Plant-derived chemotherapeutic agents are gaining attention in oncology[13, 14]. Solanine, a type of steroidal alkaloid, consists of glucose residues and lycopene, with the molecular formula of C45 H73 Ol5 N (Fig. 6), and is found in Solanum nigrum L., tomatoes, and other nightshade plants, particularly in immature fruits and potato buds. It possesses anti-inflammatory, antiviral, antibacterial, and antitumor activities[15]. Studies have shown that solanine have strong inhibitory effects on human breast cancer MCF-7[16], pancreatic cancer PANC-1 cells[12], hepatocellular carcinoma HepG2 cells[17], lung cancer A549 cells[18], prostate cancer PC-3 cells[11], cervical cancer HeLa cells[19], colon cancer HT-29 cells[20], rectal cancer HCT116 and SW480 CRC cells[21], etc. The inhibitory effects are as follows: affecting the structure and function of the cell membranes; affecting the immune function of erythrocytes; inducing apoptosis of tumor cells; inhibiting tumor angiogenesis. Scholars have studied its anti-tumor action mechanism. For instance, Sun HW et al. found that the inhibitory effect of solanine on pancreatic cancer cells was concentration-time dependent by different concentrations of solanine acting on pancreatic cancer SW1990 and Panc-1 cell lines, and the apoptosis rate detected by CCK-8[12]. Similarly, Mohsenikia et al. demonstrated that alpha-solanine significantly inhibited breast cancer cell proliferation and induced apoptosis, as well as inhibited angiogenesis in breast cancer-bearing mice[10]. Lu MK found that a-solanine inhibited the invasive migration of melanoma A2058 cells, and the inhibitory effect may be achieved by decreasing the expression of MMP-2/9, inhibiting the JNK and PI3K/Akt signaling pathways, and NF-κB activity[22].
For investigating the effect of solanine on the proliferation inhibition of melanoma cells, CCK-8 assay was used to assess the effects of different concentrations of solanine on the viability of A375 and A2058 cells. As shown in Fig. 1, the proliferation inhibitory effect of solanine on melanoma was time-concentration dependent. Fluorescence microscopy after DAPI staining revealed that the nuclei of the control group were uniform in size and uniformly distributed, while the experimental group showed typical morphological changes of apoptotic cell nuclei such as nuclear consolidation, nuclear border clustering and the formation of apoptotic vesicles (Fig. 3). The change was more obvious with the increase of concentration. Thus, it is proved that the apoptosis of melanoma A375 cells can be induced by solanine, and the apoptosis phenomenon is more obvious with the increase of solanine concentration.
Currently, two classical pathways for apoptosis are recognized: the mitochondrial pathway (endogenous pathway) and the death receptor pathway (exogenous pathway)[23]. The death receptor pathway is triggered by triggering the cell-surface death receptor ligand to bind to the cell-surface receptor, and its pathways are mainly Fas/FasL, TRAIL, and TNFR[24], which are interconnected with each other, and the crossroads of the different pathways are manifested in the eventual activation of Caspase-8, which initiates the cascade reaction of caspases and perpetuates apoptosis. The mitochondrial pathway is triggered by the activation of a series of apoptosis-stimulating factors within the mitochondria. Among them, the Bcl-2 family is currently the gene family that has received the most attention in the regulation of apoptosis[25]. The members of the Bcl-2 family can be functionally categorized into pro-apoptotic (e.g., Bax) and anti-apoptotic (e.g., Bcl-2) factors[26]. Many anticancer drugs trigger the mitochondria-mediated apoptosis pathway by down-regulating the expression of Bcl-2 and up-regulating Bax, causing apoptosis in cancer cells. Bcl-2/Bax can form homo- or heterodimers, and the apoptosis program is initiated based on the ratio of Bcl-2/Bax to Bax[27]. Normally, the Bcl-2/Bax ratio is fixed, but it can change when cells are subjected to external stimuli. When Bcl-2 expression is enhanced, Bax and Bcl-2 form a heterodimer, and Bcl-2 binds to the mitochondrial membrane to inhibit the release of cytochrome C by inhibiting the opening of the MPTP channel of the mitochondrial membrane, thus preventing the activation of caspase and exerting anti-apoptotic effects. Conversely, enhanced Bax expression can form homodimers to promote apoptosis. Therefore, Bcl-2/Bax is a key factor in apoptosis[28]. In the present study, we provided evidences that solanine induced apoptosis by increasing mitochondrial membrane permeability and initiating mitochondrial signaling pathways, such as RT-qPCR and Western Blot in human melanoma cells, suggesting that within a certain range, as the concentration of solanine increased, Bax increased, Bcl-2 decreased, and Bcl-2/Bax decreased. The Caspase family is a family of cysteine proteases, which is composed of a series of inactive zymogens that are central to the process of apoptosis[29]. Among them, Caspase-3, as an important member of the Caspase family[30], is a key execution and effector molecule of apoptosis[31]. Here, we demonstrated that caspase-3 expression was up-regulated with increasing concentrations of solanine after acting different concentrations of solanine on melanoma cells for 24 hours. According to the results, we suggested that solanine down-regulates Bcl-2/Bax, which then forms a homodimer, leading to the opening of MPTP channels in the inner mitochondrial membrane, releasing Cytc to activate the caspase family and promote apoptosis.
Taken together, solanine induces melanoma cell apoptosis by mediating the mitochondrial apoptotic pathway. These findings offer a potential mechanistic insight into solanine's anticancer effects, although further studies, particularly in vivo, are necessary to validate these results and explore the therapeutic potential of solanine in melanoma treatment.