Glioblastoma multiforme is the most aggressive form of
primary brain tumor and remains largely incurable, in large part, due to its highly invasive nature. The
phosphoinositide (
PI) 3-kinase pathway is often constitutively active in these
tumors due to activating mutations in the
epidermal growth factor receptor, or deletion/loss of function of the
tumor suppressor PTEN.
Protein kinase C type iota (
PKC iota), a member of the
atypical protein kinase C family, is activated by the
PI 3-kinase pathway and is an important downstream mediator. Here, we have assessed the role of
PKC iota in
glioblastoma cell invasion. Depletion of
PKC iota with RNA interference caused an increase in actin stress fibers and a decrease in cell motility and invasion. Gene expression microarray analysis of U87MG cells showed that
PKC iota repressed expression of
mRNA for RhoB, which has previously been shown to have a role in actin stress fiber formation. Western blot analysis showed that both
PKC iota depletion and pharmacological inhibition of
PKC iota caused an increase in the
protein levels of RhoB, as did inhibition of
PI 3-kinase. Expression of RhoB from a constitutive promoter caused changes in actin stress fibers and cell invasion that were similar to those seen with
PKC iota depletion. These data show that
PKC iota, activated as a consequence of aberrant upstream
PI 3-kinase signaling, mediates
glioblastoma cell motility and invasion, and that repression of RhoB is key downstream event in
PKC iota signaling leading to enhanced cell motility. In addition, constitutive expression of RhoB repressed
PKC iota activity, as assessed by its phosphorylation status on Thr555.
PKC iota and RhoB are, therefore, mutually antagonistic, potentially creating a sensitive switch between invasive and non-invasive phenotypes.