Abstract
Circumventing chemoresistance is crucial for effectively treating cancer including glioblastoma, a lethal brain cancer. The gap junction protein connexin 43 (Cx43) renders glioblastoma resistant to chemotherapy; however, targeting Cx43 is difficult because mechanisms underlying Cx43-mediated chemoresistance remain elusive. Here we report that Cx43, but not other connexins, is highly expressed in a subpopulation of glioblastoma and Cx43 mRNA levels strongly correlate with poor prognosis and chemoresistance in this population, making Cx43 the prime therapeutic target among all connexins. Depleting Cx43 or treating cells with αCT1–a Cx43 peptide inhibitor that sensitizes glioblastoma to the chemotherapy temozolomide–inactivates phosphatidylinositol-3 kinase (PI3K), whereas overexpression of Cx43 activates this signaling. Moreover, αCT1-induced chemo-sensitization is counteracted by a PI3K active mutant. Further research reveals that αCT1 inactivates PI3K without blocking the release of PI3K-activating molecules from membrane channels and that Cx43 selectively binds to the PI3K catalytic subunit β (PIK3CB, also called PI3Kβ or p110β), suggesting that Cx43 activates PIK3CB/p110β independent of its channel functions. To explore the therapeutic potential of simultaneously targeting Cx43 and PIK3CB/p110β, αCT1 is combined with TGX-221 or GSK2636771, two PIK3CB/p110β-selective inhibitors. These two different treatments synergistically inactivate PI3K and sensitize glioblastoma cells to temozolomide in vitro and in vivo. Our study has revealed novel mechanistic insights into Cx43/PI3K-mediated temozolomide resistance in glioblastoma and demonstrated that targeting Cx43 and PIK3CB/p110β together is an effective therapeutic approach for overcoming chemoresistance.
Highlights
Overcoming resistance to chemotherapy such as temozolomide (TMZ) has proven perplexing and remains a key unmet clinical need
connexin 43 (Cx43), but not other connexins, is highly expressed in a subpopulation of GBM and mRNA levels of Cx43 correlate with poor prognosis and chemoresistance Studies on the expression of Cx43 in normal brain and malignant glioma have shown discrepant results [15, 41,42,43,44,45,46,47,48]
Despite that Cx43 mRNA levels are usually high in normal brain and low-grade glioma and vary in high-grade glioma, whether Cx43 and other connexins (Supplemental Table S2) are expressed in GBM and important for GBM chemoresistance has not yet been explored
Summary
Overcoming resistance to chemotherapy such as temozolomide (TMZ) has proven perplexing and remains a key unmet clinical need. Given that TMZ is able to pass the blood–brain barrier [2], this drug has been used as the frontline chemotherapy for glioblastoma (GBM) an aggressive and lethal cancer that accounts for approximately half of all malignant brain tumors and has a grim prognosis with an average survival time of 14.6 months [3, 4]. Adding to this dismal outcome, nearly 90% of patients with GBM succumb to tumor recurrence and the average survival for recurrent GBM is about 5.5–7.5 months due to limited therapeutic options and resistance to TMZ [5]. MGMT repairs TMZ-induced DNA damage, conferring MGMT-dependent TMZ resistance; as such, inhibiting
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