Deregulation of hamster fibroblast proliferation by mutated ras oncogenes is not mediated by constitutive activation of phosphoinositide‐specific phospholipase C.

Stable expression of high levels of activated forms of Haras (T24) or v‐Ki‐ras by transfection of Chinese hamster lung fibroblasts (CCL39) yielded cells highly tumorigenic in nude mice. Two classes of transformed cells were distinguished, one with moderate p21 expression (10‐fold increased) had retained growth factor dependency, the second with higher level of p21 (greater than 50‐fold) appeared autonomous for growth. Neither class of transformants expressing Ki‐ras or Ha‐ras displayed a significant basal activity of polyphosphoinositide‐specific phospholipase C, measured either in serum‐starved cells or during exponential growth in the presence of growth factors of the tyrosine kinase family (EGF, FGF, insulin). In the growth‐factor‐dependent class of T24‐Ha‐ras‐transfected cells (clone 39THaB), phospholipase C could be stimulated normally by serum, thrombin and AlF‐4. In the more growth autonomous class (clones 39THaC and 39Ki9), release of inositol phosphates after stimulation with thrombin or serum was drastically reduced. This desensitization, apparently at the receptor level since the response to AlF‐4 persisted, is, however, not specific to ras expression. We observed it to the same degree in polyoma virus‐transformed CCL39 cells. Finally, expression of mutated forms of p21 ras did not abrogate the sensitivity of phospholipase C activation to pertussis toxin. We conclude that the transforming potential of activated forms of p21ras does not result from persistent activation of phospholipase C and that ras GTP‐binding proteins cannot substitute for Gp.