Protein Kinase CK2α Maintains Extracellular Signal-regulated Kinase (ERK) Activity in a CK2α Kinase-independent Manner to Promote Resistance to Inhibitors of RAF and MEK but Not ERK in BRAF Mutant Melanoma.

The protein kinase casein kinase 2 (CK2) is a pleiotropic and constitutively active kinase that plays crucial roles in cellular proliferation and survival. Overexpression of CK2, particularly the α catalytic subunit (CK2α, CSNK2A1), has been implicated in a wide variety of cancers and is associated with poorer survival and resistance to both conventional and targeted anticancer therapies. Here, we found that CK2α protein is elevated in melanoma cell lines compared with normal human melanocytes. We then tested the involvement of CK2α in drug resistance to Food and Drug Administration-approved single agent targeted therapies for melanoma. In BRAF mutant melanoma cells, ectopic CK2α decreased sensitivity to vemurafenib (BRAF inhibitor), dabrafenib (BRAF inhibitor), and trametinib (MEK inhibitor) by a mechanism distinct from that of mutant NRAS. Conversely, knockdown of CK2α sensitized cells to inhibitor treatment. CK2α-mediated RAF-MEK kinase inhibitor resistance was tightly linked to its maintenance of ERK phosphorylation. We found that CK2α post-translationally regulates the ERK-specific phosphatase dual specificity phosphatase 6 (DUSP6) in a kinase dependent-manner, decreasing its abundance. However, we unexpectedly showed, by using a kinase-inactive mutant of CK2α, that RAF-MEK inhibitor resistance did not rely on CK2α kinase catalytic function, and both wild-type and kinase-inactive CK2α maintained ERK phosphorylation upon inhibition of BRAF or MEK. That both wild-type and kinase-inactive CK2α bound equally well to the RAF-MEK-ERK scaffold kinase suppressor of Ras 1 (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. Our findings support a kinase-independent scaffolding function of CK2α that promotes resistance to RAF- and MEK-targeted therapies.

Zhou B ，Ritt DA ，Morrison DK ，Der CJ ，Cox AD ... -  《-》

MicroRNA-211 Modulates the DUSP6-ERK5 Signaling Axis to Promote BRAF(V600E)-Driven Melanoma Growth In Vivo and BRAF/MEK Inhibitor Resistance.

MicroRNAs (miRs) are important posttranscriptional regulators of cell fate in both normal and disease states. miR-211 has previously been shown to be a direct regulator of metabolism in BRAFV600E-mutant melanoma cells in vitro. Here, we report that miR-211 expression promotes the aggressive growth of BRAFV600E-mutant melanoma xenografts in vivo. miR-211 promoted proliferation through the posttranscriptional activation of extracellular signal-regulated kinase (ERK) 5 signaling, which has recently been implicated in the resistance to BRAF and MAPK/ERK kinase inhibitors. We therefore examined whether miR-211 similarly modulated melanoma resistance to the BRAF inhibitor vemurafenib and the MAPK/ERK kinase inhibitor cobimetinib. Consistent with this model, miR-211 expression increased melanoma cell resistance to both the inhibitors, and this resistance was associated with an increased ERK5 phosphorylation. miR-211 mediates these effects by directly inhibiting the expression of DUSP6, an ERK5 pathway-specific phosphatase and now shown to be an miR-211 target gene. These results dissect the role of the miR-211-DUSP6-ERK5 axis in melanoma tumor growth and suggest a mechanism for the development of drug-resistant tumors and a target for overcoming resistance.

Lee B ，Sahoo A ，Sawada J ，Marchica J ，Sahoo S ，Layng FIAL ，Finlay D ，Mazar J ，Joshi P ，Komatsu M ，Vuori K ，de Jong PR ，Ray A ，Perera RJ ... -  《-》

ROS production induced by BRAF inhibitor treatment rewires metabolic processes affecting cell growth of melanoma cells.

Cesi G ，Walbrecq G ，Zimmer A ，Kreis S ，Haan C ... -  《Molecular Cancer》

Molecular pathways: adaptive kinome reprogramming in response to targeted inhibition of the BRAF-MEK-ERK pathway in cancer.

