ECTOPIC EXPRESSION OF A MUTANT FORM OF PKC-ALPHA ORIGINALLY FOUND IN HUMAN TUMORS - ABERRANT SUBCELLULAR TRANSLOCATION AND EFFECTS ON GROWTH-CONTROL

A point mutation in PKC alpha was originally discovered in a subpopula tion of human pituitary tumors characterized by their invasive phenoty pe, and the same mutation was also seen in some thyroid neoplasms, To investigate the role of this mutation in tumorigenesis, normal and mut ant human PKC alpha cDNAs were overexpressed in Rat6 embryo fibroblast s (R6), When extracts of R6 cells that expressed either the normal or mutant PKC alpha were assayed in the presence of calcium, phosphatidyl serine and the phorbol ester TPA, for phosphorylation of either histon e 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 n ormal PKC alpha mainly translocated to the plasma membrane, but mutant PKC alpha translocated mainly to the perinuclear region and slightly to the nucleus, Furthermore, the cells that expressed the mutant PKC a lpha displayed a decreased requirement for serum when compared to the cells expressing the normal human PKC alpha, and they formed small col onies in soft agar, By contrast, the cells expressing the normal human PKC alpha failed to form colonies in soft-agar, Thus, ectopic express ion in rat fibroblasts of this mutant human PKC alpha sequence alters the growth properties of these cells and, when activated, the mutant P KC alpha displays aberrant intracellular translocation, Therefore, thi s mutation in PKC alpha could contribute to the process of tumor progr ession in certain human tumors.