Role of a tumor-suppressor gene in the negative control of anchorage-independent growth of Syrian hamster cells.

Tumor-suppressor genes control the neoplastic phenotype of tumor cells, but the function of these genes in normal cells is unknown. In this report we show that the loss of a tumor-suppressor gene function releases negative controls on the growth of cells in agar. This conclusion is based on observations of cell hybrids and studies of cell variants that have retained or lost a tumor-suppressor gene function. Nontumorigenic cell hybrids between normal Syrian hamster embryo cells and a benzo[a]pyrene-transformed tumor-cell line (BP6T) continued to secrete autocrine and/or paracrine growth factors produced by the tumor cells but failed to respond to these factors by growing in agar. Normal diploid cells also failed to grow in agar in response to the growth factors produced by the tumor cells. Clonal variants of nontumorigenic, immortal Syrian hamster cell lines were isolated that either retained (termed supB+) or had lost (termed supB-) the ability to suppress tumorigenicity of BP6T tumor cells after cell hybridization. Neither supB+ nor supB- variants grew in agar under conditions that allowed efficient growth of the tumor cells. However, supB- cells were reversibly induced to grow in agar with high colony-forming efficiencies in the presence of tumor cell-conditioned medium or by supplementation of the medium with a combination of growth factors. Under the same conditions, the supB+ cells failed to grow in agar. This enhanced growth-factor responsiveness in agar was used to select for supB- variants existing at a low frequency in the supB+ population. These two phenotypes, loss of tumor-suppressor function and enhanced growth-factor responsiveness in agar, were seen to cosegregate. These results indicate the tumor-suppressor gene function in these cells negatively regulates the growth response of cells in agar to mitogenic stimuli. This growth regulation may depend on cell shape or adhesion because supB+ and supB- cells grown attached to plastic responded similarly to growth factors.