In the present research, we showed that both LAT1 and 4F2hc were over-expressed in human brain gliomas; however, both of them were only negligibly expressed in adjacent normal brain tissues, suggesting that LAT1 and 4F2hc co-expression is closely associated with the formation and development of gliomas. These results were almost in agreement with previous reports [17, 18]. Although some previous reports have shown that LAT1 and 4F2hc are co-expressed in brain tissue vascular endothelia of mouse, rat, and cattle [20–22], to date, there is no research showing that LAT1 and 4F2hc are co-expressed in vascular endothelia of human normal tissues or human tumor tissues. To the best of our knowledge, this is the first report demonstrating that both LAT1 and 4F2hc were expressed in vascular endothelia of human brain gliomas, indicating that the LAT1/4F2hc complex plays an important role in large neutral amino acids permeating the blood-brain barrier (BBB) in human brain gliomas. In our previous research with less glioma samples, although the LAT1 immunoreactivity was clear in vascular endothelia in glioma tissues, the 4F2hc immunoreactivity was not obvious . One possible reason was that the 4F2hc immunoreactivity in glioma cells was intensive, so that the 4F2hc immunoreactivity in vascular endothelia was hidden. In this study, although both LAT1 and 4F2hc were expressed in both tumor cells and vascular endothelia of glioma tissues, and there was a significant association between LAT1 expression and 4F2hc expression, the expression level and location of LAT1 and 4F2hc was not completely parallel, i.e. in general, the 4F2hc immunostaining was more intensive than LAT1 immunostaining in glioma tissues, and moreover, LAT1 immunoreactivity was markedly stronger than 4F2hc immunoreactivity in vascular endothelia. On the contrary, 4F2hc immunoreactivity was markedly stronger than LAT1 immunoreactivity in tumor cells, and these results suggest there are different expression regulation mechanisms for LAT1 and 4F2hc [1, 2, 5].
In the present study, LAT1 but 4F2hc expression levels significantly correlated with the glioma pathological grade, although they were obviously higher in high grade gliomas than in low grade gliomas. Moreover, both LAT1 and 4F2hc expression levels were strongly associated with Ki-67 LI of glioma tissues. These results indicate that LAT1/4F2hc may play an important role in the malignant proliferation and progression of high-grade gliomas [17, 18]. The up-regulated expression of LAT1/4F2hc may benefit by providing tumor cells with essential amino acids for high levels of protein synthesis associated with cell activation or hormonal stimulation and also to support rapid growth or excessive proliferation [1, 2]. In light of both LAT1 and 4F2hc expression in vascular endothelia, it is reasonable to further analyze whether there is a certain correlation between LAT1/4F2hc expression and angiogenesis. We demonstrated here, also for the first time, that the LAT1 expression level was significantly associated with MVD, although unlike that for 4F2hc expression, in human brain gliomas. These results indicate that LAT1 may play a key role in the neovascularization of gliomas. Therefore, it is involved in multiple aspects for the mechanism of LAT1 promoting the growth and proliferation of glioma cells, that is to say, LAT1 can transport essential amino acids not only from the extracellular fluid to intracellular pool, but also from the tumor microvessel to extracellular matrix [20–22].
In this series of glioma specimens, Ki-67 LI markedly correlated with the glioma pathological grade, and Ki-67 LI in high grade glioma tissues was significantly higher than that in low grade glioma tissues, suggesting that tumor cell proliferative activity obviously enhances with the increase in the glioma pathological grade, which was in agreement with the previous reports . However, there was no statistically significant correlation between the MVD and glioma pathological grade, although MVD in high grade glioma tissues was slightly higher than that in low grade glioma tissues, and if grade I pilocytic astrocytomas are excluded, there was an increasing tendency of MVD with the glioma pathological grade ascending, suggesting that the pilocytic astrocytoma is a very specific glioma subtype with better prognosis, the lowest pathological grade and very low Ki-67 LI, but abundant neovascularization; also indicating Ki-67 LI may more objectively reflect the malignant degree of gliomas compared with MVD. Interestingly, the LAT1 expression level significantly correlated with not only Ki-67 LI but also MVD in this series of glioma specimens, perhaps indicating that the LAT1 expression level can display more biological features of gliomas, and LAT1 may become a novel valuable molecular marker for gliomas.
According to our findings, LAT1/4F2hc was over-expressed in human brain gliomas, and their over-expression was associated with the malignant progression and excessive proliferation of gliomas; furthermore, LAT1 may also play some roles in glioma angiogenesis. Combining the previous related reports that LAT1 over-expression predicts short survival of glioma patients , blocking the activity of LAT1 with a system L selective inhibitor can markedly inhibit the growth of glioma cells and induce considerable apoptosis of glioma cells , as well as the expression distribution features of LAT1/4F2hc whereby LAT1 is only definitely expressed in few human normal tissues or organs including the placenta, testis and bone marrow, whereas 4F2hc is ubiquitously expressed in various human normal tissues or organs . We consider that LAT1 may become a better potential molecular target for glioma biological therapy compared with 4F2hc.