Efficient Tandem LysC/Trypsin Digestion in Detergent Conditions.

All shotgun proteomics experiments rely on efficient proteolysis steps for sensitive peptide/protein identification and quantification. Previous reports suggest that the sequential tandem LysC/trypsin digest yields higher recovery of fully tryptic peptides than single-tryptic proteolysis. Based on the previous studies, it is assumed that the advantageous effect of tandem proteolysis requires a high sample denaturation state for the initial LysC digest. Therefore, to date, all systematic assessments of LysC/trypsin proteolysis are done in chaotropic environments such as urea. Here, sole trypsin is compared with LysC/trypsin and it is shown that tandem digestion can be carried with high efficiency in Mass Spectrometry-compatible detergents, thereby resulting in higher quantitative yields of fully cleaved peptides. It is further demonstrated that higher cleavage efficiency of tandem digests has a positive impact on absolute protein quantification using intensity-based absolute quantification (iBAQ) values. The results of the examination of divergent urea tandem conditions imply that beneficial effects of the initial LysC digest do not depend on the sample denaturation state, but, are mainly caused by different target specificities of LysC and trypsin. The observed detergent compatibility enables tandem digestion schemes to be implemented in efficient cellular solubilization proteomics procedures without the need for buffer exchange to chaotropic environments.

Deficiency in the LIM-only protein FHL2 impairs assembly of extracellular matrix proteins.

We have described the scaffolding protein FHL2 as a component of focal adhesion structures, to which it is recruited via binding to both alpha- or beta-integrin subunits. Using mesenchymal stem cells from wild-type and FHL2-knockout mice, we show here that inactivation of FHL2 leads to impaired assembly of extracellular matrix proteins on the cell surface and to impaired bundling of focal adhesions. Both altered properties can be restored by reexpression of recombinant FHL2 protein in FHL2-null cells. Molecular analysis of integrin-mediated signaling revealed a higher phosphorylation of FAK at tyrosine 925 in FHL2-knockout cells compared to their wild-type counterpart. Consequently, the activation of the mitogenic kinase ERK was more pronounced in knockout cells on cell adhesion. The growth factor-induced activation of ERK, however, was not altered. The perturbed organization of extracellular matrix on FHL2-null cells was improved when the increased activation of MAPK was inhibited. Our findings point to a role of FHL2 in bundling of focal adhesion structures, in integrin-mediated ERK activation, and subsequently in proper allocation of matrix proteins on the cell surface.

FHL2 regulates cell cycle-dependent and doxorubicin-induced p21Cip1/Waf1 expression in breast cancer cells.

The transcriptional cofactor FHL2 interacts with a broad variety of transcription factors and its expression is often deregulated in various types of cancer. Here we analyzed for the first time the molecular function of FHL2 in breast cancer. FHL2 is overexpressed in almost all human mammary carcinoma samples tested but not in normal breast tissues and only low levels of FHL2 expression were present in four premalignant ductal carcinoma in situ (DCIS). Cell cycle analysis revealed an upregulation of endogenous FHL2 towards G2/M in MDA-MB 231 cells and an accelerated G2/M transition when FHL2 expression was suppressed in these cells. In search for G2/M specific target genes regulated by FHL2, we found that expression of the cell cycle inhibitor p21Cip1/Waf1 (hereafter p21) is dependent on FHL2 in MDA-MB 231 breast cancer cells. Downregulation of FHL2 by shRNA abrogated the cell cycle dependent upregulation of p21 as well as the induction of p21 in response to treatment with the DNA damaging agent doxorubicin. FHL2-dependent p21 expression occurs in a p53-independent manner and p21 expression can be downregulated by specific inhibition of mitogen-activated protein kinases (MAPKs), implicating an involvement of MAPK signaling in this regulation. Analysis of FHL2 contribution to the MAPK signaling identified FHL2 as an important downstream effector of MAPKs in breast cancer cells, capable of transactivating endogenous AP1 target genes as well as AP1 dependent reporter genes. Finally, downregulation of FHL2 reduces the ability of MDA-MB 231 cells to form colonies in soft agar, while FHL2 overexpression enhances colony formation of breast cancer cells. Thus, our findings indicate that overexpression of the transcriptional cofactor FHL2 contributes to breast cancer development by mediating transcriptional activation of MAPK target genes known to be involved in cancer progression, such as p21.

The biological relevance of FHL2 in tumour cells and its role as a putative cancer target.

