3, 4 The activation of JNK is mediated by sequential protein phosphorylation through a MAPK module, namely, MAPK kinase kinase (MAP3K or MEKK) MAPK kinase (MAP2K or MKK) MAPK, in response to various cytokines and environmental factors.2-5 Two MAP2Ks (MKK4 and MKK7) have been identified to play a nonredundant role in the dual phosphorylation of JNK at Thr183 and Tyr185 (P-JNK).2-5 MKK4 also activates p38, whereas MKK7 specifically activates JNK.3, 5 Elevated levels of P-JNK have been frequently
observed in HCC.6-8 Pharmacologic inhibition of JNK activity suppresses the growth of both xenografted human HCC cells and chemically induced mouse/rat liver cancers.6, 9 Mice lacking JNK1 display impaired liver carcinogenesis as a result of decreased tumor cell proliferation, BGB324 whereas mice lacking p38α or IκB kinase (IKK)-β in hepatocytes exhibit
enhanced liver cancers because of augmented JNK1 activity.6, 10, 11 Thus, JNK activity, especially JNK1 activity, is essential for the proliferation of liver cancer cells.6, 11 JNK activity might also contribute to liver tumorigenesis by rendering HCC cells resistant to tumor necrosis factor–related apoptosis inducing ligand (TRAIL)- or Fas-mediated apoptosis.12, 13 Apparently, chronic inflammation contributes to the augmented levels of P-JNK in HCC. However, it remains unknown whether aberrant JNK activity also results from some cell-intrinsic defect(s).6 Numerous intracellular www.selleckchem.com/products/DAPT-GSI-IX.html signaling molecules, including receptor for activated C kinase 1 (RACK1), have been implicated in regulating the activity of the JNK pathway.14, 15 RACK1 was originally identified on the basis of its ability to anchor the activated form of protein kinase C (PKC)
and is currently recognized as a multifunctional scaffold protein.14, 15 It has been reported that RACK1 can associate with both PKC and JNK, which enables PKC to phosphorylate JNK at Ser129 and thereby facilitates medchemexpress the basal and inducible dual phosphorylation of JNK by MKK4/7 in human melanoma cells.14, 16 However, the interaction of RACK1 with JNK was not detected by another group in COS7 African green monkey kidney cells. Instead, the binding of RACK1 to MEKK4 has been revealed to be essential, but not sufficient, for MEKK4-mediated JNK activation in this cell model.15 Thus, the molecular mechanism by which RACK1 regulates the JNK pathway may be cell context dependent. Because elevated levels of RACK1 messenger RNA have been independently observed in clinical HCC samples,17, 18 it is of importance to explore how RACK1 is involved in hepatic carcinogenesis. Here, we report that overexpressed RACK1 augments JNK activity and thereby promotes HCC growth through directly binding to MKK7 and enhancing MKK7 activity.