Different small-molecule inhibitors are being tested against HSP27. Quercetin—a member of the flavonoid group of polyphenols—has exhibited potent anticancer activity against primary colon cancer cells by binding to HSP27 and inhibiting its activity
[32]. Nevertheless, no current clinical trials are testing quercetin’s effectiveness on humans
[14]. In addition, YangZheng XiaoJi—a Chinese anticancer compound—can also inhibit HSP27 phosphorylation in different cancers, including CRC
[33], by inhibiting HSP27 localization with caspase_9; the HSP27 function is inhibited in cancer cells through the inhibition of phosphorylation or its colocalization with caspase-9
[33]. Additionally, ovatodiolide (Ova) is a small-molecule inhibitor isolated from Anisomeles indica that has a potent anticancer stem cell (anti-CSC) effect on different cancers, including breast cancer and CRC
[34][35]. In breast cancer cells, Ova reduced HSP27 expression to suppress tumor growth
[35]. Although Ova has an anti-CSC effect on CRC cells, this effect was identified with a different mechanism in breast cancer
[34]. Further studies are recommended to study its effect on HSP27 in CRC cells. Another naturally occurring anticancer agent is curcumin, the effect of which on CRC cells was tested after silencing HSP27. Interestingly, CRC cells lacking HSP27 exhibited resistance to curcumin treatment and, thus, reduced apoptosis; therefore, this study suggested that HSP27 is a potential target for curcumin in CRC
[36]. A quinone-based pentacyclic derivative (3
S,3′
R) spiro[(hexahydropyrrolo[1,2-
a]pyrazine-1,4-dione)-6,3′-(2′,3′-dihydrothieno[2,3-
b]naphtho-4′,9′-dione)] (DTNQ-Pro) is a novel synthetic anti-cancer agent with broad-spectrum activity on different types of cancers, including CRC. Unlike other therapies, DTNQ-Pro did not reduce HSP27 expression, but caused its redistribution inside cancer cells to the cytoplasm compared to the perinuclear HSP27 in control cells
[37].
The second inhibition approach focuses on monoclonal antibodies (mainly cetuximab), which block epidermal growth factor receptor (EGFR) activity
[38]. Cetuximab sensitizes CRC cells to CPT-11—a chemotherapy drug—by suppressing HSP27 activity by targeting the Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway. Interestingly, cetuximab could suppress HSP27 even in RAS- or BRAF-mutated cells considered resistant to cetuximab therapy. These findings may offer novel strategies for overcoming resistance to cetuximab in RAS- and BRAF-mutated CRC cells.
The third approach utilizes aptamers that can bind to HSP27 and inhibit its dimerization. The most well-known aptamers are PA11 and PA50, which bind to HSP27 oligomers and inhibit their tumorigenic effect
[30]. The effect of these aptamers has only been studied in cancers other than CRC, including prostate cancer, small cell lung cancer (SCLC), and head and neck squamous cell carcinoma
[14][30]. The effect of aptamer on CRC requires further study.
The final approach that was recently identified involves the antisense oligonucleotide (ASO). The ASO OGX427 has been shown to inhibit the mRNA expression of HSP27
[30]. ASO approaches have been extensively studied in patients with prostate, bladder, ovarian, breast, and non-small cell lung cancers
[30]. In CRC, OGX427 activity was tested on the SW480 cell line to study the inhibition of gap junction intercellular communication (GJIC) formation mediated by HSP27. Notably, inhibition of HSP27 by OGX427 abolished the formation of GJIC and therefore altered the interaction of the CRC cell line with endothelial cells
[39].
1.4. HSP60 Inhibitors
5. HSP60 Inhibitors
Targeting various HSPs for cancer chemotherapy has recently gained attention due to their significant role in CRC progression; however, few studies have targeted HSP60 activity and its co-chaperone HSP10 for CRC treatment due to their controversial role in different cancers. Nevertheless, there are two resources for HSP60 inhibitors—natural and synthetic—and, mechanistically, these inhibitors interact with ATP binding pockets or a specific cysteine residue in Hsp60
[40]. Various natural inhibitor compounds were tested against HSP60, including mizoribine, epolactaene, and myrtucommulone A (MC). Mizoribine was reported to inhibit the ATPase activity of HSP60, while epolactaene bound to the Cys442 residue of HSP60 to inhibit its activity
[40]. Unfortunately, few synthetic compounds were able to target HSP60 activity, one of which was o-carboranylphenoxyacetanilide, but none of the above-mentioned inhibitors were tested on CRC cell lines. Overall, these studies have provided a better understanding of these inhibitors’ bioactivity and have therefore paved the way for testing on CRC.
Recently, another 24 different inhibitors were tested against HSP60 activity in CRC and showed significant inhibition of CRC cells compared to normal cells. Interestingly, the effect on CRC cell viability of those inhibitors was associated with the inhibition of expression of HSP60, thus, indicating that HSP60 might be a target for these inhibitors
[41].
1.5. HSP110 Inhibitors
6. HSP110 Inhibitors
Only limited studies have focused on the role of HSP110 in CRC, and little research has targeted its activity. Gozzi and his colleagues identified two abiotic foldamers, 33 and 52, using chemical library screening to inhibit HSP110 functioning by targeting its NBD. These inhibitors were able to reduce CRC cell growth, as confirmed by an in vivo model. This study will open opportunities for discovering more molecules to target HSP110 for CRC treatment
[42].