Autocrine growth loops dependent on peptidyl alpha-amidating enzyme as targets for novel tumor cell growth inhibitors

Many small cell lung tumors are dependent in vitro and in vivo on autocrine growth loops. The prototypical small cell lung cancer autocrine growth fac tor, gastrin-releasing peptide (GRP), is one of many peptide hormones which require post-translational carboxy-terminal alpha-amidation for bioactivit y. We have reported that neuroendocrine human lung tumor cell lines express the bifunctional enzyme PAM which catalyzes the biosynthesis of alpha-amid ated peptides in a two-step process, and have:recently shown that non-small cell lung cancer cell lines and tumors, generally considered to be non-end ocrine in nature, also express PAM. We have also shown that two chemical cl asses of PAM inhibitors, substrate analogues and specific copper chelators, inhibit amidating enzyme activity in cell-free extracts. Here we demonstra te in vitro growth inhibition of lung cancer tumor cell lines by both these classes of PAM inhibitors using the MTT assay and the clonogenic assay. Gr owth inhibition in a small cell lung cancer cell line can be overcome by ex ogenous addition of synthetic a-amidated GRP. Similar growth-suppressive ef fects are seen in cell lines stably transfected with a vector expressing an tisense PAM RNA. These data support the mechanism of inhibition for a new t ype of chemotherapeutic/intervention agent, directed at synthesis and activ ation of peptide growth factors, and support our postulate that cr-amidated peptide hormones are a common component in lung tumor autocrine growth bio logy which can be inhibited by targeting the biochemical mechanisms necessa ry for growth factor synthesis. (C) 1999 Elsevier Science Ireland Ltd. Al r ights reserved.