Osteosarcoma (OS) is the most common primary malignant bone tumour in humans and dogs. Although medicine has made dramatic progress in treating osteosarcoma by surgery, with chemotherapy given before and after surgery, drug resistance and highly metastatic spread are often responsible for the failure of current therapies. Thus, more effective therapeutic approaches for treating osteosarcoma are needed. Previous results from our laboratory and others had shown that the basic-leucine zipper (bLZip) containing transcription factor, Zhangfei/CREBZF is a potent inhibitor of a variety of other transcription factors and has a dramatic effect on the growth of several cancer cell lines, including dog OS and human medulloblastoma cells. The objective of the studies described in this thesis was to determine the molecular mechanisms by which Zhangfei exerts its effect on dog and human OS cells. Several stressors in the microenvironment of cancer cells directly or indirectly perturb the endoplasmic reticulum (ER), which then activates the Unfolded Protein Response (UPR). The UPR modulates the effects of stress and allows tumours to survive, develop, metastasize and escape therapy. The UPR is regulated by three bLZip transcription factors—ATF6, ATF4 and Xbp1s. Since Zhangfei inhibits Luman/CREB3, a bLZip structurally similar to and closely related to ATF6 and ATF4, I initially focused my efforts on this pathway. I hypothesized that Zhangfei interacts with UPR-related bLZip transcription factors and inhibits their ability to activate the UPR signaling pathways, thereby suppressing the growth of cancer cells and increasing their susceptibility to ER stressors. To test this hypothesis, we monitored cell growth as well as levels of UPR gene transcripts and proteins in several dog and human osteosarcoma cell lines infected with adenovirus vectors expressing Zhangfei, and studied the interactions between Zhangfei and the UPR-mediator, Xbp1s. The results showed that the ectopic expression of Zhangfei in cell lines derived from dog osteosarcomas potently suppressed cell growth and inhibited their ability to activate the UPR. Further studies demonstrated that Zhangfei inhibited the UPR, at least partially, by binding to Xbp1s and suppressing its ability to activate transcription from a promoter containing unfolded protein response elements (UPRE). The leucine zipper of Zhangfei was required for this interaction, which led to the subsequent proteasomal degradation of Xbp1s. However, we also found that the effects of Zhangfei were not universal. While Zhangfei had a profound effect on the growth and UPR in some OS cell lines, it either had only a partial effect, or no effect on others. This suggested that susceptibility (or resistance) to Zhangfei may be an inherent property of OS cell lines. Since the suppressive effects of Zhangfei were not universal, and it had no obvious effects on untransformed cells and some cancer cell lines, I proposed that Zhangfei mediates its effect on cell growth and the UPR through an intermediary that is either not induced or is defective in cells that are unaffected by Zhangfei. I found that this intermediary was the tumour suppressor protein p53. The inhibitory effects of Zhangfei were only observed in the wild-type p53 expressing OS cell line U2OS while Zhangfei had no effect on the p53-null OS cell line MG63. In cells with functional p53, the ectopic expression of Zhangfei caused it to displace the ubiquitin ligase mdm2 and stabilize p53. Suppression of p53 by siRNA partially inhibited the effects of Zhangfei on the UPR and cell growth. In contrast, OS cells lacking functional p53 could be made to respond to Zhangfei if they were transfected to express wild-type p53. These results explain why Zhangfei has a profound effect on some cancer cells while having no obvious effect on others. I also characterized the interaction of Zhangfei and p53 by mapping the interacting domains on both proteins, showing that the bLZip domain of Zhangfei and the N-terminal transactivation domain (NTD) of p53 were required for their interactions. My findings reveal the profoundly inhibitory effects of Zhangfei on OS growth and the UPR, a stress-response known to promote tumour survival. I also show how Zhangfei may exert its effects. My work suggests an alternative modality for the therapy of certain types of OS, and perhaps other tumours with functional p53.

The unfolded protein response (UPR) is a well conserved mechanism in eukaryotes that protects organisms from the damaging effects of endoplasmic reticulum (ER) stresses. Activation of the UPR will lead to two outcomes. It first attempts to restore cellular functions by enhancing protein folding capacity, inhibiting protein synthesis and promoting degradation of harmful proteins (the pro-survival pathway). However, if the stressful conditions are prolonged or severe, apoptosis will be induced (the pro-apoptotic pathway). The present study suggests that an isoform of the cellular protein Zhangfei (ZF, CREBZF) is linked to the pro-apoptotic pathway in the UPR by using DNA, protein and cell viability analyses. This isoform is known as the short-tail ZF (stZF). We demonstrated that stZF can be induced by prolonged ER stress. The protein of stZF is stable under ER stress and it has the ability to promote programmed cell death in the early stage of apoptosis through the induction of CHOP, a protein that plays a key role in the pro-apoptotic pathway of the UPR.