Cancer Biology

Many protein families consist of multiple highly homologous proteins, whether they are encoded by different genes or originating from the same genomic location. Predominance of certain isoforms has been linked to various pathological conditions, such as cancer. Detection and relative quantification of protein isoforms in research are commonly done via immunoblotting, immunohistochemistry, or immunofluorescence, where antibodies against an isoform-specific epitope of particular family members are used. However, isoform-specific antibodies are not always available, making it impossible to decipher isoform-specific protein expression patterns. Here, we describe the insertion of the versatile 11 amino acid HiBiT tag into the genomic location of the protein of interest. This tag was developed and is distributed by Promega (Fitchburg, WI, USA). This protocol describes precise and specific protein expression analysis of highly homologous proteins through expression of the HiBiT tag, enabling protein expression quantification when specific antibodies are missing. Protein expression can be analyzed through traditional methods such as western blotting or immunofluorescence, and also in a luciferase binary reporter system, allowing for reliable and fast relative expression quantification using a plate reader.

Graphical overview

The core planar cell polarity (PCP) protein Vang/Vangl, including Vangl1 and Vangl2 in vertebrates, is indispensable during development. Our previous studies showed that the activity of Vangl is tightly controlled by two important posttranslational modifications, ubiquitination and phosphorylation. Vangl is ubiquitinated through an endoplasmic reticulum-associated degradation (ERAD) pathway and is phosphorylated by casein kinase 1 (CK1) in response to Wnt. Here, we present step-by-step procedures to analyze Vangl ubiquitination and phosphorylation, including cell culture, transfection, sample preparation, and signal detection, as well as the use of newly available phospho-specific antibodies to detect Wnt-induced Vangl2 phosphorylation. The protocol described here can be applicable to the analysis of posttranslational modifications of other membrane proteins.

Protein synthesis is one of the most fundamental biological processes to maintain cellular proteostasis. Azidohomoalaine (AHA) is a non-radioactive and “clickable” amino acid analog of methionine which can be incorporated into newly synthesized proteins. Thus, AHA-labeled nascent proteins can be detected and quantified through fluorescent labeling by "click" chemistry. Here we describe a protocol to measure protein synthesis by AHA labeling and flow cytometry. Taking advantage of gating different cell populations, we provide a typical example of the flow cytometric-based analysis of protein synthesis during the cell cycle. While we used mouse B cells in this protocol this method can be readily applied to any cell types and organisms.

Specialized secretory cells known as goblet cells in the intestine and respiratory epithelium are responsible for the secretion of mucins. Mucins are large heavily glycosylated proteins and typically have a molecular mass higher than 10⁶ Da. These large proteins are densely substituted with short glycan chains, which have many important functional roles including determining the hydration and viscoelastic properties of the mucus gel that lines and protects the intestinal epithelium. In this protocol, we comprehensively describe the method for extraction of murine mucus and its analysis by agarose gel electrophoresis. Additionally we describe the use of High Iron Diamine-Alcian Blue, Periodic Acid Schiff’s-Alcian Blue and immune–staining methods to identify and differentiate between the different states of glycosylation on these mucin glycoproteins, in particular with a focus on sulphation and sialylation.

In eukaryotic cells transcriptional processes are carried out by three different RNA polymerases: RNA polymerase I which specifically transcribes ribosomal RNA (rRNA), RNA polymerase II which transcribes protein-coding genes to yield messenger RNAs (mRNAs) and small RNAs, while RNA polymerase III transcribes the genes for transfer RNAs and for the smallest species of ribosomal RNA (5S rRNA). This protocol describes an in vitro assay to evaluate the rRNA transcriptional activity of RNA polymerase I. The method measures the quantity of radiolabelled uridine 5’ triphosphate incorporated in ex novo synthesized rRNA molecules by RNA polymerase I, in optimal conditions for the enzyme activity and in the presence of a toxin, α-amanitin, which inhibits RNA polymerase II and III without affecting RNA polymerase I (Novello and Stirpe, 1970).

Histone acid extraction assay is a popular method to determine histone modification levels in mammalian cells. It includes three steps: first, histones are released from chromatin by sulfuric acid; trichloroacetate (TCA) is then added to precipitate histones; and finally, histones are dissolved in double-distilled H₂O (ddH₂O). Here we present a detailed histone acid extraction assay in our laboratory using a colon cancer cell line, HCT116, as a model.

