Ras‐Dependent and Ras‐Independent Activation Pathways for the Stress‐Activated‐Protein‐Kinase Cascade

Abstract

We have previously shown that osmotic stress activates both the mitogen‐activated protein kinase (MAPK) cascade and the stress‐activated protein kinase (SAPK, also known as JNK) cascade in rat fibroblastic 3Y1 cells and rat PC12 cells. Here, we show that treatment of these cells with sodium arsenite, a chemical compound that mimics the effects of heat shock, or anisomycin, a protein synthesis inhibitor, induces activation of SAPKs potently. These chemical compounds also stimulated the activity of SEKU MKK4/JNKK, SAPK activator, and the activity of MEKK, SEK1 activator. Expression of a dominant negative mutant of Ras blocked the anisomycin‐induced activation of SAPK and SEK1, but did not affect markedly the arsenite‐induced or heat shock‐induced activation in PC12 cells. The osmotic‐stress‐induced activation of SAPK was insensitive to the expression of a dominant negative Ras, but was partly sensitive to down‐regulation of protein kinase C. These results suggest the existence of Ras‐dependent and Ras‐independent activation pathways for the SAPK cascade triggered by environmental stresses including chemical stress in PC12 cells. Cell staining with a specific anti‐SAPK serum showed that SAPKs were present in both the cytoplasm and the nucleus under normal conditions, and became located mainly in the nucleus after osmotic stress or ultraviolet treatment, suggesting the nuclear translocation of SAPKs.