal administration of 1,25(OH)$_{2}$ vitamin D$_{3}$ (2 days, once per day) in wild type and intestinal-specific Slc34a2 deficient mice, and analyzed trans- vs paracellular routes of phosphate absorption. We found that treatment stimulated active transport of phosphate only in jejunum of wild type mice, though NaPi-IIb protein expression was upregulated in jejunum and ileum . In contrast, 1,25(OH)$_{2}$ vitamin D$_{3}$ administration had no effect in Slc34a2 deficient mice, suggesting that the hormone specifically regulates NaPi-IIb expression. In both groups, 1,25(OH)$_{2}$ vitamin D$_{3}$ elicited the expected increase of plasma FGF23 and reduction of PTH. Treatment resulted in hyperphosphaturia (and hypercalciuria) in both genotypes, though mice remained normophosphatemic. While increased intestinal absorption and higher FGF23 can trigger the hyperphosphaturic response in wild types, only the second one can explain the renal response in Slc34a2 deficient mice. Thus, 1,25(OH)$_{2}$ vitamin D$_{3}$ stimulates intestinal phosphate absorption by acting on the active transcellular pathway mostly mediated by NaPi-IIb while the paracellular pathway appears not to be affected. Intestinal absorption of phosphate proceeds via an active/transcellular route mostly mediated by NaPi-IIb/Slc34a2 and a poorly characterized passive/paracellular pathway. Intestinal phosphate absorption and expression of NaPi-IIb are stimulated by 1,25(OH)$_{2}$ vitamin D$_{3}$ but whether NaPi-IIb is the only target under hormonal control remains unknown. We report that administration of 1,25(OH)$_{2}$ vitamin D$_{3}$ to wild type mice results in the expected increase in active transport of phosphate in jejunum, without changing paracellular fluxes. Consequently, treatment failed to alter phosphate transport in intestinal-depleted Slc34a2 mice. In both genotypes, 1,25(OH)$_{2}$ vitamin D$_{3}$ induced similar hyperphosphaturic responses and changes in FGF23 and PTH. While urinary phosphate loss induced by administration of 1,25(OH)$_{2}$ vitamin D$_{3}$ did not alter plasma phosphate, further studies should investigate whether chronic administration would lead to phosphate imbalance in mice with reduced active intestinal absorption. This article is protected by copyright. All rights reserved.

AIM Several Na$^{+}$ -dependent phosphate cotransporters namely NaPi-IIb/SLC34A3, Pit-1/SLC20A1 and Pit-2/SLC20A2, are expressed at the apical membrane of enterocytes but their contribution to active absorption of phosphate is unclear. The aim of this study was to compare their pattern of mRNA expression along the small and large intestine and to analyse the effect of intestinal depletion of Pit-2 on phosphate homeostasis. METHODS Intestinal epithelial Pit-2 deficient mice were generated by crossing floxed Pit-2 with villin-Cre mice. Mice were fed two weeks standard or low phosphate diets. Stool, urine, plasma and intestinal and renal tissue were collected. Concentration of electrolytes and hormones, expression of mRNAs and proteins and intestinal transport of tracers were analysed. RESULTS Intestinal mRNA expression of NaPi-IIb and Pit-1 is segment-specific whereas the abundance of Pit-2 mRNA is comparable along the whole intestine. In ileum, NaPi-IIb mRNA expression is restricted to enterocytes whereas Pit-2 mRNA is found in epithelial and non-epithelial cells. Overall, their mRNA is not regulated by dietary phosphate. The absence of Pit-2 from intestinal epithelial cells does not affect systemic phosphate-homeostasis under normal dietary conditions. However, in response to dietary phosphate restriction, Pit-2 deficient mice showed exacerbated hypercalciuria and sustained elevation of 1,25(OH)$_{2}$ vitamin D$_{3}$ . CONCLUSIONS In mice, the intestinal Na$^{+}$ /phosphate cotransporters are not coexpressed in all segments. NaPi-IIb but not Pit-2 mRNA is restricted to epithelial cells. Intestinal epithelial Pit-2 does not contribute significantly to absorption of phosphate under normal dietary conditions. However, it may play a more significant role upon dietary phosphate restriction.

Mutations in the human homologue of Drosophila Patched1 (PTCH1) have been found in several common tumours including basal cell carcinoma, medulloblastoma, and rhabdomyosarcoma (RMS). Medulloblastoma and RMS are also present in the murine model for Ptch1 deficiency. Tumours in heterozygous Ptch1(neo67/+) mice consistently exhibit elevated transcript levels of the proto-oncogene Gli1, of Ptch1 itself, and of the insulin-like growth factor 2 (Igf2). The present study has investigated additional molecular changes in RMSs of Ptch1 mutant mice by means of microarray analysis and protein expression analysis. The data show activation of the cell survival-promoting Akt/protein kinase B (Pkb). Furthermore, RMSs express increased levels of the anti-apoptotic protein Bcl-2 and of genes and proteins known to inhibit cell proliferation, including Gadd45a and p27kip1. Taken together, the data suggest that the formation of RMSs in Ptch1 mutants is associated with the ability of tumour cells to resist apoptosis.