Zhou Bingying, Ritt Daniel A, Morrison Deborah K, Der Channing J, Cox Adrienne D
From the Department of Pharmacology.
Laboratory of Cell and Developmental Signaling, Center for Cancer Research, NCI, National Institutes of Health, Frederick, Maryland 21702.
J Biol Chem. 2016 Aug 19;291(34):17804-15. doi: 10.1074/jbc.M115.712885. Epub 2016 May 17.
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.
蛋白激酶酪蛋白激酶2（CK2）是一种多效性且组成型激活的激酶，在细胞增殖和存活中起关键作用。CK2的过表达，尤其是α催化亚基（CK2α，CSNK2A1），与多种癌症有关，并且与较差的生存率以及对传统和靶向抗癌疗法的耐药性相关。在此，我们发现与正常人黑素细胞相比，黑色素瘤细胞系中CK2α蛋白水平升高。然后，我们测试了CK2α在对美国食品药品监督管理局批准的黑色素瘤单药靶向疗法的耐药性中的作用。在BRAF突变的黑色素瘤细胞中，异位表达的CK2α通过一种不同于突变型NRAS的机制降低了对维莫非尼（BRAF抑制剂）、达拉非尼（BRAF抑制剂）和曲美替尼（MEK抑制剂）的敏感性。相反，敲低CK2α可使细胞对抑制剂治疗敏感。CK2α介导的RAF-MEK激酶抑制剂耐药性与其对ERK磷酸化的维持紧密相关。我们发现CK2α以激酶依赖性方式在翻译后调节ERK特异性磷酸酶双特异性磷酸酶6（DUSP6），降低其丰度。然而，我们意外地发现，通过使用CK2α的激酶失活突变体，RAF-MEK抑制剂耐药性并不依赖于CK2α激酶催化功能，并且在抑制BRAF或MEK后，野生型和激酶失活的CK2α均能维持ERK磷酸化。野生型和激酶失活的CK2α与Ras 1的RAF-MEK-ERK支架激酶抑制因子（KSR1）结合能力相同，这表明CK2α增加了KSR对ERK磷酸化的促进作用。因此，CK2α不会导致对ERK1/2选择性抑制剂SCH772984直接抑制ERK产生耐药性。我们的研究结果支持CK2α的激酶非依赖性支架功能，该功能促进对RAF和MEK靶向疗法的耐药性。
