From recent progress in cancer research, the hypothesis has been proposed that cancer cells have the ability to produce growth factors and to stimulate tumorigenesis by responding to these factors. This mechanism is now known as the autocrine growth mechanism of cancer cells, and factors responsible for this mechanism are called autocrine growth factors. We have recently examined biologic and clinical implications of possible autocrine growth factors for lung cancers. Here, we review our observations indicating that transforming growth factor, TGFa, is a possible autocrine growth factor for lung adenocarcinoma and that gastrin-releasing peptide (GRP) could serve as a reliable tumor marker for patients with small cell lung carcinoma (SCLC).
TGFa and Adenocarcinoma
By using human TGFa radioimmunoassay (RIA) as well as TGFa radioreceptor assay (RRA), production of TGFa was examined in 9 lung cancer cell lines of various histologic types; these were 3 small cell carcinomas, 4 adenocarcinomas, and 2 squamous cell carcinomas. Immunoreactive TGFa was detected in the conditioned media of 2 adenocarcinoma cell lines, A-549 and PC-9, at the concentrations of 190 and 130 pM, respectively, when approximately 4x 104 cells/ml were cultured for 72-92 h. When the same samples were assayed by RRA, these conditioned media were found to contain TGFa-like activity equivalent to 190 and 87 pM, respectively. The presence of TGFa receptor was examined using labeled recombinant human TGFa, and the receptor was identified in these 2 cell lines producing TGFa. The growth of these 2 cell lines was stimulated by exogenously added recombinant human TGFa at concentrations of 100 pM or greater, indicating that the concentrations of exogenously added TGFa that were sufficient to stimulate cellular growth were in a range similar to those detected in the conditioned media of these cell lines. this
To find out whether TGFa functions as an autocrine growth factor in these cell lines, we developed an antihuman TGF0 monoclonal antibody (MoAb) and determined its effects on the growth of these cell lines in vitro. Figure 1 shows that a higher dose of anti-TGFa MoAb significantly inhibits the cellular growth of these 2 cell lines. The possibility of nonspecific toxic effects of the MoAb on these cell lines can be excluded, because the excess amount of TGFtt incubated concomitantly with this MoAb restored cellular growth to the control levels. Furthermore, the same dose of MoAb against epidermal growth factor (EGF) did not inhibit cellular growth, suggesting that anti-TGFa MoAb specifically neutralize the effect of TGFa produced by these cell lines. These results indicate that TGFa can serve as an autocrine growth factor in these 2 adenocarcinoma cell lines. Further studies will be required to know the effects of exogenously added anti-TGFa MoAb on the tumor growth of human lung cancer xenografts in nude mice and to know the frequency of production of TGFa in a large number of fresh cancer tissues obtained from lung cancer patients.
Figure 1. Effects of MoAbs on cellular growth of lung adenocarcinoma cell lines A-549 (A) and PC-9 (B). Bars representing increased rate of absorbance (%) determined by MTT dye assay indicate the following: control (a); anti-TGF-a MoAb 100 μg/ml (b); anti-TGFa MoAb 1,000 μg/ml (c); anti-TGFa MoAb 1,000 μg/ml plus hTGFa 100 nM (d); and anti-EGF MoAb 1,000 μg/ml (e). Data represent means (±SD, brackets) of triplicate determinations. , p<.01. (Imanishi K, et al. JNCI 1989; 81:222. Reproduced by permission of the Journal of the National Cancer Institute.)