Prepro-thyrotropin-releasing hormone (TRH) contains five TRH progenitor sequences and at least six other potential
peptides (Lechan, R. M., Wu, P., Jackson, I. M. D., Wolf, H., Cooperman, S., Mandel, G., and Goodman, R. H. (1986a) Science 231, 159-161). Previous studies using radioimmunoassays developed against discrete regions of
prepro-TRH have demonstrated that several of the potential
peptides are present in rat brain and pancreas (Wu, P., Lechan, R. M., and Jackson, I. M. D. (1987) Endocrinology 121, 108-115; Wu, P. and Jackson, I. M. D. (1988a) Brain Res. 456, 22-28; Wu, P., and Jackson, I. M. D. (1988b) Regul.
Pept. 22, 347-360). However, the low level of
peptides present in intact tissues has made isolation of the
peptides difficult. CA77 cells, a medullary
thyroid carcinoma cell line, also express
prepro-TRH and display processing similar to that found in tissues. However,
peptide content in this tumor cell line is enhanced only 3-fold compared with normal tissues (Sevarino, K. A., Wu, P., Jackson, I. M. D., Roos, B. A., Mandel, G., and Goodman, R. H. (1988) J. Biol. Chem. 263, 620-623). To achieve higher levels of expression for facilitating
peptide sequencing studies and to see if alternate processing of
prepro-TRH could be detected in different cell types, we transfected into 3T3, GH4, AtT20, and RIN 5F cells a
cDNA vector under control of the cytomegalovirus immediate-early promoter. 3T3 and GH4 cells failed to process
prepro-TRH beyond cleavage of the
signal sequence. Both AtT20 and RIN 5F cells efficiently cleaved the precursor at dibasic sites to generate mature TRH and the non-TRH
peptides previously identified in vivo.
Peptide content was up to 30 times greater than in hypothalamic extracts and 10 times greater than in CA77 cells. Secretion experiments with transfected AtT20 cells demonstrated that both mature TRH and the non-TRH
peptides were secreted via a regulated secretory pathway similar to that utilized by endogenously synthesized
peptides. We isolated several of the non-TRH
peptides synthesized by transfected AtT20 cells and characterized these
peptides by sequential Edman degradation. These studies identified the
signal sequence cleavage site and determined that the non-TRH
peptides are generated by cleavage at the dibasic sites flanking the five TRH progenitor sequences. Further, we determined that processing occurs at the Arg51-Arg52 site located in the amino-terminal portion of the precursor, the only dibasic site not flanking a TRH progenitor sequence.