Background: Engineered cell therapies expressing a chimeric antigen receptor (CAR) in human T cells have demonstrated great promise for the treatment of hematological malignancies. However, unique challenges remain in translating the success of CAR T cell therapies to solid tumors, namely: (1) specificity: there are few truly unique antigens present on solid tumors, leading to off-target effects that damage normal tissue and pose safety risks; and (2) suppression: solid tumors develop microenvironments that are immunosuppressive and decrease T cell survival and tumor-directed cytotoxicity. To address these challenges and improve the safety and potency of solid tumor cell therapy, we engineered cells with synthetic genetic circuits containing a previously validated hypoxia biosensor (HBS) that responds to upregulation of the native HIF1α and HIF2α transcription factors by hypoxia, enabling the cells to sense and respond to the hypoxic tumor microenvironment ubiquitous among solid tumors. Methods: We evaluated CAR expression from circuit architectures containing the HBS promoter driving expression of both CAR and native HIF1α, HIF2α, as well as from circuit architectures containing the HBS promoter and our toolkit of synthetic transcription factors and promoters. Circuits were integrated into HEK293 cells via PiggyBac transposon vector to generate stable cell lines. Cells were subsequently cultured in normoxia (21% O₂) and hypoxia (1% O₂), and harvested for flow cytometric analysis to evaluate CAR surface expression across a three day time frame. Results: We identified novel circuits providing fast reporter output with minimal background signal under hypoxic conditions, and elucidated design rules that enable or restrict amplification of transgene in native HIF1α and HIF2α-containing circuits. We observed differences in CAR transgene expression in hypoxia between circuits containing HIF1α and HIF2α-mediated feedback, suggesting mechanistic differences in regulation of HIF1α and HIF2α when overexpressed. Additionally, we observed greatly amplified transgene expression in circuit topologies containing feedback mediated by our synthetic transcription factor and promoter toolkit compared to native HIF1α and HIF2α-containing circuits and cells constitutively expressing CAR. Conclusions: We present a new therapeutic technology that senses hypoxic environments and enables high-output sense and respond behaviors with therapeutic outputs. We envision that this technology will enable development of safer, more effective cell therapies for solid tumors, as well as provide new biological and mechanistic insights that can inform the future development of hypoxia-responsive cell therapies.