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

Abstract

A point mutation in PKCα 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 PKCα cDNAs were overexpressed in Rat6 embryo fibroblasts (R6). When extracts of R6 cells that expressed either the normal or mutant PKCα 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 PKCα mainly translocated to the plasma membrane, but mutant PKCα translocated mainly to the perinuclear region and slightly to the nucleus. Furthermore, the cells that expressed the mutant PKCα displayed a decreased requirement for serum when compared to the cells expressing the normal human PKCα, and they formed small colonies in soft agar. By contrast, the cells expressing the normal human PKCα failed to form colonies in soft-agar. Thus, ectopic expression in rat fibroblasts of this mutant human PKCα sequence alters the growth properties of these cells and, when activated, the mutant PKCα displays aberrant intracellular translocation. Therefore, this mutation in PKCα could contribute to the process of tumor progression in certain human tumors.