The LIM-only protein FHL2 (four-and-a-half LIM-domain protein 2) belongs to the FHL protein family of transcriptional cofactors present in various cell lines. FHL2 interacts with a variety of transcription factors known to be involved in tumour development. Furthermore, FHL2 expression is often deregulated in cancer including overexpression and down-regulation in various types of tumours. The function of FHL2 in cancer is particularly intriguing, since it may act as an oncoprotein or as a tumour suppressor in a tissue-dependent fashion. This dual nature of FHL2 is also reflected by the finding that it can function as repressor or activator of transcriptional activity depending on the cell-type. The ability of FHL2 to exert functional diversity lies within its structural composition as a LIM-only protein. LIM-domains are enzymatically inactive protein-to-protein interaction domains, which determine the function of LIM-only proteins as adaptor molecules or scaffolding proteins. By selectively using different LIM-domains for protein-to-protein interactions, FHL2 is capable to interact with a broad spectrum of functionally unrelated proteins, thereby triggering different signalling pathways. In this review, the current knowledge of FHL2 expression in different cancers was summarized and the interaction of FHL2 with transcription factors and other proteins involved in cancer development was examined. Since transcription factors control all fundamental developmental and homeostatic processes, transcriptional cofactors like FHL2 are likely to contribute to human carcinogenesis and are of clinical importance in various forms of cancer.

Polo-like kinase 1: target and regulator of transcriptional control.

Deregulated cell cycle control is a hallmark of cancer cells. Developmental or other mitogenic stimuli activate the proliferation of normal cells in response to the requirements of growing tissues. In contrast, cancer cells liberate from proliferative restrictions exerted by anti-proliferative signals arising from the stroma and by endogenous genetic programs that correlate to the terminal differentiation of cells. The study of cyclin-dependent kinases (Cdks) and polo-like kinases (Plks) as evolutionary conserved regulators of the cell cycle has contributed significantly to our current understanding of the mechanisms that underlie the proliferation of mammalian cells. Given the importance of Plk1 for mitotic progression the temporal expression of Plk1 is crucial and has to be tightly regulated. It is known that steady-state Plk1 mRNA and protein levels are coordinately regulated during cell cycle progression, being low during interphase but high in mitosis. This review will summarize the current knowledge on how cell cycle-dependent transcriptional regulation of the Plk1 gene is achieved. While binding sites for various transcriptional activators are dispersed throughout the entire Plk1 promoter region, the cell cycle-dependent regulation of the Plk1 gene expression seems to be regulated by G1-specific repression rather than by G2/M-specific activation of the Plk1 transcriptional unit. The tumor suppressor gene p53 was identified as a key player in the precise restriction of Plk1 gene expression to the G2/M phase. The activity of p53 is in turn controlled by Plk1 itself indicating the existence of an auto-regulatory mechanism involved in the cell cycle-dependent regulation of the Plk1 gene. Furthermore, transcription factors regulated by Plk1 will also be subject of discussion.

Polo-like kinase 1-mediated phosphorylation stabilizes Pin1 by inhibiting its ubiquitination in human cells.

The Polo-like kinase 1 (Plk1) is a key regulator of mitosis. It is reported that the human peptidyl-prolyl cis/trans-isomerase Pin1 binds to Plk1 from mitotic cell extracts in vitro. Here we demonstrate that Ser-65 in Pin1 is the major site for Plk1-specific phosphorylation, and the polo-box domain of Plk1 is required for this phosphorylation. Interestingly, the phosphorylation of Pin1 by Plk1 does not affect its isomerase activity but rather is linked to its protein stability. Pin1 is ubiquitinated in HeLa S3 cells, and substitution of Glu for Ser-65 reduces the ubiquitination of Pin1. Furthermore, inhibition of Plk1 activity by expression of a dominant negative form of Plk1 or by transfection of small interfering RNA targeted to Plk1 enhances the ubiquitination of Pin1 and subsequently reduces the amount of Pin1 in human cancer cells. Since previous reports suggested that Plk1 is a substrate of Pin1, our work adds a new dimension to this interaction of two important mitotic regulators.

The LIM-only protein FHL2 interacts with beta-catenin and promotes differentiation of mouse myoblasts.

FHL2 is a LIM-domain protein expressed in myoblasts but down-regulated in malignant rhabdomyosarcoma cells, suggesting an important role of FHL2 in muscle development. To investigate the importance of FHL2 during myoblast differentiation, we performed a yeast two-hybrid screen using a cDNA library derived from myoblasts induced for differentiation. We identified beta-catenin as a novel interaction partner of FHL2 and confirmed the specificity of association by direct in vitro binding tests and coimmunoprecipitation assays from cell lysates. Deletion analysis of both proteins revealed that the NH2-terminal part of beta-catenin is sufficient for binding in yeast, but addition of the first armadillo repeat is necessary for binding FHL2 in mammalian cells, whereas the presence of all four LIM domains of FHL2 is needed for the interaction. Expression of FHL2 counteracts beta-catenin-mediated activation of a TCF/LEF-dependent reporter gene in a dose-dependent and muscle cell-specific manner. After injection into Xenopus embryos, FHL2 inhibited the beta-catenin-induced axis duplication. C2C12 mouse myoblasts stably expressing FHL2 show increased myogenic differentiation reflected by accelerated myotube formation and expression of muscle-specific proteins. These data imply that FHL2 is a muscle-specific repressor of LEF/TCF target genes and promotes myogenic differentiation by interacting with beta-catenin.