IMMUNOCYTOCHEMICAL EVALUATION OF PROTEIN-KINASE-C TRANSLOCATION TO THE INNER NUCLEAR MATRIX IN 3T3 MOUSE FIBROBLASTS AFTER IGF-I TREATMENT

The complex pathway which links the agonist-cell membrane receptor bin ding to the response at the genome level involves? among other element s, protein kinase C (PKC). Agonists acting at the cell membrane can af fect an autonomous nuclear polyphosphoinositide signaling system induc ing an activation of nuclear phosphoinositidase activity and a subsequ ent translocation of PKC to the nuclear region. The fine localization of PKC has been investigated by means of electron microscopy quantitat ive immunogold labeling in 3T3 mouse fibroblasts, mitogenically stimul ated by IGF-I. The enzyme, which in untreated cells is present in the cytoplasm, except for the organelles, and in the nucleoplasm, after IG F-I treatment is reduced in the cytoplasm and almost doubled in the nu cleus. The PKC isoform translocated to the nucleus is the ex isozyme, which is found not only associated with the nuclear envelope but mainl y with the interchromatin domains. By using in situ matrix preparation s, PKC appears to be retained at the nuclear matrix level, both at the nuclear lamina and at the inner nuclear matrix, suggesting a direct i nvolvement in the phosphorylation of nuclear proteins which are respon sible for the regulation of DNA replication.