HCaRG, a Novel Calcium-regulated Gene Coding for a Nuclear Protein, Is Potentially Involved in the Regulation of Cell Proliferation

Abstract

Since a negative calcium balance is present in spontaneously hypertensive rats, we searched for the gene(s) involved in this dysregulation. A cDNA library was constructed from the spontaneously hypertensive rat parathyroid gland, which is a key regulator of serum-ionized calcium. From seven overlapping DNA fragments, a 1100-base pair novel cDNA containing an open reading frame of 224 codons was reconstituted. This novel gene, named HCaRG (hypertension-related, calcium-regulated gene), was negatively regulated by extracellular calcium concentration, and its basal mRNA levels were higher in hypertensive animals. The deduced protein showed no transmembrane domain, 67% ␣-helix content, a mutated calcium-binding site (EF-hand motif), four putative "leucine zipper" motifs, and a nuclear receptor-binding domain. At the subcellular level, HCaRG had a nuclear localization. We cloned the human homolog of this gene. Sequence comparison revealed 80% homology between rats and humans at the nucleotide and amino acid sequences. Tissue distribution showed a preponderance in the heart, stomach, jejunum, kidney (tubular fraction), liver, and adrenal gland (mainly in the medulla). HCaRG mRNA was significantly more expressed in adult than in fetal organs, and its levels were decreased in tumors and cancerous cell lines. We observed that after 60-min ischemia followed by reperfusion, HCaRG mRNA declined rapidly in contrast with an increase in c-myc mRNA. Its levels then rose steadily to exceed base line at 48 h of reperfusion. HEK293 cells stably transfected with HCaRG exhibited much lower proliferation, as shown by cell count and [ 3 H]thymidine incorporation. Taken together, our results suggest that HCaRG is a nuclear protein potentially involved in the control of cell proliferation.Ionized calcium concentration in plasma is maintained within a very narrow range. The major players maintaining extracellular calcium homeostasis are calciotropic hormones, parathyroid hormone (PTH), 1 1,25-dihydroxyvitamin D, calcitonin, and calcium itself. Indeed, extracellular calcium regulates its own concentration as an extracellular messenger by acting on calcium receptors or calcium sensors. The calciumsensing receptor is linked to several intracellular second messenger systems via guanylyl nucleotide-regulating G proteins and activates phosphoinositide-specific phospholipase C, leading to accumulation of inositol 1,4,5-trisphosphate and diacylglycerol (1-5).

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