Expression of enzymes of covalent protein modification during regulated and dysregulated proliferation of mammary epithelial cells: PKA, PKC and NMT

The role of N-myristoyltransferase 1 in tumour development

N-myristoyltransferase 1 (NMT1) is an indispensable eukaryotic enzyme that catalyses the transfer of myristoyl groups to the amino acid terminal residues of numerous proteins. This catalytic process is required for the growth and development of many eukaryotes and viruses. Elevated expression and activity of NMT1 is observed to varying degrees in a variety of tumour types (e.g. colon, lung and breast tumours). Furthermore, an elevated level of NMT1 in tumours is associated with poor survival. Therefore, a relationship exists between NMT1 and tumours. In this review, we discuss the underlying mechanisms by which NMT1 is associated with tumour development from the perspective of oncogene signalling, involvement in cellular metabolism, and endoplasmic reticulum stress. Several NMT inhibitors used in cancer treatment are introduced. The review will provide some directions for future research.Key MessagesElevated expression and activity of NMT1 is observed to varying degrees in a variety of tumour types which creates the possibility of targeting NMT1 in tumours.NMT1-mediated myristoylation plays a pivotal role in cancer cell metabolism and may be particularly relevant to cancer metastasis and drug resistance. These insights can be used to direct potential therapeutic avenues for NMT1 inhibitors.

Discovery of Novel Myristic Acid Derivatives as N-Myristoyltransferase Inhibitors: Design, Synthesis, Analysis, Computational Studies and Antifungal Activity

In recent years, N-Myristoyltransferase (NMT) has been identified as a new target for the treatment of fungal infections. It is observed that at present, there are increased rates of morbidity and mortality due to fungal infections. Hence, a series of novel myristic acid derivatives were designed via molecular docking studies and ADMET studies by targeting NMT (N-Myristoyltransferase). The designed myristic acid derivatives were synthesized by converting myristic acid into myristoyl chloride and coupling it with aryl amines to yield corresponding myristic acid derivatives. The compounds were purified and characterized via FTIR, NMR and HRMS spectral analyses. In this study, we carried out a target NMT inhibition assay. In the NMT screening assay results, the compounds 3u, 3m and 3t showed better inhibition compared to the other myristic acid derivatives. In an in vitro antifungal evaluation, the myristic acid derivatives were assessed against Candida albicans and Aspergillus niger strains by determining their minimal inhibitory concentrations (MIC₅₀). The compounds 3u, 3k, 3r and 3t displayed superior antifungal capabilities against Candida albicans, and the compounds 3u, 3m and 3r displayed superior antifungal capabilities against Aspergillus niger compared to the standard drug FLZ (fluconazole). Altogether, we identified a new series of antifungal agents.

Protein myristoylation in health and disease

N-myristoylation is the attachment of a 14-carbon fatty acid, myristate, onto the N-terminal glycine residue of target proteins, catalysed by N-myristoyltransferase (NMT), a ubiquitous and essential enzyme in eukaryotes. Many of the target proteins of NMT are crucial components of signalling pathways, and myristoylation typically promotes membrane binding that is essential for proper protein localisation or biological function. NMT is a validated therapeutic target in opportunistic infections of humans by fungi or parasitic protozoa. Additionally, NMT is implicated in carcinogenesis, particularly colon cancer, where there is evidence for its upregulation in the early stages of tumour formation. However, the study of myristoylation in all organisms has until recently been hindered by a lack of techniques for detection and identification of myristoylated proteins. Here we introduce the chemistry and biology of N-myristoylation and NMT, and discuss new developments in chemical proteomic technologies that are meeting the challenge of studying this important co-translational modification in living systems.

Angiotensin II activates extracellular signal regulated kinases via protein kinase C and epidermal growth factor receptor in breast cancer cells

Angiotensin II (Ang II) induces, through AT1, intracellular Ca(2+) increase in both normal and cancerous breast cells in primary culture (Greco et al., 2002 Cell Calcium 2:1-10). We here show that Ang II stimulated, in a dose-dependent manner, the 24 h-proliferation of breast cancer cells in primary culture, induced translocation of protein kinase C (PKC)-alpha, -beta1/2, and delta (but not -epsilon, -eta, -theta, -zeta, and -iota), and phosphorylated extracellular-regulated kinases 1 and 2 (ERK1/2). The proliferative effects of Ang II were blocked by the AT1 antagonist, losartan. Also epidermal growth factor (EGF) had mitogenic effects on serum-starved breast cancer cells since induced cell proliferation after 24 h and phosphorylation of ERK1/2. The Ang II-induced proliferation of breast cancer cells was reduced by (a) Gö6976, an inhibitor of conventional PKC-alpha and -beta1, (b) AG1478, an inhibitor of the tyrosine kinase of the EGF receptor (EGFR), and (c) downregulation of 1,2-diacylglycerol-sensitive PKCs achieved by phorbol 12-myristate 13-acetate (PMA). A complete inhibition of the Ang II-induced cell proliferation was achieved using the inhibitor of the mitogen activated protein kinase kinase (MAPKK or MEK), PD098059, or using Gö6976 together with AG1478. These results indicate that in human primary cultured breast cancer cells AT1 regulates mitogenic signaling pathways by two simultaneous mechanisms, one involving conventional PKCs and the other EGFR transactivation.

Targeting of PKA in mammary epithelial cells. Mechanisms and functional consequences

Targeting of protein kinases, promoting association with specific partner-molecules and localisation to particular sites within the cell, has come to be recognised as a key mechanism for attributing specificity to these enzymes. In mammary epithelial cells, the repertoire of acute regulatory roles played by cyclic AMP-dependent protein kinase (PKA) differs from that in other lipogenic cell-types. Furthermore, PKA is implicated in the regulation of mammary-specific function, mediating a tonic stimulation of the flux of newly-synthesised casein through its basal secretory pathway. Both these observations imply mammary-specific properties of either PKA targeting systems or of PKA itself. Evidence for the latter is currently lacking. Pulse-chase labelling experiments in the presence and absence of selective effectors of PKA have enabled the site(s) of action of this protein kinase on casein secretion to be localised to the early stages of the secretory pathway. Possible mechanisms are considered for the physical targeting of PKA to the membrane-enclosed components of the secretory pathway and evidence for their occurrence in mammary epithelial cells is presented.