A rat kidney-specific calcium transporter in the distal nephron

CaT1 expression correlates with tumor grade in prostate cancer

Ca(2+) signaling is important for growth and survival of prostatic carcinoma (PCa) cells. Here we report that the gene for CaT1, a channel protein highly selective for Ca(2+), is expressed at high levels in human PCa and in the LNCaP PCa cell line. CaT1 mRNA levels were elevated in PCa specimens in comparison to benign prostatic hyperplasia (BPH) specimens and positively correlated with Gleason grade in a PCa series. CaT1 mRNA was suppressed by androgen and was induced by a specific androgen receptor antagonist in LNCaP cells, suggesting that the gene is negatively regulated by androgen. These findings are the first to implicate a Ca(2+) channel in PCa progression and suggest that CaT1 may be a novel target for therapy.

CaT1 manifests the pore properties of the calcium-release-activated calcium channel

The calcium-release-activated Ca2+channel, ICRAC, is a highly Ca2+-selective ion channel that is activated on depletion of either intracellular Ca2+ levels or intracellular Ca2+ stores. The unique gating of ICRAC has made it a favourite target of investigation for new signal transduction mechanisms; however, without molecular identification of the channel protein, such studies have been inconclusive. Here we show that the protein CaT1 (ref. 4), which has six membrane-spanning domains, exhibits the unique biophysical properties of ICRAC when expressed in mammalian cells. Like ICRAC, expressed CaT1 protein is Ca2+ selective, activated by a reduction in intracellular Ca2+ concentration, and inactivated by higher intracellular concentrations of Ca2+. The channel is indistinguishable from ICRAC in the following features: sequence of selectivity to divalent cations; an anomalous mole fraction effect; whole-cell current kinetics; block by lanthanum; loss of selectivity in the absence of divalent cations; and single-channel conductance to Na+ in divalent-ion-free conditions. CaT1 is activated by both passive and active depletion of calcium stores. We propose that CaT1 comprises all or part of the ICRAC pore.

Inhibition of CaT1 channel activity by a noncompetitive IP3 antagonist

A newly cloned, human epithelial Ca2+ transport protein (CaT1) was expressed in Xenopus laevis oocytes, and its single channel characteristics were examined. The CaT1 channel shows a strong dependence upon hyperpolarizing voltages, being activated by very negative voltages. The probability of channel opening and mean open times increase substantially at more negative voltages in the range of -90 to -160 mV. In addition, CaT1 channel activity was markedly inhibited by micromolar levels of a noncompetitive antagonist of the IP3 receptor originally isolated from a marine sponge, Xestospongin C. This inhibitory effect could be mediated indirectly via the binding of Xestospongin C to the inositol-trisphosphate (IP3) receptor or, alternatively, by a direct action on the CaT1 channel itself. Independent of its mechanism of action in inhibiting CaT1, Xestospongin C will provide a useful tool for elucidating the physiological role(s) of this novel epithelial Ca2+ channel.

Human calcium transport protein CaT1

Transcellular calcium transport occurs in many epithelial tissues including intestine, kidney, and placenta. We identified the human ortholog (hCaT1) of a recently cloned rat calcium transport protein, CaT1, that mediates intestinal calcium uptake. hCaT1 messenger RNA is present in the gastrointestinal tract, including esophagus, stomach, duodenum, jejunum, ileum, and colon. High levels of hCaT1 transcripts are also present in pancreas, placenta, prostate, and salivary gland, while moderate levels are present in liver, kidney, and testis. hCaT1 mRNA is also expressed in the colorectal cancer cell line, SW480, and the chronic myelogenous leukemia cell line, K-562. The hCaT1 gene was assigned to the long arm of chromosome 7, bands q33-34, by fluorescence in situ hybridization. When expressed in Xenopus laevis oocytes, hCaT1 promotes saturable Ca(2+) uptake with a Michaelis constant of 0.25 mM. Our studies suggest a role for hCaT1 in cellular calcium uptake in a variety of tissues, including the transcellular calcium transport pathway in intestine.