Changes in calcium responsiveness and handling during keratinocyte differentiation. Potential role of the calcium receptor

Intracellular calcium oscillations in cell populations of ras-transfected I-7 subline of human HaCaT keratinocytes

We have observed oscillations of intracellular Ca2+ (Ca[i]) concentration in populations of ras-transfected HaCaT keratinocytes of I-7 subline. In postconfluent monolayers of I-7 keratinocytes, an increase in extracellular Ca2+ (Ca[o]) concentration to 0.25-0.5 mM induced sinusoidal Ca(i) oscillations, which persisted longer than 1 h with amplitudes of 50-150 nM and periods of 5-10 min. Thapsigargin, which depletes internal Ca2+ stores, did not prevent Ca(o)-induced Ca(i) oscillations, and it also induced Ca(i) oscillations in the ras-transfected I-7 line. Removal of extracellular Ca2+ or addition of Ca2+-entry blocker La3+ or SK&F 96365 inhibited Ca(i) oscillations, suggesting that Ca(i) oscillations in ras-transfected HaCaT keratinocytes were dependent on Ca2+ influx across the plasma membrane. Because the Ca(o)-induced Ca(i) oscillations have been observed only in ras-transfected I-7 subline and not in its nontransfected parental HaCaT line, this may provide a partial explanation for the divergent responses of ras-transfected and nontransfected keratinocytes to Ca(o) signal for control of growth and differentiation.

Human squamous-cell-carcinoma cell line (DJM-1) cells synthesize P-cadherin molecules via an elevation of extracellular calcium: calcium regulates P-cadherin-gene expression at the translational level via protein tyrosine phosphorylation

Spatially-regulated P-cadherin expression is crucial for maintaining the normal epidermal architecture. P-cadherin expression in cutaneous squamous-cell carcinomas (SCC) is altered, and may participate in tumor progression. We therefore investigated how P-cadherin expression was regulated in a cultured cutaneous SCC cell line (DJM-1). At low calcium concentration (0.05 mM), DJM-1 cells expressed P-cadherin weakly in the cytoplasm. At a higher calcium concentration, P-cadherin was promptly translocated to the cell surface within 30 min, gradually increased on the cell surface for up to 48 hr, and was continuously expressed for at least 7 days. During this time course, the total amount of P-cadherin protein had increased, whereas the steady-state mRNA levels for P-cadherin had not changed. The inhibition of protein synthesis by cycloheximide, but not the inhibition of gene transcription by actinomycin-D, completely suppressed the expression of P-cadherin. The effect of calcium was inhibited by tyrphostins but not by H-7, cholera toxin, or dibutylic cyclic AMP. Increments in the extracellular calcium concentration did not mobilize the intracellular calcium pool, and were accompanied by the tyrosine phosphorylation of a 62-kDa protein. In addition, DJM-1 cells expressed mRNA for a calcium-sensing receptor originally demonstrated in the parathyroid gland. The results suggest an unique mechanism for regulating P-cadherin gene expression in DJM-1 cells by extracellular calcium, which stimulates the de novo synthesis of P-cadherin at the translational level through protein tyrosine phosphorylation.

Identification of putative transmembrane receptor sequences homologous to the calcium-sensing G-protein-coupled receptor

The sensing of extracellular calcium is a general paradigm for regulating diverse cellular functions in many tissues. A calcium-sensing receptor (Casr) belonging to the metabotropic glutamate family of G-protein-coupled receptors (GPCR) that transduces the effects of extracellular calcium in the parathyroid gland as well as other tissues has been identified. The diversity of GPCR families and the recent finding of calcium sensing in cells lacking the known Casr suggest the existence of additional receptors related to Casr. By polymerase chain reaction (PCR) amplification and screening of genomic libraries, we have identified multiple Casr-related sequences (Casr-rs) in the mouse. Using primers designed to regions of the first and third intracellular loops of Casr, we initially PCR amplified a 497-bp Casr-related sequence (Casr-rs1) with high homology to Casr. The deduced protein sequence of Casr-rs1 is 63% similar and 40% identical to Casr over the available transmembrane region. We screened a mouse genomic library with a Casr-rs1 probe and identified two additional Casr-related sequences (Casr-rs2 and Casr-rs3). In the predicted transmembrane domain, Casr-rs2 and Casr-rs3 are 95% identical to Casr-rs1. We mapped Casr-rs1 to mouse Chromosome (Chr) 7 by interspecific backcross analysis, whereas the known Casr localizes to mouse Chr 16. By fluorescence in situ hybridization, Casr-rs2 also localized to mouse Chr 7 and Casr-rs3 mapped to mouse Chr 4. We were able to distinquish Casr-rs1 from Casr-rs2 by PCR using specific primers, suggesting that they are distinct genes clustered on Chr 7. By RT-PCR, we identified additional Casr-rs transcripts in mouse kidney, brain, testis, embryo, and MC3T3-E1 osteoblasts, but not in lung or liver. The homologous sequence in mouse kidney, embryo, and MC3T3-E1 osteoblasts, designated Casr-rs4, has a deduced amino acid sequence that is 100% similar and 97% identical to that of Casr-rs1. The sequence amplified from mouse brain, Casr-rs5, has a deduced protein sequence that is 96% similar and 92% identical to that of Casr-rs1. Our findings establish the existence of a novel multimembered family of Casr-related sequences in the mouse which may encode receptors that transduce responses to diverse extracellular cations.

