Evasion of immune system is a hallmark of cancer, which enables cancer cells to escape the attack from immune cells. Cancer cells can express many immune inhibitory signalling proteins to cause immune cell dysfunction and apoptosis. One of these inhibitory molecules is programmed death-ligand-1 (PD-L1), which binds to programmed death-1 (PD-1) expressed on T-cells, B-cells, dendritic cells and natural killer T-cells to suppress anti-cancer immunity. Therefore, anti-PD-L1 and anti-PD-1 antibodies have been used for the treatment of cancer, showing promising outcomes. However, only a proportion of patients respond to the treatments. Further understanding of the regulation of PD-L1 expression could be helpful for the improvement of anti-PD-L1 and anti-PD-1 treatments. Studies have shown that PD-L1 expression is regulated by signalling pathways, transcriptional factors and epigenetic factors. In this review, we summarise the recent progress of the regulation of PD-L1 expression in cancer cells and propose a regulatory model for unified explanation. Both PI3K and MAPK pathways are involved in PD-L1 regulation but the downstream molecules that control PD-L1 and cell proliferation may differ. Transcriptional factors hypoxia-inducible factor-1α and signal transducer and activation of transcription-3 act on the promoter of PD-L1 to regulate its expression. In addition, microRNAs including miR-570, miR-513, miR-197, miR-34a and miR-200 negatively regulate PD-L1. Clinically, it could increase treatment efficacy of targeted therapy by choosing those molecules that control both PD-L1 expression and cell proliferation.

We previously demonstrated that AKT2, a member of protein kinase B family, is activated by a number of growth factors via Ras and PI 3-kinase signaling pathways. Here, we report the frequent activation of AKT2 in human primary ovarian cancer and induction of apoptosis by inhibition of phosphoinositide-3-OH kinase (PI 3-kinase)/Akt pathway. In vitro AKT2 kinase assay analyses in 91 ovarian cancer specimens revealed elevated levels of AKT2 activity (43-fold) in 33 cases (36.3%). The majority of tumors displaying activated AKT2 were high grade and stages III and IV. Immunostaining and Western blot analyses using a phospho-ser-473 Akt antibody that detects the activated form of AKT2 (AKT2 phosphorylated at serine-474) con®rmed the frequent activation of AKT2 in ovarian cancer specimens. Phosphorylated AKT2 in tumor specimens localized to the cell membrane and cytoplasm but not the nucleus. To address the mechanism of AKT2 activation, we measured in vitro PI 3-kinase activity in 43 ovarian cancer specimens, including the 33 cases displaying elevated AKT2 activation. High levels of PI 3-kinase activity were observed in 20 cases, 15 of which also exhibited AKT2 activation. The remaining ®ve cases displayed elevated AKT1 activation. Among the cases with elevated AKT2, but not PI 3-kinase activity (18 cases), three showed down-regulation of PTEN protein expression. Inhibition of PI 3-kinase/AKT2 by wortmannin or LY294002 induces apoptosis in ovarian cancer cells exhibiting activation of the PI 3-kinase/AKT2 pathway. These ®ndings demonstrate for the ®rst time that activation of AKT2 is a common occurrence in human ovarian cancer and that PI 3-kinase/Akt pathway may be an important target for ovarian cancer intervention.

Past studies have shown that activation of mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) kinase (MEK)/ERK is a common cause for resistance of melanoma cells to death receptor-mediated or mitochondriamediated apoptosis. We report in this study that inhibition of the MEK/ERK pathway also sensitizes melanoma cells to endoplasmic reticulum (ER) stress-induced apoptosis, and this is mediated, at least in part, by caspase-4 activation and is associated with inhibition of the ER chaperon glucoseregulated protein 78 (GRP78) expression. Treatment with the ER stress inducer tunicamycin or thapsigargin did not induce significant apoptosis in the majority of melanoma cell lines, but resistance to these agents was reversed by the MEK inhibitor U0126 or MEK1 small interfering RNA (siRNA). Induction of apoptosis by ER stress when MEK was inhibited was caspase dependent with caspase-4, caspase-9, and caspase-3 being involved. Caspase-4 seemed to be the apical caspase in that caspase-4 activation occurred before activation of caspase-9 and caspase-3 and that inhibition of caspase-4 by a specific inhibitor or siRNA blocked activation of caspase-9 and caspase-3, whereas inhibition of caspase-9 or caspase-3 did not inhibit caspase-4 activation. Moreover, overexpression of Bcl-2 inhibited activation of caspase-9 and caspase-3 but had minimal effect on caspase-4 activation. Inhibition of MEK/ERK also resulted in down-regulation of GRP78, which was physically associated with caspase-4, before and after treatment with tunicamycin or thapsigargin. In addition, siRNA knockdown of GRP78 increased ER stressinduced caspase-4 activation and apoptosis. Taken together, these results seem to have important implications for new treatment strategies in melanoma by combinations of agents that induce ER stress and inhibitors of the MEK/ERK pathway.