Key peptide processing enzymes are expressed by a variant form of small-cell carcinoma of the lung

Small-cell carcinoma of the lung (SCCL) is a neuroendocrine tumor character ized by having the capacity to produce and secrete a number of small neurop eptides. These peptides serve the tumor as autocrine growth factors. SCCL i s known to undergo a process of dedifferentiation to a variant (drug-resist ant) form, and this process is associated with loss of marker enzymes such as neuron-specific enolase (NSE) and dopa decarboxylase (DDC). The current study was designed to discover if variant SCCL, represented by cell line NC I H82, retains some capacity to generate active neuropeptides (like vasopre ssin) from their precursors by continuing to express the three key classes of enzymes necessary for such conversions, namely prohormone convertases (P Cs), carboxypeptidases (CPs), and peptidylglycine ac-amidating monooxygenas e (PAM). RT-PCR for mRNAs representing PC1, PC2, CPE, and PAM was performed on total RNA extracted from NCI H82. The primers selected for PCR and part ial sequencing were synthetic 20, 21, 22, and 24 oligomers designed to yiel d products of 533, 880, 405, and 560 base pairs (bp) for PC1, PC2, CPE, and PAM, respectively. For the conditions used, we were able to demonstrate pr oducts for all four enzymes. Each of the four products generated were of th e expected size. Cloning and sequencing of these products revealed that eac h had a structure identical to that published for the human form of the res pective enzyme. Western analysis with antibodies against PC1, PC2, CPE, and PAM, provided evidence that mRNAs for the four enzymes are translated into proteins that could represent functional forms. Our findings therefore dem onstrate that key enzymes involved in the generation of active neuropeptide s, unlike the marker enzymes NSE and DDC, continue to be expressed by varia nt SCCL. (C) 1998 Elsevier Science Inc.