Ca2+-sensing receptors in intestinal epithelium

Expression of Ca2+-sensing receptors (CaR) was demonstrated in several human intestinal epithelial cell lines (T84, HT-29, and Caco-2) and in rat intestinal epithelium by both reverse transcriptase-polymerase chain reaction (PCR) and Northern blotting of RNA. Restriction patterns of the PCR products were of the sizes predicted by the human and rat sequences. CaR agonists (Ca2+, poly-L-arginine, protamine) mediated an increase in intracellular Ca2+ in HT-29-18-C1 cells (monitored by changes in fura 2 fluorescence), which was dependent on release from thapsigargin-sensitive stores. U-73122, an inhibitor of phosphatidylinositol-phospholipase C, eliminated the CaR agonist-mediated rise in intracellular Ca2+, whereas its inactive analog, U-73343, had no effect. Pertussis toxin pretreatment had no effect on CaR agonist-mediated modulation of intracellular Ca2+. Taken together, these studies demonstrate that CaR are expressed in intestinal epithelial cells and couple to mobilization of intracellular Ca2+. The presence of CaR in intestinal epithelial cells presents a new locus for investigations into the role(s) of extracellular Ca2+ in modulating intestinal epithelial cell differentiation and transepithelial Ca2+ transport.

Alternatively spliced forms of the cGMP-gated channel in human keratinocytes

Alternatively spliced forms of the alpha subunit of the cGMP-gated channel have been cloned from human keratinocytes. One form encodes a complete channel which is almost identical to the rod photoreceptor. A second spliced variant would encode a protein missing a portion of the intracellular hydrophilic domain and the putative first transmembrane domain. Both complete and spliced variants of the channel also were found in epidermis. The expression of the complete form of the channel was induced by levels of extracellular calcium which promote keratinocyte differentiation. The cGMP-gated channel may play an important role in calcium induced keratinocyte differentiation by mediating Ca2+ entry.

Expression of extracellular calcium-sensing receptor by human lens epithelial cells

The extracellular calcium-sensing receptor (CaR) confers the capacity to sense small changes in the extracellular Ca2+ concentration (Ca2+o) not only upon cells involved in maintaining systemic Ca2+ homeostasis but also upon those not directly involved in this process. Since high Ca2+o is known to affect various physiological processes in lens epithelium both in health and in disease states (e.g., the formation of cataracts in hypocalcemic states), we investigated the expression and function of the CaR in these cells. By RT-PCR and immunocytochemistry the CaR is expressed in human lens epithelial cells in culture. In addition, the open state probability of a Ca(2+)-activated potassium (K+) channel with a conductance of 82 +/- 3 pS is significantly increased by elevating Ca2+o to 3.0 mM or by application of 100 microM neomycin, both effective CaR agonists. Therefore, our data suggest that human lens-epithelial cells express the CaR, which may be functionally linked to Ca(2+)-activated K+ channels and, perhaps, to other ion channels involved in ionic homeostasis in the lens.

A distinct cation-sensing mechanism in MC3T3-E1 osteoblasts functionally related to the calcium receptor

The presence of a cation-sensing mechanism in osteoblasts is suggested by the ability of specific cations to stimulate osteoblastic proliferation in culture and to induce de novo bone formation in some experimental models. Our study examines whether extracellular cations stimulate osteoblasts through the recently identified G protein-coupled calcium receptor (CaR). We found that CaR agonists, calcium (Ca2+), gadolinium (Gd3+), aluminum (Al3+), and neomycin, stimulated DNA synthesis in murine-derived MC3T3-E1 preosteoblasts, whereas magnesium (Mg2+), nickel (Ni2+), cadmium (Cd2+), and zinc (Zn2+) had no effect. With the exception of Mg2+, the cation specificities and apparent affinities were similar to that reported for CaR. CaR agonists also stimulated DNA synthesis in C3HT10(1/2) fibroblasts, but not in mesangial PVG, CHO, hepatic HTC, COS-7 cells, or malignant transformed ROS17/2.8 and UMR-106 osteoblasts. In addition, similar to other growth factors, CaR agonists activated transcription of a serum response element luciferase reporter construct (SRE-Luc) stably transfected into MC3T3-E1 osteoblasts, but had no effect on SRE-Luc transfected into CHO and COS-7 cells. We were unable to detect CaR expression by Northern analysis using a mouse CaR-specific probe or to amplify CaR mRNA by reverse transcribed polymerase chain reaction in MC3T3-E1 osteoblasts. These findings suggest that an extra-cellular cation-sensing mechanism is present in murine-derived osteoblasts that is functionally similar to but molecularly distinct from CaR.