Vaccinia growth factor-dependent modulation of the mTORC1-CAD axis upon nutrient restriction

The molecular mechanisms by which vaccinia virus (VACV), the prototypical member of the poxviridae family, reprograms host cell metabolism remain largely unexplored. Additionally, cells sense and respond to fluctuating nutrient availability, thereby modulating metabolic pathways to ensure cellular homeostasis. Understanding how VACV modulates metabolic pathways in response to nutrient signals is crucial for understanding viral replication mechanisms, with the potential for developing antiviral therapies. In this study, we establish the importance of <i>de novo</i> pyrimidine synthesis during VACV infection. We report the significance of vaccinia growth factor (VGF), a viral early protein and a homolog of cellular epidermal growth factor (EGF), in enabling VACV to phosphorylate the key enzyme CAD of the <i>de novo</i> pyrimidine pathway at serine 1859, a site known to positively regulate CAD activity. Although nutrient-poor conditions typically inhibit mTORC1 activation, VACV activates CAD via the mTORC1-S6K1 signaling axis in a VGF-dependent manner, especially upon glutamine and asparagine limitation. However, unlike its cellular homolog EGF, the VGF peptide alone, in the absence of VACV infection, has minimal ability to activate CAD. This suggests the involvement of other viral factors yet to be identified. Our research provides a foundation for understanding the regulation of a significant metabolic pathway, <i>de novo</i> pyrimidine synthesis during VACV infection, shedding new light on viral regulation under distinct nutritional environments. This study not only has the potential to contribute to the advancement of antiviral treatments but also improve the development of VACV as an oncolytic agent and vaccine vector.IMPORTANCEViruses often reprogram host cell metabolism to facilitate replication. How poxviruses, such as the prototype member, vaccinia virus (VACV), modulate host cell metabolism is not well understood. Understanding how VACV affects these metabolic pathways is key to learning about viral replication and developing antiviral treatments. This study highlights the importance of <i>de novo</i> pyrimidine synthesis during VACV infection. We found that the vaccinia growth factor (VGF), a viral protein similar to the cellular epidermal growth factor (EGF), helps VACV activate the enzyme CAD of the <i>de novo</i> pyrimidine pathway. Upon nutrient limitation, VGF is needed for the activation of CAD through mTORC1-S6K signaling. VGF peptide alone is unable to activate this pathway independent of infection, suggesting the involvement of other viral factor(s). Our research not only sheds light on how VACV regulates metabolism but also holds promise for improving VACV as a cancer treatment and vaccine.