Preston Adhikari

Introduction. Glioblastoma (GBM), accounting for almost 80% of all malignant brain tumors, is one of the most aggressive and lethal tumors with a median survival time of only 12-15 months, and a five-year survival rate close to zero^1,4. Due to the physiological and pathological properties of GBM, the efficacies of standard treatment strategies, primarily temozolomide (TMZ) treatment following surgical resection, are limited, and all patients inevitably relapse^1,4. Recent evidence indicates that tumor cells in GBM upregulate molecules like programmed cell death 1 (PD-L1) that function as immune checkpoints, which may contribute to host immune evasion mechanisms and further tumor proliferation^2,4,5. These findings suggest that immune checkpoint inhibitor therapies should be explored as potential novel therapeutic strategies for GBM. Methods. GBM cells (U87 and U251) were treated with TMZ (200 uM for U87 and 50 uM for 251) for 0-72 hours. The cells were then lysed by RIPA lysis buffer and immunoblotting analysis was performed with PD-L1 antibodies^2,5. Additional GBM cells were also treated with 200 uM TMZ for 0-48 hours, followed by WB analysis using STAT3pY705, STAT3, HIF-1, c-Myc and Tubulin antibodies². Results. There was a significant increase in PD-L1 expression on GBM cells after TMZ treatment. TMZ elevated mRNA levels of CD274, encoding PD-L1, after TMZ challenge in a gradual and time dependent manner². PD-L1 protein levels were also evaluated in TMZ-stimulated U87 and U251 cells, which demonstrated that PD-L1 expression was increased with time. Activation of several transcription factors were investigated to see if they could up-regulate CD274 mRNA levels and only STAT3pY705 was highly increased after TMZ treatment². A STAT3 inhibitor was used to validate whether STAT3 was involved in TMZ-induced PD-L1 expression. Use of inhibitor not only blocked phosphorylation of tyrosine 705 in STAT3 but also ameliorated induction of PD-L1². This demonstrated that TMZ can trigger STAT3 activation and STAT3 acts as a key transcription factor to upregulate PD-L1 expression. Conclusions. It has been demonstrated that TMZ triggers STAT3 which causes increased PD-L1 expression^2,3. There are several consequences of this increased PD-L1 expression, including reduction of key pro-inflammatory cytokines, further TMZ resistance, and restored cell proliferation ability of treatment sensitive GBM cells despite therapy². Therefore, inhibition of this STAT3 pathway is an attractive target to bypass treatment resistance for regulating PD-L1 expression in cancer cells and should be explored further in glioblastoma^2,3.

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