Previous researchers have reported that the accumulation of LDs drives EMT and the metastatic spreading of tumor cells in SCC, such as cervical, nasopharyngeal, and colon cancers [13, 15, 16]. The present study demonstrated an accumulation of LDs in OPMDs and OSCC tissues compared to the normal oral epithelium. In general, an increasing trend in caveolin-2, perilipin-3, and FABP5 expression was observed from normal to OPMDs to OSCC groups, but the increased expression of perilipin-3 was not significant in OPMDs compared to the normal control group. Instead, the E-cadherin protein positive staining showed a decreasing trend from normal to OPMDs to OSCC groups. Caveolin-2, perilipin-3, and FABP5 expressions were positively correlated with epithelial dysplasia in OPMDs, and independent of histopathological grade in OSCC. In contrast, E-cadherin positivity was negatively correlated with epithelial dysplasia and histopathological grade in OPMDs and OSCC, respectively. Caveolin-2 and FABP5 were negatively correlated with E-cadherin level, and caveolin-2 and perilipin-3 were positively correlated with FABP5 expression. Furthermore, OSCC local recurrence was positively correlated with caveolin-2 and FABP5 expression, and lymph node metastasis was correlated with high FABP5 expression but low E-cadherin expression.
Comparisons with other studies and what does the current work add to the existing knowledge
Lipid metabolism has emerged as a target for cancer management. In the process of cancer development, lipids are ingested and stored in LDs to support cancer cell proliferation, invasion, and metastasis; however, the LDs have not been discussed in precancerous lesions [17]. This study showed increased LDs in both OPMDs and OSCC tissues, indicating the accumulation of LDs as early as the precancerous stage. A previous study showed that perilipin-2, the surface protein of LDs, in CD68+ tumor-associated macrophages in OSCC could predict disease prognosis [18]. The surface of LDs is decorated with various proteins that might play a decisive role in regulating LDs’ function. Caveolin-2 and perilipin-3 are two relatively understudied coat proteins of LDs. Fujimoto et al. suggested that LDs were related to lipid molecule trafficking, mediated by caveolin-2 [11]. It has been reported that a high level of caveolin-2 promoted pancreatic cancer progression and was a poor indicator of prognosis [19]. Zhang et al. studied the expression of the perilipin family in breast cancer, and only the expression of perilipin-3 was found to be up-regulated compared to normal breast tissues [20]. It has also been demonstrated that downregulating perilipin-3 can repress prostate cancer progression, and perilipin-3 can act as a prognostic predictor for renal cancer [21, 22]. The present study found that the caveolin-2 and perilipin-3 expression was positively associated with epithelial dysplasia in OPMDs, independent of histopathological grade in OSCC, indicating that the formation of LDs might mainly occur in the precancerous stage. OSCC local recurrence and worse DFS was associated with high expression of caveolin-2, which might be a prognostic marker in OSSC patients.
As a lipid storage and transportation center, LDs interact with peridroplet mitochondria and function as a lipid regulation system in response to metabolic changes (17). Through β-oxidation in the mitochondria, free fatty acids released from LDs can be utilized as a substrate, generating ATP to fulfill immediate cellular needs. Fatty acid β-oxidation is strongly associated with stem cell properties and is abnormally activated in various cancer cells [23]. An intracellular fatty acid-binding protein, FABP5, is involved in lipid transport and mitochondrial oxidation during lipid metabolism processes [24]. A growing body of research has revealed that increased FABP5 expression is essential for changes in lipid metabolism and the promotion of the spread of cancer [25]. This study suggested that increased FABP5 levels were significantly associated with epithelial dysplasia in OPMDs, independent of histopathological grade in OSCC, indicating that the FABP5-related lipid metabolism might be essential for the progression of precancerous lesions of the oral cavity. In addition, the lipid-related protein caveolin-2 and perilipin-3 staining were positively correlated with FABP5, suggesting that LDs might regulate lipid metabolism. Fang et al. claimed that FABP5 promoted cell proliferation and invasion in OSCC [26]. Consistently, we found that OSCC local recurrence and lymph node metastasis were associated with high expression of FABP5.
Synergistic loss of E-cadherin is a distinguishing characteristic of EMT, which is essential for epithelial dysplasia. The detachment of epithelial cells from one another and the underlying basement membrane occurs due to the reconstruction of cell-to-cell and cell-to-extracellular matrix contacts, promoting mesenchymal transformation [4]. Previous research suggested that E-cadherin could be a potential marker to detect oral diseases with a risk of malignant transformation [5]. Similarly, the current study showed that the E-cadherin protein expression was gradually enhanced in normal, OPMDs, and OSCC tissues and negatively associated with epithelial dysplasia and histopathological grade in OPMDs and OSCC, respectively. A significant association was detected between low E-cadherin expression and lymph node status. Furthermore, activation of the fatty acid β-oxidation pathway was demonstrated to be associated with EMT cellular program [27]. EMT was correlated with FABP5 expression in low-grade gliomas [28]. Moreover, Li et al. reported that caveolin-2 might regulate the Mir-4723/Wnt7A pathway through EMT, speeding up glioma cell proliferation and invasion and pancreatic cancer metastasis [12]. The present study revealed a negative correlation of E-cadherin with caveolin-2 and FABP5 protein expression, but not with the expression of perilipin-3, suggesting that LDs regulating lipid metabolism could be closely related to the process of oral epithelial dysplasia, and caveolin-2 might be the key functional protein.
Strength and limitations
The current study aimed to quantify the accumulation of LDs and the expression of caveolin-2, perilipin-3, FABP5, and E-cadherin in the carcinogenesis processes in the oral cavity. Then, the relationship between the protein expression and clinicopathological characteristics was explored. This is the first study to reveal the function of LDs in precancerous lesions. It explored the relationship between LDs, lipid metabolism, and EMT, providing an important basis for the early prevention and control of oral cancers.
Nevertheless, the specific molecular mechanisms of lipid metabolism regulated by LDs affecting the EMT process are still unclear. Therefore, future research should focus on the role of LDs in mitochondrial β-fatty acid oxidation and the downstream mechanisms of their influence on EMT in oral malignant lesions in vitro and in vivo.