Ectopic expression of a mutant form of PKCalpha originally found in human tumors: aberrant subcellular translocation and effects on growth control

A point mutation in PKCalpha was originally discovered in a subpopulation of human pituitary tumors characterized by their invasive phenotype, and the same mutation was also seen in some thyroid neoplasms. To investigate the role of this mutation in tumorigenesis, normal and mutant human PKCalpha cDNAs were overexpressed in Rat6 embryo fibroblasts (R6). When extracts of R6 cells that expressed either the normal or mutant PKCalpha were assayed in the presence of calcium, phosphatidylserine and the phorbol ester TPA, for phosphorylation of either histone IIIS or the EGF-receptor peptide, both extracts gave similar results. However, the subcellular localization of the two proteins differed. Immunohistochemistry studies indicated that after treatment with TPA normal PKCalpha mainly translocated to the plasma membrane, but mutant PKCalpha translocated mainly to the perinuclear region and slightly to the nucleus. Furthermore, the cells that expressed the mutant PKCalpha displayed a decreased requirement for serum when compared to the cells expressing the normal human PKCalpha, and they formed small colonies in soft agar. By contrast, the cells expressing the normal human PKCalpha failed to form colonies in soft-agar. Thus, ectopic expression in rat fibroblasts of this mutant human PKCalpha sequence alters the growth properties of these cells and, when activated, the mutant PKCalpha displays aberrant intracellular translocation. Therefore, this mutation in PKCalpha could contribute to the process of tumor progression in certain human tumors.