We sought to determine the functional significance of the c-kit receptor (Kit) in melanoma, breast carcinoma, and non-small cell lung cancer (NSCLC). To explore these issues, we first screened cell lines of each type for c-kit mRNA expression using a reverse-transcription polymerase chain reaction. We found that WM-39 melanoma cells, HTB-22 breast carcinoma cells, and A549 NSCLC cells all expressed c-kit mRNA. Of interest, all of these cells expressed the c-kit ligand, Steel factor (SF). We then assessed the functional significance of c-kit and SF expression by disrupting the gene's expression with antisense (AS) oligodeoxynucleotides (ODN) targeted to c-kit mRNA codons 1-6 and SF mRNA codons 2-7, respectively. Nonhybridizing sequences [sense (5) and scrambled (SCR)] were also employed as controls. WM-39, HTB-22, and A549 cells were exposed to ODN (approximately 25 microM) for 5-7 days. Downregulation of c-kit and SF mRNA, and c-kit protein was demonstrated in cells treated with AS ODN. Effects on viable cell growth were demonstrated by the 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) or 3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4 -sulfophenyl)- 2H-tetrazolium (MTS) assay. In fact, c-kit antisense ODN inhibited the viable cell growth of A549 cells 66% and 79% compared to sense and untreated controls (P = .0003; P < .0001). Additionally, WM-39 cell growth was inhibited 48% and 21% (P < .0001, P < .03) and HTB-22 cell growth was inhibited 50% (P < .001) compared to sense and untreated controls. Viable cell growth was also significantly inhibited by SF AS ODN compared to S and SCR controls in all cell lines. These results demonstrate that WM-39, HTB-22, and A549 NSCLC cells all express the c-kit and SF protooncogenes and suggest that the encoded receptor and ligand are important for cell growth. By finding the presence, and functional importance, of both the receptor and ligand in these cells, this study suggests the existence of an autocrine loop growth mechanism worthy of further study.