Resistance to Src inhibition alters the BRAF-mutant tumor secretome to promote an invasive phenotype and therapeutic escape through a FAK>p130Cas>c-Jun signaling axis

Few therapy options exist for patients with advanced papillary and anaplastic thyroid cancer. We and others have previously identified c-Src as a key mediator of thyroid cancer pro-tumorigenic processes and a promising therapeutic target for thyroid cancer. To increase the efficacy of targeting Src in the clinic, we sought to define mechanisms of resistance to the Src inhibitor, dasatinib, to identify key pathways to target in combination. Using a panel of thyroid cancer cell lines expressing clinically relevant mutations in BRAF or RAS, which were previously developed to be resistant to dasatinib, we identified a switch to a more invasive phenotype in the BRAF-mutant cells as a potential therapy escape mechanism. This phenotype switch is driven by FAK kinase activity, and signaling through the p130Cas>c-Jun signaling axis. We have further shown this more invasive phenotype is accompanied by alterations in the secretome through the increased expression of pro-inflammatory cytokines, including IL-1β, and the pro-invasive metalloprotease, MMP-9. Furthermore, IL-1β signals via a feedforward autocrine loop to promote invasion through a FAK>p130Cas>c-Jun>MMP-9 signaling axis. We further demonstrate that upfront combined inhibition of FAK and Src synergistically inhibits growth and invasion, and induces apoptosis in a panel of BRAF- and RAS-mutant thyroid cancer cell lines. Together our data demonstrate that acquired resistance to single-agent Src inhibition promotes a more invasive phenotype through an IL-1β>FAK>p130Cas>c-Jun >MMP signaling axis, and that combined inhibition of FAK and Src has the potential to block this inhibitor-induced phenotype switch.

Sustained activation of the AKT/mTOR and MAP kinase pathways mediate resistance to the Src inhibitor, dasatinib, in thyroid cancer

New targeted therapies are needed for advanced thyroid cancer. Our lab has shown that Src is a key mediator of tumorigenic processes in thyroid cancer. However, single-agent Src inhibitors have had limited efficacy in solid tumors. In order to more effectively target Src in the clinic, our lab has previously generated four thyroid cancer cell lines that are resistant to dasatinib through gradual dose escalation. We further tested two additional Src inhibitors and shown the dasatinib-resistant (DasRes) cells exhibit cross-resistance to saracatinib, but are sensitive to bosutinib, suggesting that unique off-targets of bosutinib play an important role in mediating sensitivity to bosutinib. To identify the kinases targeted by dasatinib and bosutinib, we utilized an unbiased compound centric chemical proteomics screen. We identified 33 kinases that were enriched in the bosutinib pull down. Using the STRING database to map protein-protein interactions of the unique bosutinib targets, we identified a signaling axis which included mTOR, FAK, and MEK. Inhibition of the mTOR, MEK, and Src/FAK nodes simultaneously was the most effective at reducing cell growth and survival. Overall, these studies have identified key mediators of Src inhibitor resistance, and show that targeting these signaling nodes are necessary for anti-tumor efficacy.

Resistance to RET-Inhibition in RET-Rearranged NSCLC Is Mediated By Reactivation of RAS/MAPK Signaling

Oncogenic rearrangements in RET are present in 1%-2% of lung adenocarcinoma patients. Ponatinib is a multi-kinase inhibitor with low-nanomolar potency against the RET kinase domain. Here, we demonstrate that ponatinib exhibits potent antiproliferative activity in RET fusion-positive LC-2/ad lung adenocarcinoma cells and inhibits phosphorylation of the RET fusion protein and signaling through ERK1/2 and AKT. Using distinct dose escalation strategies, two ponatinib-resistant LC-2/ad cell lines, PR1 and PR2, were derived. PR1 and PR2 cell lines retained expression, but not phosphorylation of the RET fusion and lacked evidence of a resistance mutation in the RET kinase domain. Both resistant lines retained activation of the MAPK pathway. Next-generation RNA sequencing revealed an oncogenic NRAS p.Q61K mutation in the PR1 cell. PR1 cell proliferation was preferentially sensitive to siRNA knockdown of NRAS compared with knockdown of RET, more sensitive to MEK inhibition than the parental line, and NRAS dependence was maintained in the absence of chronic RET inhibition. Expression of NRAS p.Q61K in RET fusion expressing TPC1 cells conferred resistance to ponatinib. PR2 cells exhibited increased expression of EGFR and AXL. EGFR inhibition decreased cell proliferation and phosphorylation of ERK1/2 and AKT in PR2 cells, but not LC-2/ad cells. Although AXL inhibition enhanced PR2 sensitivity to afatinib, it was unable to decrease cell proliferation by itself. Thus, EGFR and AXL cooperatively rescued signaling from RET inhibition in the PR2 cells. Collectively, these findings demonstrate that resistance to ponatinib in RET-rearranged lung adenocarcinoma is mediated by bypass signaling mechanisms that result in restored RAS/MAPK activation. Mol Cancer Ther; 16(8); 1623-33. ©2017 AACR.

The Mitogen-Activated Protein Kinase Pathway Facilitates Resistance to the Src Inhibitor Dasatinib in Thyroid Cancer

Advanced stages of papillary and anaplastic thyroid cancer represent a highly aggressive subset, in which there are currently few effective therapies. We and others have recently demonstrated that c-SRC is a key mediator of growth, invasion, and metastasis, and therefore represents a promising therapeutic target in thyroid cancer. However, clinically, Src inhibitor efficacy has been limited, and therefore further insights are needed to define resistance mechanisms and determine rational combination therapies. We have generated four thyroid cancer cell lines with a greater than 30-fold increase in acquired resistance to the Src inhibitor dasatinib. Upon acquisition of dasatinib resistance, the two RAS-mutant cell lines acquired the c-SRC gatekeeper mutation (T341M), whereas the two BRAF-mutant cell lines did not. Accordingly, Src signaling was refractory to dasatinib treatment in the RAS-mutant dasatinib-resistant cell lines. Interestingly, activation of the MAPK pathway was increased in all four of the dasatinib-resistant cell lines, likely due to B-Raf and c-Raf dimerization. Furthermore, MAP2K1/MAP2K2 (MEK1/2) inhibition restored sensitivity in all four of the dasatinib-resistant cell lines, and overcame acquired resistance to dasatinib in the RAS-mutant Cal62 cell line, in vivo Together, these studies demonstrate that acquisition of the c-SRC gatekeeper mutation and MAPK pathway signaling play important roles in promoting resistance to the Src inhibitor dasatinib. We further demonstrate that up-front combined inhibition with dasatinib and MEK1/2 or ERK1/2 inhibitors drives synergistic inhibition of growth and induction of apoptosis, indicating that combined inhibition may overcome mechanisms of survival in response to single-agent inhibition. Mol Cancer Ther; 15(8); 1952-63. ©2016 AACR.

Disorder and residual helicity alter p53-Mdm2 binding affinity and signaling in cells

Levels of residual structure in disordered interaction domains determine in vitro binding affinities, but whether they exert similar roles in cells is not known. Here, we show that increasing residual p53 helicity results in stronger Mdm2 binding, altered p53 dynamics, impaired target gene expression and failure to induce cell cycle arrest upon DNA damage. These results establish that residual structure is an important determinant of signaling fidelity in cells.