PAK5 (p21 activated kinase 5) is upregulated in human colorectal carcinoma cells and is a known tumor promoter in carcinogenesis of the colon. Little is known regarding the mechanisms underlying the downstream targets of PAK5, and information concerning its biological significance in glioma is lacking. In this study, we investigated the effects of PAK5 on proliferation, migration, invasion, and apoptosis in human U87 and U251 glioma cells and examined the underlying molecular mechanism. We performed cell growth assays and cell cycle analysis to observe the cell proliferation. Flow cytometry analysis was performed to evaluate apoptosis, and in vitro scratch assays, cell migration assays, and gelatin zymography were performed to examine cell migration. Western blot analysis was performed to examine signal transduction in the cells. We demonstrated that suppression of PAK5 in glioma cells significantly inhibited cell migration and invasion. We also observed that suppression of PAK5 in human glioma cell lines inhibited cell growth because of G1 phase arrest. Additionally, flow cytometry and Western blot analysis indicated that PAK5 could inhibit cell apoptosis. These results suggest that the PAK5-Egr1-MMP2 signaling pathway is involved in tumor progression and may have a potential role in cancer prevention and treatment.

p21-activated kinases (PAKs) are activated by various extracellular stimuli and, in turn, activate other kinases by phosphorylating them at specific serine/threonine residues or through protein-protein interaction. As a recently identified member of the group B PAK family, the role of PAK5 in cancer is poorly understood. In this study, we investigated the effect of PAK5 on the malignant phenotype, such as proliferation, cell cycle, apoptosis, migration, and invasion. Cell growth assay and cell cycle analysis consistently showed that knockdown of PAK5 could significantly inhibit the proliferation of breast cancer cells. Wound healing assay. migration assay, and invasion assay showed that PAK5 promoted cell migration. Furthermore, in order to elucidate the underlying mechanism of PAK5 on cellular growth and migration, we examined the protein expressions of cyclin D1, p21, early growth response protein 1 (Egr1), and matrix metalloproteinase 2 (MMP2). Our work further reveals the PAK5-Egr1-MMP2 signaling pathway to be a critical regulator of cell migration and invasion. These results suggest that PAK5 may be a potential therapeutic target for breast cancer.

CSN6 is one subunit of the constitutive photomorphogenesis 9 (COP9) signalosome (CSN), which is an evolutionarily conserved multiprotein complex found in plants and animals and originally described as a repressor of light-dependent growth and transcription in Arabidopsis. CSN is homologous to the 19S lid subcomplex of the 26S proteasome, thus it has been postulated to be a regulator of the ubiquitin-proteasome pathway. In mammalian cells, it consists of eight subunits (CSN1-CSN8). Among the CSN subunits, CSN5 and CSN6 are the only two that each contains an MPN (Mpr1p and Pad1p N-terminal) domain. The deneddylating activity of an MPN domain toward cullin-RING ubiquitin ligases (CRL) may coordinate CRL-mediated ubiquitination activity. More and more studies about CSN6 are emerging, and its overexpression is found in many types of cancers. Evidence has shown that CSN6 is a molecule platform between protein degradation and signal transduction. Here, we provide a summary of human CSN6, especially its roles in cancer, hoping that it can lay the groundwork for cancer prevention or therapy.

In order to investigate the inhibitory effects and mechanisms of troglitazone (TGZ), a peroxisome proliferator-activated receptor γ (PPARγ) agonist, and retinoid X receptor (RXR) agonist (9-cis-retinoic acid (RA)) on gastric carcinoma cells SGC7901, SGC7901 cells were treated with TGZ and 9-cis-RA, respectively, or in combination. Then, the cell growth, apoptosis, morphological changes, and the expression of PPARγ, RXRγ, Bcl-2, and Bax were detected by MTT assay, flow cytometry, HE staining, immunocytochemistry staining, and Western blot assay, respectively. Our results showed that the growth of SGC7901 cells was inhibited and the cells got sparser at the concentrations of 50 μmol/L TGZ, 20 μmol/L 9-cis-RA, or combination of TGZ (25 μmol/L) and 9-cis-RA (10 μmol/L). Immunocytochemistry and Western blot showed that after 72 h, the expression of PPARγ, RXRγ, and Bax were upregulated; Bcl-2 was downregulated compared with the negative control group. These data indicated that PPARγ agonist and RXR agonist could inhibit the proliferation of SGC7901 cells via inducing the apoptosis, which involved the increase in the level of Bax/Bcl-2. The combination of RXR agonist and PPARγ agonist could induce the maximal inhibitory effects on tumor growth and apoptosis via promoting the formation of RXR/PPARγ heterodimer.

Gamma-aminobutyric acid (GABA), an inhibitory neurotransmitter in central nervous system, has yet been found to widely exist in tumor tissues to regulate tumor cells growth. However, the function of GABA on inducing tumor cells apoptosis and the potential mechanism are still unclear. In order to detect whether GABA via GABAB receptor GABABR1 would activate c-Jun N-terminal kinases (JNKs) to promote tumor cells apoptosis, co-immunoprecipitation assay was used to investigate the association of β-arrestins with GABABR1 and JNKs in the different four cancer cell lines. Our observation demonstrated that β-arrestins, in addition to their role in G protein-coupled receptors desensitization, had an additional function as adapter proteins to recruit JNKs to GABABR1, thereby conferring distinct enzymatic activities upon the receptor, which may trigger JNKs signal pathway involved in the regulation of cellular growth. Activated JNKs subsequently phosphorylated downstream c-Jun to transcribe a wide variety of pro-apoptotic genes. Additionally, GABA up-regulated the ratio of pro-apoptotic protein Bax to anti-apoptotic protein Bcl-2, and thus facilitated caspase-3 cleavage, leading to tumor cells apoptosis in a mitochondrial-dependent pathway. In contrast, GABABR antagonist CGP35348 reversed GABA-induced JNKs phosphorylation and its downstream proteins activation, which consequently restrained tumor cells apoptosis. Taken together, our study suggested that GABA via its receptor GABABR1 recruited β-arrestins to facilitate the activation of JNKs cascade, resulting in tumor cells growth inhibition.