RAS signaling and therapeutic resistance in melanoma Public Deposited

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Last Modified
  • March 20, 2019
Creator
  • Zhou, Bingying
    • Affiliation: School of Medicine, Department of Pharmacology
Abstract
  • Increasing appreciation of the complexity of RAS signaling in cancer has led to a renewed wave of RAS research. I have focused on two key areas: the role of wild-type RAS isoforms in RAS-mutant cancers, and mechanisms of resistance to molecularly targeted therapies directed against RAS effector pathways. Melanoma, the most aggressive form of skin cancer, presents an excellent model to study RAS signaling; ~90% of melanomas have a driver mutation in NRAS (26%) or BRAF (63%), thus hyper-activating the canonical RAS-RAF-MEK-ERK effector signaling pathway. An army of small molecule inhibitors has emerged to target this pathway, and several are FDA-approved for melanoma treatment. However, all targeted therapies face the challenge of resistance. Most validated mechanisms of resistance to these inhibitors involve either reactivation of ERK signaling or other bypass routes that result in cancer cell survival. In my investigation of resistance mechanisms in BRAF-mutant melanoma. I found that CK2α was sufficient to drive resistance to inhibitors of BRAF (BRAFi) and of MEK (MEKi). CK2α facilitated rebound of ERK phosphorylation in the presence of BRAFi, and maintained ERK phosphorylation upon treatment with MEKi. Surprisingly, by using a kinase-inactive mutant of CK2α, I showed that RAF-MEK inhibitor resistance did not rely on CK2α kinase catalytic function. That both wild-type and kinase-inactive CK2α bound equally well to the RAF-MEK-ERK scaffold KSR1 suggested that CK2α increases KSR facilitation of ERK phosphorylation. Accordingly, CK2α did not cause resistance to direct inhibition of ERK by the ERK1/2-selective inhibitor SCH772984. These findings support a new mechanism whereby a kinase-independent scaffolding function of CK2α promotes resistance to RAF- and MEK-targeted therapies. Another pressing issue is understanding the biological activities of wild-type RAS isoforms in RAS-mutant tumors. Most studies have investigated KRAS-mutant cancers, but little is known about NRAS-mutant cancers. NRAS-mutant melanomas comprise the second largest subgroup of melanoma patients, and no targeted therapy is approved for these patients. Exploring the roles of wild-type RAS isoforms in NRAS-mutant melanoma cells, I found that WT KRAS is essential for their proliferation and survival. Interestingly, depletion of KRAS resulted in a unique cellular morphology and in signaling outcomes distinct from those due to silencing of NRAS or HRAS. Moreover, KRAS knockdown stabilized p53 protein, which was accompanied by an increase in p53 target genes. Intriguingly, by using reverse phase protein array analysis, I found that KRAS knockdown severely impaired phosphorylation of ribosomal protein S6, dependent on S6K1 (p70 S6K) activity, but independent of Akt-mTOR, or ERK-p90RSK activity. These results may shed light on the potential efficacy of pan-RAS inhibition in NRAS-mutant cancers. Together, my findings uncover a novel kinase-independent scaffolding function of CK2α in promoting resistance to inhibitors of BRAF and MEK in BRAF-mutant melanoma; highlight the critical importance of WT KRAS in NRAS-mutant melanoma; and describe the unique role of WT KRAS in maintaining phosphorylation of ribosomal protein S6. How CK2α engages in scaffolding to maintain ERK signaling, and how KRAS circumvents Akt-mTOR to regulate ribosomal protein S6, are issues to be addressed in future studies of RAS signaling.
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Rights statement
  • In Copyright
Advisor
  • Major, Michael
  • Graves, Lee
  • Cox, Adrienne
  • Der, Channing
  • Johnson, Gary
Degree
  • Doctor of Philosophy
Degree granting institution
  • University of North Carolina at Chapel Hill Graduate School
Graduation year
  • 2016
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