We have previously demonstrated that most solid tumor cell lines of man have the ability to growth in protein-free media (R0). These include neoplasms of a diverse range of cell types encompassing lung, colon, breast, ovarian, pancreas, prostate, and brain. In contrast, none of the hemopoietic cell lines (lymphocytic and monocytic) we have studies were able to accomplish this R0 adaptation. We have noted that some cell lines undergo a morphological shift when adapted to R0. For example, the small cell lung cancer (SCLC) classic cell line NCI-H345, which grows as loose floating aggregates in serum supplemented media, will form tight floating spheroids under R0 conditions. The SCLC variant cell line NCI-N417 grows in loose floating aggregates in serum containing media also, but forms dendritic shaped adherent cell colonies in R0. Additionally, the adherent breast cancer cell line MCF-7 appears as a fibroblastoid shaped cell in serum but rapidly converts to a floating spheroid colony during R0 adaptation. The exact reason for the resulting morphological shift associated with growth in an R0 environment is not known but may possibly be a result of maximizing paracrine/autocrine growth factor availability. Consistent with this idea, we have identified a variety of peptide/protein growth factors which are produced by R0 grown tumor cell lines; the most commonly found is insulin-like growth factor I (IGF-I) and transferrin (Tf). Utilizing an immune induction screening technique to assay the components of R0 conditioned media, we have identified immuno reactive species related to transforming growth factor ` and ~, epidermal growth factor, gastrin releasing peptide (GRP), insulin-like growth factor II, calcitonin, glucagon, nerve growth factor, fibroblast growth factor (acidic), arginine vasopressin, and vasoactive intestinal peptide. Interestingly, the production of such factors appears to be an inducible phenomenon which directly relates to the cell's ability to survive harsh nutritional surroundings. For example, we have demonstrated that the bronchioloalveolar carcinoma cell line A549, which normally does not produce GRP in serum supplimented media, will express GRP mRNA and secrete a functional growth-promoting peptide when adapted to R0 media (Siegfried et al., JBC 269:8596-8603, 1994). In parallel studies, we have also shown that the message for IGF-I and Tf are upregulated during R0 adaptation. When concomitantly produced, both of these protein factors are potent inducers of cell proliferation which can mediate autocrine/paracrine growth of the tumor cell. By characterizing the cellular products responsible for R0 growth, we have begun to define the possible factors involved in the clonal expansion of tumor cells during carcinogenesis and reveal potential target sites for the disruption of this malignant process.
