Evaluating TBK1 as a Therapeutic Target in Cancers with Activated IRF3

Abstract

TBK1 (TANK-binding kinase 1) is a noncanonical IkB protein kinase that phosphorylates and activates downstream targets such as IRF3 and c-Rel and, mediates NF-kB activation in cancer. Previous reports demonstrated synthetic lethality of TBK1 with mutant KRAS in non-small cell lung cancer (NSCLC); thus, TBK1 could be a novel target for treatment of KRAS-mutant NSCLC. Here, the effect of TBK1 on proliferation in a panel of cancer cells by both genetic and pharmacologic approaches was evaluated. In KRAS-mutant cancer cells, reduction of TBK1 activity by knockdown or treatment with TBK1 inhibitors did not correlate with reduced proliferation in a two-dimensional viability assay. Verification of target engagement via reduced phosphorylation of S386 of IRF3 (pIRF3 S386 ) was difficult to assess in NSCLC cells due to low protein expression. However, several cell lines were identified with high pIRF3 S386 levels after screening a large panel of cell lines, many of which also harbor KRAS mutations. Specifically, a large subset of KRAS-mutant pancreatic cancer cell lines was uncovered with high constitutive pIRF3 S386 levels, which correlated with high levels of phosphorylated S172 of TBK1 (pTBK1 S172 ).Finally, TBK1 inhibitors dose-dependently inhibited pIRF3 S386 in these cell lines, but this did not correlate with inhibition of cell growth. Taken together, these data demonstrate that the regulation of pathways important for cell proliferation in some NSCLC, pancreatic, and colorectal cell lines is not solely dependent on TBK1 activity.Implications: TBK1 has therapeutic potential under certain contexts and phosphorylation of its downstream target IRF3 is a biomarker of TBK1 activity.

Drugging the undruggable RAS: Mission Possible?

“…Although the protein kinase STK33 was initially proposed to be a druggable target in KRAS -mutant cells 144 , subsequent studies found that neither genetic or pharmacologic inhibition of STK33 selectively inhibited the growth of such cells 162 . Genetic and pharmacologic evaluations of TBK1 found no consistent requirement for TBK1 in the growth of KRAS -mutant tumour cell lines in vitro 163 , although combined TBK1 and MEK inhibition led to partial regression of Kras;p53 mutant lung tumours in mice 164 . This result highlights a limitation of high throughput screening - hits must be rigorously validated.…”

KRAS: The Critical Driver and Therapeutic Target for Pancreatic Cancer

“…Therefore, it was very intriguing when TBK1 was identified as a synthetic lethal interactor of mutant KRAS (Barbie et al 2009). However, subsequent studies found that TBK1 function was not tightly linked with RAS -mutant cancer (Muvaffak et al 2014). Another effector implicated in RALB-dependent PDAC invasion is RALBP1/RLIP76 (Neel et al 2012), a multidomain protein that can act as a GTP-activating protein (GAP) for the Cdc42 and RAC1 RHO family small GTPases.…”

Ral small GTPase signaling and oncogenesis: More than just 15minutes of fame

“…RalB binding to Sec5 leads to an interaction of Sec5 with TBK1, a protein kinase known to regulate NF-κB signaling. Intriguingly, TBK1 has recently been identified in siRNA screens as a synthetic lethal partner of activated K-Ras [62], although a subsequent study failed to support this relationship [63]. …”