Microfilament bundles, LETS protein and growth control in somatic cell hybrids

Hybrid cell lines between normal rat embryo fibroblasts and TA3B mouse tumour cells, or between TA3B and BI hamster sarcoma cells, have been examined for the expression of the cell surface large external-transformation-sensitive (LETS) protein and the organization of microfilament bundles. LETS protein was detected by lactoperoxidase-catalysed radioiodination and microfilament bundles were visualized by indirect immunofluorescence with antibodies directed against actin or myosin. Hybrids which exhibited normal growth control characteristics had high levels of LETS protein and extensive microfilament bundles. Neoplastic transformation appears to be suppressed in these hybrids. Hybrids which had the growth control characteristics typical of transformed cells had reduced or zero levels of LETS protein and few microfilament bundles. These results confirm previous studies on the expression of the transformed phenotype in these hybrids and demonstrate that there is a good correlation between normal growth control and the presence of LETS protein and microfilament bundles. However, the changes in cell surface LETS protein and in the organization of microfilament bundles often appeared to be quantitative reductions rather than all-or-none effects. The magnitude of the alterations in the levels of LETS protein and in the organization of microfilaments appeared to correlate with the range of transformed characteristics exhibited by the cells. One transformed hybrid in particular, selected for growth in agar, had some surface LETS protein, some microfilament bundles and retained density-dependent inhibition of growth.