Insulin-like growth factor-I-dependent stimulation of nuclear phospholipase C-beta1 activity in Swiss 3T3 cells requires an intact cytoskeleton and is paralleled by increased phosphorylation of the phospholipase.
Swiss 3T3 mouse fibroblasts were exposed to 10 microM colchicine to disrupt microtubules, then stimulated with insulin-like growth factor-I. Immunoprecipitation experiments showed that insulin-like growth factor-I receptor and insulin receptor substrate-1 were tyrosine phosphorylated to the same extent in both cells treated with colchicine and in those not exposed to the drug. Moreover, the activity of phosphatidylinositol 3-kinase was not affected by incubation with colchicine. While in nuclei prepared from cells not exposed to colchicine it was possible to detect an insulin-like growth factor-I-dependent increase in the mass of diacylglycerol, as well as stimulation of phospholipase C activity, no similar changes were observed in nuclei obtained from cells treated with colchicine. Activation of the nuclear phospholipase activity was paralleled by an increase of its phosphorylation. Immunofluorescent studies revealed that mitogen-activated protein kinase did not translocate towards the nucleus when the cytoskeleton was depolymerized. These results show that in Swiss 3T3 cells some as yet unknown events necessary for the insulin-like growth factor-I-dependent activation of nuclear polyphosphoinositide metabolism require the presence of an intact cytoskeleton and are situated down-stream the activation of insulin receptor substrate-1 and phosphatidylinositol 3-kinase. Activation of nuclear phospholipase C-beta1 might be linked to its phosphorylation and translocation of mitogen-activated protein kinase to the nucleus.