The migration of membrane receptors upon exposure to different stimulants/inhibitors is of great importance. Among others, the internalization of membrane receptors affects their accessibility to ligands and cell responsiveness to environmental cues. Experimentally, receptor internalization can be used as a measure of their activation. In our studies, we employed this approach to explore cross-talk between a seven transmembrane domain receptor for neuropeptide Y (NPY), Y5R, and a tyrosine kinase receptor for brain-derived neurotrophic factor (BDNF), TrkB. To this end, we measured the internalization of Y5R upon stimulation with the TrkB ligand, BDNF. Upon treatment with BDNF, the cells were exposed to a membrane impermeable, biotinylation reagent that selectively labels surface proteins. Subsequently, the biotinylated membrane proteins were affinity-purified on columns with avidin resins and analyzed by Western blot. Differences in the fraction of receptors present on the cell surface of control and ligand-treated cells served as a measure of their internalization and response to particular stimuli.

Galectin-3 is a member of a class of proteins termed Galectins, characterized by their ability to bind glycans containing β-galactose (Cummings and Liu, 2009). Galectin-3 binds preferentially to proteoglycans terminating with N-acetyllactosamine (LacNAc) chains (i.e., tandem repeats of galactose) (Newlaczyl and Yu, 2011). Galectin-3 is unique among the galectins in its chimeric structure. It shares a conserved carbohydrate recognition domain (CRD) with the other galectins, but has a long amino-terminal tail that is thought to be involved in protein aggregation. It can also form homodimers through its CRD (Cummings and Liu, 2009). Galectin-3 has been found to have diverse functions in tumorigenesis including: signaling, apoptosis inhibition, immune suppression, cell growth, and metastasis among others. Galectin-3 is frequently upregulated in cancers (Nangia-Makker et al., 2008). Its function largely depends on its expression and localization properties (Newlaczyl and Yu, 2011). Because of its many roles in cancer-associated processes, establishing a method for Galectin-3 production is valuable for further study of its functions in cancer. Here, we describe how Galectin-3 purification was achieved by cloning of the human Galectin-3 gene into pGEX-2T vector containing the gene for glutathione-S-transferase (GST) upstream of its cloning site. The Galectin-3 gene was cloned into this vector via restriction digests of both the plasmid and the Galectin-3 gene by restriction enzymes BamHI and EcoRI, followed by ligation of the two fragments. The resulting plasmid was then used to transform BL21, an Escherichia coli (E. coli) strain specialized for protein expression. Finally, we discuss how the GST fusion protein was isolated and the recombinant Galectin-3 protein was further purified from the GST.

Protein-protein interaction networks provide a global picture of cellular function and biological processes, and the dysfunction of some interactions causes many diseases, including cancer. The in situ proximity ligation assay (PLA) is a powerful technology capable of detecting the interactions among proteins in fixed tissue and cell samples. The interaction between two proteins is detected using the corresponding two primary antibodies raised in different species. Species-specific secondary antibodies (PLA probes), each with a unique short DNA strand attached to it, bind to the primary antibodies. When the PLA probes are in close proximity (<40 nm), the DNA strands can interact through a subsequent addition of two other circle-forming DNA oligonucleotides. Several-hundredfold replication of the DNA circle can occur after the amplification reaction, and a fluorescent signal is generated by labelled complementary oligonucleotide probes. Therefore, each detected signal is visualized as an individual fluorescent dot, which can be quantified and assigned to a specific subcellular location based on microscopy images. This revolutionary technique enables us to study the protein complex formation with high specificity and sensitivity compared to the other traditional methods, such as co-immunoprecipitation (Co-IP).

KRAS is the oncogene most frequently mutated in human solid tumors especially in pancreas, colon, small intestine, biliary tract and lung. We have recently demonstrated that oncogenic KRAS needs S181 phosphorylation to fully display its oncogenic features suggesting its inhibition as a therapeutic treatment against KRAS-driven tumors. Due to the importance to detect KRAS phosphorylation in human tumors and the absence of specific antibodies against phosphorylated KRAS, we developed a new protocol based on the Phos-tag SDS methodology to detect this post-translational modification for KRAS. Phos-tag is a molecule that binds specifically to phosphorylated proteins, decreasing their migration speed in SDS-PAGE and allowing its separation from the non-phosphorylated forms.