O-GlcNAcylation of proteins regulates important cellular processes. A few reports noted that O-GlcNAcylation exhibits cross-talk with tyrosine phosphorylation. With an activity-based microarray analysis of 256 tyrosine kinase peptide substrates, we found that phosphorylation of six peptides by Jak2 inhibits their subsequent O-GlcNAcylation. However, O-GlcNAcylation has no detectable effect on their subsequent phosphorylation. A specific peptide (ZO3_357_371), derived from the ZO-3 protein, was studied in detail. Kinetic results show that the presence of a phosphate at Tyr364 of ZO3_357_371 slows the O-GlcNAcylation of nearby Ser369, while the presence of a GlcNAc at Ser369 has no significant effect on the phosphorylation of this peptide at Tyr364. These findings provide a glimpse into the new paradigm for cellular signaling control by cross-talk.

O-GlcNAcylation is a reversible and dynamic protein post-translational modification in mammalian cells. The O-GlcNAc cycle is catalyzed by O-GlcNAc transferase (OGT) and O-GlcNAcase (OGA). O-GlcNAcylation plays important role in many vital cellular events including transcription, cell cycle regulation, stress response and protein degradation, and altered O-GlcNAcylation has long been implicated in cancer, diabetes and neurodegenerative diseases. Recently, numerous approaches have been developed to identify OGT substrates and study their function, but there is still a strong demand for highly efficient techniques. Here we demonstrated the utility of the peptide microarray approach to discover novel OGT substrates and study its specificity. Interestingly, the protein RBL-2, which is a key regulator of entry into cell division and may function as a tumor suppressor, was identified as a substrate for three isoforms of OGT. Using peptide Ala scanning, we found Ser 420 is one possible O-GlcNAc site in RBL-2. Moreover, substitution of Ser 420, on its own, inhibited OGT activity, raising the possibility of mechanism-based development for selective OGT inhibitors. This approach will prove useful for both discovery of novel OGT substrates and studying OGT specificity.

O-GlcNAcylation is a post-translational modification resulting from the addition of an N-acetylglucosamine moiety to the hydroxyl groups of serine and threonine residues of nuclear and cytoplasmic proteins. In addition, O-GlcNAcylated proteins can be phosphorylated, which suggests the possibility for crosstalk between O-GlcNAcylation and phosphorylation. Dysregulation of O-GlcNAcylation affects cell signaling, transcriptional regulation, cell cycle control and can e.g. lead to tumorigenesis and tumor metastasis. There is a strong demand for efficient analytical techniques to better detect and investigate this abundant modification and its role in cancer. Herein we demonstrated the utility of an O-GlcNAcylated peptide array to examine O-GlcNAcase (OGA) activity and substrate specificity of both purified protein as well cell lysates of different cancer cell lines. Using this microarray, we clearly observed OGA activity and also inhibition thereof by OGA inhibitor thiamet G. Interestingly, different levels of OGA activity were observed of lysates derived from different cancer cell lines. This suggests that the tool may be useful in cancer research and biomarker development.

O-GlcNAcylation, like phosphorylation, is a dynamic and rapid posttranslational modification which regulates many cellular processes. Phosphorylation on tyrosine and O-GlcNAcylation on nearby serine or threonine residues may modulate each other. Indeed, by using a microarray with a peptide model system based on the ZO-3 protein, extensive cross talk between O-GlcNAcylation by OGT and phosphorylation by kinases was observed. However, studying the effects of kinases and OGT without the reverse processes catalyzed by phosphatases and O-GlcNAcase (OGA) does not provide a complete picture of the cross talk. The study of the missing part showed that nearby phosphorylation affects the de-O-GlcNAcylation by OGA, but not to the same extent as it affects the O-GlcNAcylation by OGT. Both the phosphorylation and de-phosphorylation processes were only slightly affected by the presence of an O-GlcNAc residue on a nearby serine.

This thesis includes two parts. Part I, ‘Development of a peptide microarray method to study post-translational modifications’ and part II, ‘Toxicity of galectin-9 toward KRAS mutated CRC cells: pathway elucidation’. Part I aimed to study the posttranslational modifications (PTMs), O-GlcNAcylation/ de- O-GlcNAcylation, phosphorylation/ dephosphorylation, crosstalk between de-O-GlcNAcylation and dephosphorylation and finally O-GlcNAcylation of JAK2 JH1 kinase using short peptide substrates derived from their parental proteins. Part II of this thesis, ‘Toxicity of galectin-9 toward KRAS mutated CRC cells: pathway elucidation’, aimed to profile the STK and PTK activity of galectin-9 sensitive DLD-1 cancer cells using peptide microarrays. The goal was to identify the kinase pathways which are involved upon galectin-9 treatment and includes two chapters.