The central role of the BRAF-MEK-ERK pathway in controlling cell fate has made this pathway a primary target for deregulated activation in cancer. BRaf is activated by Ras proteins allowing Ras oncogenes to constitutively activate the pathway. Activating BRaf mutations are also frequent in several cancers, being the most common oncogenic mutation in thyroid carcinoma and melanoma. There are currently two inhibitors, vemurafenib and dabrafenib, approved for treatment of malignant melanoma having activating BRaf mutations. Concurrent administration of BRAF and MAP-ERK kinase (MEK) inhibitor (trametinib) is significantly more active in patients with BRAF-mutant melanoma than either single agent alone, but progression to resistance ultimately occurs by different mechanisms that increase the activation of extracellular signal-regulated kinase (ERK). Such adaptive changes in tumor cell signaling networks allow bypass of targeted oncoprotein inhibition. This is true with targeted inhibitors for BRaf and MEK as well as specific inhibitors for AKT, mTOR, and many receptor tyrosine kinases such as EGF receptor (EGFR) and HER2. It is this adaptive response to targeted kinase inhibitors that contributes to the failure of single-agent kinase inhibitors to have durable responses. This failure is seen in virtually all cancers treated with single-agent kinase inhibitors, most of which are not as dependent on a single signaling pathway such as BRaf-MEK-ERK in melanoma. Thus, understanding the breadth of adaptive reprogramming responses to specific targeted kinase inhibition will be critical to develop appropriate combination therapies for durable clinical responses.

Johnson GL ，Stuhlmiller TJ ，Angus SP ，Zawistowski JS ，Graves LM ... -  《-》

Targeting oncogenic Raf protein-serine/threonine kinases in human cancers.

The Ras-Raf-MEK-ERK signal transduction cascade is arguably the most important oncogenic pathway in human cancers. Ras-GTP promotes the formation of active homodimers or heterodimers of A-Raf, B-Raf, and C-Raf by an intricate process. These enzymes are protein-serine/threonine kinases that catalyze the phosphorylation and activation of MEK1 and MEK2 which, in turn, catalyze the phosphorylation and activation of ERK1 and ERK2. The latter catalyze the regulatory phosphorylation of dozens of cytosolic and nuclear proteins. The X-ray crystal structure of B-Raf-MEK1 depicts a face-to-face dimer with interacting activation segments; B-Raf is in an active conformation and MEK1 is in an inactive conformation. Besides the four traditional components in the Ras-Raf-MEK-ERK signaling module, scaffolding proteins such as Kinase Suppressor of Ras (KSR1/2) play an important role in this signaling cascade by functioning as a scaffold protein. RAS mutations occur in about 30% of all human cancers. Moreover, BRAFV600E mutations occur in about 8% of all cancers making this the most prevalent oncogenic protein kinase. Vemurafenib and dabrafenib are B-RafV600E inhibitors that were approved for the treatment of melanomas bearing the V600E mutation. Coupling MEK1/2 inhibitors with B-Raf inhibitors is more effective in treating such melanomas and dual therapy is now the standard of care. Vemurafenib and cobimetanib, dabrafenib and trametinib, and encorafenib plus binimetinib are the FDA-approved combinations for the treatment of BRAFV600E melanomas. Although such mutations occur in other neoplasms including thyroid, colorectal, and non-small cell lung cancers, these agents are not as effective in treating these non-melanoma neoplasms. Vemurafenib and dabrafenib produce the paradoxical activation of the MAP kinase pathway in wild type BRAF cells. The precise mechanism for this activation is unclear, but drug-induced Raf activating side-to-side dimerization appears to be an essential step. Although 63%-76% of all people with advanced melanoma with the BRAF V600E mutation derive clinical benefit from combination therapy, median progression-free survival lasts only about nine months and 90% of patients develop resistance within one year. The various secondary resistance mechanisms include NRAS or KRAS mutations (20%), BRAF splice variants (16%), BRAFV600E/K amplifications (13%), MEK1/2 mutations (7%), and non-MAP kinase pathway alterations (11%). Vemurafenib and dabrafenib bind to an inactive form of B-Raf (αC-helixout and DFG-Din) and are classified as type I½ inhibitors. LY3009120 and lifirafenib, which are in the early drug-development stage, bind to a different inactive form of B-Raf (DFG-Dout) and are classified as type II inhibitors. Besides targeting B-Raf and MEK protein kinases, immunotherapies that include ipilimumab, pembrolizumab, and nivolumab have been FDA-approved for the treatment of melanomas. Current clinical trials are underway to determine the optimal usage of targeted and immunotherapies.

Roskoski R Jr 《-》