HCaRG, a novel calcium-regulated gene coding for a nuclear protein, is potentially involved in the regulation of cell proliferation

Since a negative calcium balance is present in spontaneously hypertensive r ats, we searched for the gene(s) involved in this dysregulation. A cDNA lib rary was constructed from the spontaneously hypertensive rat parathyroid gl and, which is a key regulator of serum-ionized calcium. From seven overlapp ing DNA fragments, a 1100-base pair novel cDNA containing an open reading f rame of 224 codons was reconstituted. This novel gene, named HCaRG (hyperte nsion-related, calcium-regulated gene), was negatively regulated by extrace llular calcium con centration, and its basal mRNA levels were higher in hyp ertensive animals. The deduced protein showed no transmembrane domain, 67% alpha-helix content, a mutated calcium-binding site (EF-hand motif), four p utative "leucine zipper" moths, and a nuclear receptor-binding domain. At t he subcellular level, HCaRG had a nuclear localization. We cloned the human homolog of this gene. Sequence comparison revealed 80% homology between ra ts and humans at the nucleotide and amino acid sequences. Tissue distributi on showed a preponderance in the heart, stomach, jejunum, kidney (tubular f raction), 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 6 0-min ischemia followed by reperfusion, HCaRG mRNA declined rapidly in cont rast with an increase in c-myc mRNA. Its levels then rose steadily to excee d base Line at 48 h of reperfusion. HEK293 cells stably transfected with HC aRG exhibited much lower proliferation, as shown by cell count and [H-3]thy midine incorporation. Taken together, our results suggest that HCaRG is a n uclear protein potentially involved in the control of cell proliferation.