Differential growth regulation of a metastatic human lung carcinoma cell line through activation of phosphatidyl inositol turnover signal transduction pathway

Until recently, the signal transduction pathways involved in the processes of tumor growth have been poorly understood. In the present study, we investigated cell surface receptors which utilize phosphatidylinositol (Pl) turnover/Ca²⁺ mobilization as a signal transduction pathway to regulate cell growth in a metastatic human lung carcinoma cell line, PG. We found that purinoceptor agonists, including ATP and its analogs, and bombesin, an amphibian tetradeca-peptide of mammalian homology gastrin-releasing peptide, induced rapid transient increase of cytoplasmic-free Ca²⁺ in PG cells loaded with fura-2. The Ca ²⁺ responses were derived both from release from internal stores and the opening of plasma membrane Ca ²⁺ channels. HPLC analysis of inositol 1,4,5-triphosphate (Ins(1,4,5)P3) and its isomers showed a receptor-linked phospholipase C activation by ATP and bombesin. Although ATP and bombesin were both able to induce PI turnover and Ca²⁺ mobilization in PG cells, they had differential growth regulatory effects on PG cells. Treatment with bombesin stimulated PG cell growth while treatment with ATP inhibited significantly PG cell growth. Pharmacological studies showed that the purinoceptors on PG cells were of the P2 subtype. Other hydrolysis-resistant P2 purinoceptor agonists, including ATPγS and AMP-PNP, were as effective as ATP in stimulating PI turnover and Ca²⁺ mobilization as well as in inhibiting PG cell growth in vitro, suggesting the potential usefulness of such ATP analogs in clinical trials. Preliminary results suggest G protein involvement in the differential regulation of ATP and bombesin signal transduction pathways.