Activating mutations in the RAC1 gene have recently been discovered as driver events in malignant melanoma. Expression of this gene is associated with melanocyte proliferation, and melanoma cells bearing this mutation are insensitive to BRAF inhibitors such as vemurafenib and dabrafenib, and also may evade immune surveillance due to enhanced expression of PD-L1. Activating mutations in RAC1 are of special interest, as small molecule inhibitors for the RAC effector p21-activated kinase (PAK) are in late-stage clinical development and might impede oncogenic signaling from mutant RAC1. In this work, we explore the effects of PAK inhibition on RAC1P29S signaling in zebrafish embryonic development, in the proliferation, survival, and motility of RAC1P29S-mutant human melanoma cells, and on tumor formation and progression from such cells in mice. We report that RAC1P29S evokes a Rasopathy-like phenotype on zebrafish development that can be blocked by inhibitors of PAK or MEK. We also found and that RAC1 mutant human melanoma cells are resistant to clinical inhibitors of BRAF but are uniquely sensitive to PAK inhibitors. These data suggest that suppressing the PAK pathway might be of therapeutic benefit in this type of melanoma.

RHO GTPases, members of the RAS superfamily of small GTPases, are adhesion and growth-factor activated molecular switches that play important roles in tumor development and progression. When activated, RHO-family GTPases such as RAC1, CDC42, and RHOA, transmit signals by recruiting a variety of effector proteins, including the protein kinases PAK, ACK, MLK, MRCK, and ROCK. Genetically-induced loss of RHO function impedes transformation by a number of oncogenic stimuli, leading to an interest in developing small molecule inhibitors that either target RHO GTPases directly, or that target their downstream protein kinase effectors. Although inhibitors of RHO GTPases and their downstream signaling kinases have not yet been widely adopted for clinical use, their potential value as cancer therapeutics continues to facilitate pharmaceutical research and development and is a promising therapeutic strategy.

Key Points Bone marrow-specific deletion of Pak2 is associated with macrothrombocytopenia and abnormal megakaryocyte morphology and function. Pak2 deletion is associated with defects in megakaryocyte endomitosis and the activation of Aurora-A and LIM kinase.

p21-activated kinases (PAKs) are Cdc42/Rac–activated serine-threonine protein kinases that regulate of several key cancer-relevant signaling pathways, such as the Mek/Erk, PI3K/Akt, and Wnt/b-catenin signaling pathways. Pak1 is frequently overexpressed and/or hyperactivated in different human cancers, including human breast, ovary, prostate, and brain cancer, due to amplification of the PAK1 gene in an 11q13 amplicon. Genetic or pharmacological inactivation of Pak1 has been shown to reduce proliferation of different cancer cells in vitro and reduce tumor progression in vivo. In this work, we examined the roles of Pak1 in cellular and animal models of PAK1-amplified ovarian cancer. We found that inhibition of Pak1 leads to decreased proliferation and migration in PAK1 amplified/overexpressed ovarian cancer cells, and has no effect in cell that lack such amplification/overexpression. Further, we observed that loss of Pak1 function causes 11q13 amplified ovarian cancer cells to arrest in the G2/M phase of the cell cycle. This arrest correlates with activation of p53 and p21Cip and decreased expression of cyclin B1. These findings suggest that small molecule inhibitors of Pak1 may play a therapeutic role in the ~25% of ovarian cancers characterized by PAK1 gene amplification.

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Summary The protein kinases Mst1 and Mst2 have tumor suppressor activity, but their mode of regulation is not well established. Mst1 and Mst2 are broadly expressed and may have certain overlapping functions in mammals, as deletions of both Mst1 and Mst2 together are required for tumorigenesis in mouse models [1–3]. These kinases act via a three-component signaling cascade comprising Mst1/2, the protein kinase Lats1/2, and the transcriptional coactivators Yap and Taz [4–6]. Mst1/2 contain C-terminal SARAH domains that mediate their homodimerization as well as heterodimerization with other SARAH-domain containing proteins, which may regulate Mst1/2 activity. Here, we show that, in addition to forming homodimers, Mst1 and Mst2 heterodimerize in cells, that this interaction is mediated by their SARAH domains and is favored over homodimers, and that these heterodimers have much reduced protein kinase activity compared to Mst1 or Mst2 homodimers. Mst1/Mst2 heterodimerization is strongly promoted by oncogenic H-ras, and this effect requires activation of the Erk pathway. Cells lacking Mst1, in which Mst1/Mst2 heterodimers are not possible, are resistant to H-ras-mediated transformation and maintain active hippo pathway signaling compared to wild-type cells or cells lacking both Mst1 and Mst2. Our results suggest that H-ras, via an Erk-dependent mechanism, down-regulates Mst1/2 activity by inducing the formation of inactive Mst1/Mst2 heterodimers.

p21-activated kinases (Paks) play an important role in oncogenic signaling pathways and have been considered as potential therapeutic targets in various cancers. Most studies of Pak function employ gene knock-out or knock-down methods, but these approaches result in loss of both enzymatic and scaffolding properties of these proteins, and thus may not reflect the effects of small molecule inhibitors. Here we use a transgenic mouse model in which a specific peptide inhibitor of Group I Paks is conditionally expressed in response to Cre recombinase. Using this model, we show that inhibition of endogenous Paks impedes the transition of adenoma to carcinoma in an Apc-driven mouse model of colorectal cancer. These effects are mediated by inhibition of Wnt signaling through reduced β-catenin activity as well as suppression of an epithelial-mesenchymal transition program mediated by miR-200 and Snai1. These results highlight the potential therapeutic role of Pak1 inhibitors in colorectal cancer.

Background:-Glucuronyl C5-epimerase (GLCE) is a key enzyme involved in the biosynthesis of heparan sulphate proteoglycans, which has an important role in cell–cell and cell–matrix interactions and signalling. Decreased GLCE expression in human breast tumours and its anti-proliferative effects in breast cancer cells suggest that it may be a candidate tumour-suppressor gene. The aim of this study was to investigate the involvement of GLCE in lung carcinogenesis.Methods:-Glucuronyl C5-epimerase expression in different lung cancer cell lines was determined and the gene was ectopically re-expressed in U2020 small-cell lung cancer cells. Cellular proliferation in vitro and tumour growth in vivo were then examined.Results:Ectopic re-expression of GLCE in U2020 cells did not affect cell viability but did influence morphology. Cellular proliferation in vitro and tumour formation in vivo were both suppressed. These effects were mediated via downregulation of several pro-angiogenic growth factors and their receptors, including VEGF-A, TGFB1, FGFR2, PDGF-A and PDGF-B, and TNFa and its receptors. Expression of matrix metalloproteinase2, MTA1, PLAU, TIMP3, S100A4, SERPINE1 and TWIST1 was also downregulated.Conclusion:The anti-tumour effects associated with ectopic GLCE re-expression suggest that it may be a potential tumour-suppressor gene and a possible target for lung cancer diagnosis and treatment.