Depleting HIF-1α attenuates the progression of osteosarcoma, but tumorigenicity is sustained through HIF-independent pathways

Poor survival of patients with osteosarcoma means that novel therapeutic targets are needed. A previously developed osteosarcoma mouse model revealed that HIF-1 target genes are upregulated under anchorage-independent growth conditions. HIF-1α is highly expressed at the hypoxic invasion front in vivo. Knockout of HIF-1α attenuates cell growth under hypoxic and non-adherent conditions in vitro, as well as growth of primary and metastatic osteosarcoma in C57BL/6 mice, suggesting key roles for HIF-1α in osteosarcoma progression. However, tumors with a rich vasculature develop in the absence of HIF-1α. Thus, the HIF-independent survival pathways on which HIF-KO clones depend needs to be identified. The present study revealed that expression of glycolysis-related genes, which are targets of HIF, decreased in HIF-KO clones, but the sensitivity of each clone to inhibitors varied: Some were less sensitive than HIF wild-type cells under hypoxic conditions. Compound screening revealed that the pathways upon which KO clones depend for survival differ. Indeed, inhibiting the mitochondrial electron transport chain, PI3K or mTOR further reduced growth of KO clones under hypoxic conditions, although one clone was less sensitive to these treatments and retained high proliferation capacity under hypoxic conditions. This clone was extremely sensitive to inhibition of the mevalonate synthesis pathway, suggesting that this might be the mechanism underlying resistance to HIF-targeted therapies. Thus, although HIF-1 is an attractive therapeutic target for osteosarcoma, it is necessary to identify and inhibit heterogenous HIF-independent pathways upon which individual tumor cells rely.