The Serine/Threonine Protein Kinase subfamily includes AGC kinases and Microtubule-associated serine/threonine kinase (MASTL) or Great wall kinase (GWL) is a member of the AGC family of kinases. The huge number of proteins that AGC kinases can phosphorylate demonstrates their importance in a variety of cellular activities. Serine /threonine kinases are essential regulators of cell adhesion and contraction, which is important for cancer growth and metastasis [1]. Many human disorders, including cancer, are caused by mutations or dysregulation of AGC kinases, which mediates a wide range of critical cellular processes [2].
MASTL is a unique AGC kinase that lacks a hydrophobic motif, unlike most AGC kinases, despite the presence of a hydrophobic pocket that specifies its particular method of regulation [3]. It features a unique T-loop region with insertion of roughly 500 amino acids. It has received far less attention as compared to other AGC kinases. Furthermore, in immortalized normal breast epithelial cells, overexpression of MASTL slows cell cycle progression, causes abnormal cell division, DNA damage response, affects migration, the actin cytoskeleton, and cell–cell junctions, tumor resistance in response to anticancer treatments and leads to enhanced invasion and metastasis in vitro and in vivo, leading to cancer development [4]. Moreover, MASTL triggered mitotic cell death in a variety of cancer cells while normal cells were less affected [5]. It is known to be a key player in the cell division, growth, metabolism, and differentiation. It accelerates the cell cycle progression by phosphorylating Endosulfine Alpha (ENSA) and Arpp19, which limits PP2A-B55 phosphatase activity and hence maintains the phosphorylated status of Cyclin-dependent kinase 1 (CDK1) substrates [6]. MASTL is phosphorylated during mitosis which is an essential requirement for its activation [7,8]. MASTL inhibition of PP2A-B55 is essential to sustain the mitotic state during cell division, whereas MASTL inactivation and PP2A reactivation is required for mitotic departure [9]. In vitro and in vivo studies in breast cancer found that inhibiting MASTL reduced tumour development and metastasis. These findings suggest that MASTL is a new breast cancer oncogene that may overcome contact inhibition, invasion, and chromosomal instability [10]. Overall, the evidence revealed that MASTL can be a promising target for selective anticancer treatment [11].
Research have focused on the role of natural origin drugs in anticancer treatment due to the limited number of adverse effects and the wide range of targets of naturally-derived components [12]. Till date, a few studies have reported MASTL inhibitors like GK-1, MKI-1, MKI-2 and a thieno-pyrimidinone based tricyclic derivative. These have been identified and validated using virtual screening and in-vitro analysis GKI-1 inhibited MASTL in vitro and interrupted mitotic events by lowering phosphorylated ENSA with µM range potency in HeLa cells. Moreover, in other breast cancer cells, GKI-1 exhibited negligible anti-cancer effect [13]. In in vitro and in vivo models of breast cancer, MKI-1 was found to have anticancer and radiosensitizer properties. MKI-1 demonstrated µM range potency and efficacy for MASTL inhibition. In addition, MKI-1 reduced the amount of c-Myc protein in breast cancer cells through increasing PP2A activity [14]. Recently, it was found that MKI-2 decreased recombinant MASTL activity and induced mitotic catastrophe in breast cancer cells through modulating the MASTL-PP2A axis. In mouse oocytes that were employed as a model to validate MKI-2 activity, the MKI-2 treatment displayed phenocopies with MASTL-null oocytes. MKI-2 inhibited MASTL in breast cancer cells with potency and effectiveness in the nM range [15]. All these identified MASTL inhibitors are synthetic and their toxicity profiles and long-term effects are yet to be studied. Our group has also previously identified various compounds from both natural and synthetic sources, ZINC85597499 and ZINC53845290 using virtual screening that proved to be significant leads for further experimental validation [16].
The present study is focused on the identification of new MASTL inhibitors from natural sources. We have identified Enzastaurin and Palbociclib as MASTL kinase inhibitors using in-vitro kinase assay. Moreover, these compounds also demonstrated an anti-proliferative effect on breast cancer cells. Both the compounds exhibited potency and inhibition efficacy in the µM range (1.56µM to 50µM). In addition, we investigated the influence on cell-cycle progression and apoptosis as an anti-proliferative effect was observed in MCF-7 cells treated with these natural products. Cell cycle study revealed that the Enzastaurin and Palbociclib are capable of arresting the cells in G2/M and G1 phases respectively. Thus, in the present study we have identified naturally derived compounds Enzastaurin and Palbociclib as novel MASTL kinase inhibitors with significant antitumor effect as potential therapeutic leads against breast cancer that can further be validated in animal models.