A SMALL PROLINE-RICH PROTEIN REGULATED BY VITAMIN-A IN TRACHEAL EPITHELIAL-CELLS IS INDUCED IN LUNG-TUMORS

In cell-free translations of RNA from primary cultures of pig trachea surface epithelial cells, we observed that a 20 kD proline-rich protei n (sPRP) is induced during culturing (Biochem. Biophys. Res. Commun. 1 990; 172:1304-1309). Subsequently, a cDNA encoding sPRP has been clone d from pig tracheal cell mRNA and sequenced. This cDNA shows a high si milarity to cDNAs cloned from monkey tracheal cells cultured in vitami n A-free medium and from UV-irradiated human epidermal keratinocytes. Amino acid sequences from these cDNAs are exceptionally rich in prolin e, glutamine, cysteine, and lysine but contain no aromatic amino acids . Two repeats of 12 amino acids on the N-terminus are followed by mult iple 8 amino acid repeats. When compared with monkey trachea and human keratinocyte cDNAs, the sPRP cDNA from pig trachea has an additional 24 bp nucleotide repeat. Antiserum raised to a synthetic peptide (23 a mino acids) on the C-terminus of sPRP (C23-antiserum) reacted with the 20 kD sPRP in immunoprecipitations from cell-free translations. On No rthern blot analysis, sPRP cDNA hybridized to RNAs of similar sizes in tracheal cells from cat, rabbit, and lamb. sPRP was not detected in t racheal cells that were cultured with 10(-9) M arotinoid. Since sPRP i s considered a putative squamous cell differentiation marker, experime nts using lung tumors were performed. sPRP mRNA levels were dramatical ly increased in squamous lung tumors that were induced by injecting ha msters with 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone, a tobacco- specific nitrosamine. In situ hybridization with tissue sections prepa red from these lung tumors revealed that cells around the keratin pear ls contained high levels of sPRP mRNA.