Targeting aPKC disables oncogenic signaling by both the EGFR and the proinflammatory cytokine TNFα in glioblastoma

Abstract

Grade IV glioblastoma is characterized by increased kinase activity of epidermal growth factor receptor (EGFR); however, EGFR kinase inhibitors have failed to improve survival in individuals with this cancer because resistance to these drugs often develops. We showed that tumor necrosis factor–α (TNFα) produced in the glioblastoma microenvironment activated atypical protein kinase C (aPKC), thereby producing resistance to EGFR kinase inhibitors. Additionally, we identified that aPKC was required both for paracrine TNFα-dependent activation of the transcription factor nuclear factor κB (NF-κB) and for tumor cell–intrinsic receptor tyrosine kinase signaling. Targeting aPKC decreased tumor growth in mouse models of glioblastoma, including models of EGFR kinase inhibitor–resistant glioblastoma. Furthermore, aPKC abundance and activity were increased in human glioblastoma tumor cells, and high aPKC abundance correlated with poor prognosis. Thus, targeting aPKC might provide an improved molecular approach for glioblastoma therapy.

EGFR and NF-κB: partners in cancer

“…TNFa, mainly synthesized by tumor-associated macrophages upon activation of TLR-dependent pathways, acts as a paracrine signal to trigger NF-kB activation in glioma cells [57]. This pathway involves the activation of the serine/threonine kinase (1) The production of TNFa by macrophages promotes resistance to EGFR inhibitors through NF-kB activation in glioma cells.…”

“…Tumor-associated macrophages sense dying cells and pathogen-associated molecular patterns (PAMPs) through TLRs to trigger IKK-dependent NF-kB and ERK1/2 activation. These pathways promote TNFa synthesis, which acts in a paracrine fashion to activate NF-kB through TNFR1-dependent aPKC phosphorylation on residues 403/410 in glioma cells [57]. aPKC, whose activation requires the scaffold protein p62, triggers IKKb (not illustrated) and IkBa phosphorylation to promote the nuclear translocation of NF-kB subunits p50 and p65.…”

“…Interestingly, another pool of aPKC interacting with the scaffold protein Par6 is specifically activated upon EGF stimulation, and drives the expression of another set of genes such as CD44 and MMP9 (Figure 4). Targeting aPKC activity causes tumor regression in EGFR kinase inhibitor-resistant glioblastoma [57].…”

Loss of the Polarity Protein PAR3 Activates STAT3 Signaling via an Atypical Protein Kinase C (aPKC)/NF-κB/Interleukin-6 (IL-6) Axis in Mouse Mammary Cells

“…This observation, combined with evidence that aPKC/ has oncogenic functions (21), indicates that inappropriate aPKC/ activation may favor tumor aggression when PAR3 is silenced. Although the literature suggests several mechanisms by which aPKC activation could promote STAT3 signaling, such as potentiating TNF-␣ signaling (22), interacting with NF-B (23), promoting ERK activity (24,25), and transducing signals downstream of EGFR or RAS (16,26,27), it is not obvious which is involved when PAR3 expression is disrupted. In the present study, we use gene silencing to rigorously test the role of aPKC/ following Pard3 knockdown in transformed mouse mammary cells and to identify the mechanism through which it induces STAT3.…”

Protein Scaffolds Control Localized Protein Kinase Cζ Activity

“…aPKC in vitro (41) and already used for studies in vivo (42) for selective modulation of aPKC substrate readouts in cells. Using a genetically encoded FRET reporter, the C kinase activity reporter (CKAR) (34) previously validated for measurement of aPKC activity in real time in live cells (5, 6, 40), we examined inhibition of endogenous aPKC or mCherry-tagged constructs of aPKC by PZ09.…”