The mTOR pathway has been widely studied and many of its protein members are acknowledged as the therapeutic target. The mTORC1 is one of the most extensively studied one and is referred to as the most potential therapeutic target for cancer as it has been found to be deregulated in almost all of the cancer hallmarks [Mahoney et al. 2018]. The mTORC1 inactivation by mTOR kinase inhibition using small inhibitors is one of the basic approaches to prevent these hallmarks.
Various mTOR inhibitors of mTOR have been designed so far, including Rapamycin and its derivatives (rapalogs). All of these have been found to have very serious side effects of the and also inhibit the activities of the mTORC2 complex which may lead to improper cellular functions of healthy cells [Mahoney et al. 2018]. Hence, the mTOR-RHEB binding interruption using peptides could be a good approach to inactivate the mTORC1. Also, anticancer peptide therapy has gained momentum in the last two decades as the peptides can be designed to target any protein. The main rationale behind it is that the peptides can be designed by utilizing the amino acid chains or the helices from the interacting domain. Also, these peptides can be synthesized very easily and modified accordingly using some biological and chemical techniques.
The most important thing required to proceed with this work was the knowledge about the mTOR-RHEB interface which was priorly also determined by Yang et al in 2017. According to Yang and colleagues, the RHEB binds to the mTOR kinase at HEAT domains and FAT domain. The Switch I (residues 33D-41N) and Switch II (residues 63G-79N) along with residues 5K-7R and 106M-111Q of the RHEB GTPase binds at the residues 60S-157G of N-Heat domain, residues 966H-1020V of M-Heat domain and residues 1277K-1307A of FAT domain of the mTOR kinase. But this information was not residue-specific and thus insufficient to proceed with. Hence, the Ligplot predicted residue-specific interactions between the mTOR and the RHEB were preceded with. The results from the Ligplot matched the findings from Yang's work. According to Yang et al, the major interactions between the mTOR and the RHEB are formed by the N-Heat of the mTOR and the Switch II of the RHEB [Yang et al. 2017].
Further peptides were to be designed to dock with the RHEB GTPase and ultimately use them for therapeutic uses. The two rationals which were used for this peptide designing were: one based on the merging of structural and combinatorial chemistry technology and the second one is based on mutations and evolutionary changes.
The latter has a problem with its originality and precision as it uses the Genetic Algorithms for predicting the evolutionary changes in the given protein sequence [Pirogova et al. 2011]. In this study, the mutations and the evolutionary changes in the mTOR kinase within vertebrates were recorded. Surprisingly, most of the evolutionary mutations in the mTOR kinase lay outside the mTOR-RHEB interface and nearly 40 evolutionary changes in the residues were noted at the interface as well (Fig. 5).
As shown in figure A) All of the natural and the experimental mutations of the mTOR kinase laid away from the mTOR-RHEB interface. So, they can’t participate in the peptide designing process. B) Most of the residues which show evolutionary changes in different vertebrates lay in the interfacial region of the mTOR and the RHEB. So, changes related to these residues can be helpful during peptide designing for the peptide enhancement process. The identified evolutionary changes in residues of the mTOR kinase were induced as mutations in the native form of peptides which resulted in new peptides. As expected, the newly designed peptides were even more effective and showed a better binding with the RHEB.
The targeted therapies have been a new generation in cancer-preventing drugs and this sentence fits in. This approach aims at interfering with a specific molecule (mainly protein) that plays a significant role in tumorigenesis or cancer progression. Detailed knowledge about the molecular changes involved in cancer initiation and progression helps in selecting these targets [Sawyers 2004]. The prior knowledge about mTORC1 signaling and the role of RHEB in mTORC1 signaling has been utilized very significantly. Mahoney and colleagues in 2018, selected the RHEB GTPase as their target for interrupting the mTORC1 signaling using a small molecule (NR1) which successfully interrupted the mTORC1 signaling by binding to the Switch II residues of the RHEB. [Mahoney et al. 2018].
Similarly, this work also presents the RHEB as a possible target for halting the cascade process of the mTORC1 but in a very different yet effective manner i.e. using anticancer peptides. The use of anticancer peptides has been increasing day by day, one such example of exploring peptides as anticancer therapy is the work performed by Matthew Pincus. His group worked on the Ras, a member of the GTPase family, to inhibit its interactions with various partners. In another study on Ras, Chung et al, tried to inhibit oocyte maturation using peptides from the interface of its interacting partners [Bidwell III and Raucher 2009]. This shows that the use of peptides to inhibit some signaling pathway or the process holds a strong position and hence provides a strong base to the current work.
Although, the utility of the peptides to treat cancerous conditions is limited due to their poor pharmacokinetics so far. During in-vivo applications of the peptides, these are degraded by the action of various proteases in the serum. Also, their relatively bigger size and charged nature make them impermeable to the cell membrane. Yet, the recent advances in drug delivery e.g. use of macromolecules to deliver peptides to the target site and the retro-D-inverso approach of peptide designing have made it easy to overcome such problems [Bidwell III and Raucher 2009].
Also, selecting the RHEB as a target molecule for mTORC1 inactivation is not a bad idea, as previously successful works have been done on the same.