Molecular cloning and functional expression of a third isoform of the human calcitonin receptor and partial characterization of the calcitonin receptor gene.

We have cloned and expressed two isoforms of the human calcitonin (hCT) receptor. Primers designed from the published sequence of a CT receptor cloned from an ovarian small cell carcinoma line were used for the polymerase chain reaction amplification of related products from human breast carcinoma MCF-7 cells. Two complementary DNAs were isolated. One clone lacks a 16-amino acid insert in the first intracellular loop and is virtually identical to the receptor recently cloned from the T47D human breast carcinoma cell line. The second clone is another splice variant lacking both the 16-amino acid insert in the first intracellular domain as well as the first 47 amino acids of the amino-terminus extracellular domain. COS-7 cells transfected with either receptor isoform bound [125I]salmon CT with high affinity and responded to hCT with increases in cAMP. Tissue distribution studies revealed the truncated extracellular domain 1 isoform transcripts in human skeletal muscle, kidney, brain, and lung. Analysis of a hCT receptor genomic clone demonstrated an exon/intron organization similar to that of the porcine CT receptor gene, except for a distinct exon coding for the alternatively spliced insert in the first intracellular domain.