BIOL 463 NandV presentation – Anik
Please click the link to see the slide I used to do my New and Views presentation. It was really difficult and apparently I was way above the time alloted 🙁
The Hippo effector YAP promotes resistance to RAF- and MEK-targeted cancer therapies
Increased activation of the RAF-MEK-ERK signaling pathway (also known as the MAPK signaling pathway) is a common hallmark of many human cancers. This is usually due to activating mutations in the RAF (serine-threonine protein kinase) or RAS (small GTPase) genes. MAPK-pathway targeted therapies such as BRAF and MEK inhibitors have been developed to counteract these cancers. However, these therapies are only transiently effective due to resistance. Thus, there is a need to identify the molecular targets which provide resistance to BRAF and MEK inhibitors within cancer cells. This will lead to the development of new therapeutic strategies which can enhance the treatment response.
To determine the molecular targets which regulate the response against RAF and MEK inhibitors, the authors devised a short hair-pin RNA (shRNA) screen in human non-small cell lung carcinoma (NSCLC) cells. These cells express an endogenous BRAF V600E (valine to glutamate substitution at position 600) allele. Their aim was to identify genes which, when inhibited, would display an enhanced response to the BRAF inhibitor Vermurafenib; this would indicate that the gene’s normal function is to provide resistance against the BRAF inhibitor. The best candidate from the shRNA screen was the Hippo signaling pathway effector YAP1. Based on this result, the authors hypothesized that YAP inhibition may increase the efficacy of RAF-targeted therapy.
Independent shRNA knockdown of YAP1 in HCC364 (NSCLC cell line with BRAF V600E allele) cells resulted in increased sensitivity of these cells to both Vermurafenib and the MEK inhibitor Trametinib. Stable overexpression of YAP1 in HCC364 cells resulted in substantially decreased sensitivity to both Vermurafenib and Trametinib, which confirmed the initial screening results.
To determine whether the YAP suppression effect was only limited to BRAF V600E forms of cancer, the authors silenced the YAP1 gene in Cal-12T human NSCLC cells which have normal MEK-ERK activation but encodes a BRAF with a G466V (glycine to valine) substitution. This cell line also displayed enhanced efficacy to Vermurafenib and Trametinib, which shows that YAP1 suppression can be a target to counteract RAF and MEK inhibitor resistance in human NSCLC.
Next, the authors wanted to investigate whether YAP1 regulates the response against inhibition of BRAF signaling in BRAF in other BRAF mutation-driven cancers such as human melanoma, colon, and thyroid cancer. YAP1 suppression increased the efficacy of both Vermurafenib and Trametinib in A2058/WM793 human melanoma cell lines, HT29/WiDr colon cancer cell lines, and KHM-5M and HTC-C3 thyroid cancer cell lines. This indicates that YAP1 suppression can be broadly used as a target in a wide spectrum of BRAF mutant cancers.
RAS mutant cancers work partly through MEK-ERK signalling. There is no effective therapy against this type of cancer, with MEK inhibitors providing variable results. The authors determined that YAP1 suppression also enhanced the sensitivity of multiple KRAS and NRAS mutant human NSCLC, melanoma, and pancreatic adenocarcinoma cell lines to Trametinib. This indicates that YAP1 suppression in conjunction with MEK inhibitors could be an effective therapeutic strategy in patients with mutant RAS.
The authors then wanted to determine the mechanism by which YAP regulates the response against RAF- and MEK-targeted therapy. Previous work had indicated that YAP regulates apoptosis; the authors postulated that YAP might regulate the expression of an anti-apoptotic factor such as BCL-xL to provide resistance against RAF and MEK inhibitors. YAP1 suppression indeed led to a decrease in BCL-xL levels; overexpression of BCL-xL rescued the effect of YAP1 silencing on the response to RAF and MEK inhibitors in HCC364 cells. Pharmacological BCL-xL inhibition using the drug Navitoclax and Tw37 led to increased efficacy of treatment with RAF and MEK inhibitors in several NSCLC, melanoma, and colon cancer models. Overall, these data indicate that YAP, via BCL-xL, acts as a signaling mechanism alongside the MAPK signaling pathway to promote resistance to RAF and MEK inhibitors. This research may lead to a new therapeutic strategy to enhance the efficacy of RAF and MEK inhibitors for patients suffering from a broad range of BRAF-and RAS-mutant cancers.
Lin, L., Sabnis, A.J, Chan, E… Bivona, T.G. (2015). The Hippo effector YAP promotes resistance to RAF-and MEK-targeted cancer therapies. Nature Genetics, 47, 250-256. doi:10.1038/ng.3218