Although PKC in cancer cells

Although PKCδ in cancer Deferoxamine mesylate receptor is known to promote antiapoptotic signaling, a cleaved form of PKCδ, δCF, has been reported to play a contrasting role in enhancing apoptosis (Reyland, 2007), and these complex functions appear to be cell-type dependent (Basu and Pal, 2010). In NSCLC, full-length PKCδ has been shown to promote cell survival (Basu and Pal, 2010). In our study, two antibodies, Ab182126 and LS-C199448 recognizing the C- and N-terminal domain of PKCδ, respectively, detected PKCδ protein in the nucleus, suggesting a full-length PKCδ in the nucleus of EGFR-mutant TKI-resistant lung cancer cells. Ectopic expression of full-length PKCδ in EGFR-mutant NSCLC cells led to TKI resistance without inducing apoptosis. Moreover, all our clinical and preclinical data supported the oncogenic role of PKCδ, particularly in TKI resistance. Abera and Kazanietz (2015) demonstrated that the PKC isozyme PKCα is involved in resistance to TKI using H1650 cells as a model and generating an H1650-M3 resistant clone by treatment with high doses of erl (1–10 μM). However, the parental H1650 cells themselves are considered erl resistant as previously reported (Bivona et al., 2011, Sos et al., 2009), and our data confirmed that H1650 cells are also resistant to gef (IC50 > 1 μM) compared with TKI-sensitive HCC827 (IC50 ∼ 0.006 μM). erl at 0.1 μM was sufficient to completely suppress EGFR phosphorylation in H1650 cells (Bivona et al., 2011). However, the IC50 of erl in H1650 and H1650-M3 cells were about 3 and 20 μM, respectively. Such high doses of erl are thought to induce off-target effects. In addition, the clinical role of PKCα was not validated in the study by Abera and Kazanietz (2015). Therefore, the PKCα-mediated TKI resistance (H1650-M3 versus H1650) may represent different conditions from our PKCδ study. PKC inhibitor as a single-agent treatment or in combination with chemotherapy has not been successful in tumors (Mochly-Rosen et al., 2012). Consistently, our results showed that sotra alone did not affect tumor growth in all xenograft tumor models. However, the combination of sotra with gef led to significant tumor regression without significant changes in mouse body weight or liver and kidney functions. These results suggested a potential and safe therapeutic strategy by combining TKI with PKC inhibitor for EGFR-mutant NSCLC patients with resistance to TKI. Upregulation of RTKs is a common mechanism underlying TKI resistance (Camidge et al., 2014, Minari et al., 2016). A single tumor with heterogeneous resistance mechanisms may upregulate multiple RTKs and cause failure of the combination therapy of TKI and specific RTK inhibitors (Figure 6G). Here, we identified nPKCδ as a common downstream molecule of the resistance-mediated RTKs, and a common upstream molecule of resistant survival signaling in TKI-resistant cells. Thus, inhibition of PKCδ has the potential to overcome the heterogeneity of TKI resistance. Because nPKCδ was highly expressed in human EGFR-mutant NSCLC with intrinsic and acquired TKI resistance, and was correlated with poor TKI response in patients, and because the combined inhibition of PKCδ and EGFR induced significant tumor regression in TKI-resistant EGFR-mutant NSCLC xenografts and PDX models, Permian Period may be worthwhile to evaluate such combinations as therapeutic options for patients with EGFR-mutant NSCLC with TKI resistance.
STAR★Methods
Acknowledgments This study was funded in part by the following: grants from the US National Institutes of Health (MD Anderson Cancer Center support grant P30CA016672); The MD Anderson Cancer Center-China Medical University and Hospital Sister Institution Fund; the Ministry of Health and Welfare, China Medical University Hospital Cancer Research Center of Excellence (MOHW107-TDU-B-212-114024 and MOHW107-TDU-B-112015); Center for Biological Pathways; and Chang Gung Memorial Hospital (grant CMRPG3D1911 to Y.-F.F.). S.-S.C. was supported by the CPRIT Research Training Program (RP140106 and RP170067).