This invention relates to a
novel gene that shows
tissue specific expression and increased expression in a
low calcium concentration medium. Low renin hypertension is characterized by decreased levels of serum ionized
calcium in the presence of increased levels of
parathyroid hormone. It is hypothesized that hypertensive factor(s) are co-secreted with PTH in SHR, a model of low renin hypertension, the parathyroid hypertensive factor being one of them. As a negative
calcium balance is present in spontaneously hypertensive rats (SHR), we searched for
gene(s) involved in this dysregulation. A
cDNA library was constructed from the SHR
parathyroid gland which is a key
regulator of serum ionized
calcium. From 7 overlapping
DNA fragments, a 1100-bp 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% a
helix content, a mutated calcium-
binding site (EF-hand motif), 4 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 baseline 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.