Type I interferons (IFNs) are key initiators and effectors of the immune response against malignant cells and can also directly inhibit tumor growth. IFN is highly effective in the treatment of myeloproliferative neoplasms (MPNs), but the mechanisms of action are unclear and it remains unknown why some patients respond to IFN therapy and others do not. We have identified and characterized a novel pathway involving PKC upstream of the kinase ULK1. Engagement of the Type I IFN receptor triggers PKC-dependent phosphorylation of ULK1 on serines 341 and 495, required for subsequent engagement of p38 MAPK that regulates transcription of IFN-stimulated genes and subsequent growth inhibitory responses. We show that this pathway is essential for IFN-suppressive effects on primary malignant erythroid precursors from patients with MPNs in vitro, while increased levels of ULK1 and p38 MAPK correlate with clinical response to IFN therapy in MPN patients. IFN treatment also induces cleavage/activation of the ULK1-interacting ROCK1/2 proteins in vivo, triggering a negative feedback loop that suppresses IFN responses. Both ROCK homologs are overexpressed in MPN patients and their genetic or pharmacological inhibition enhances IFN-anti-neoplastic responses in malignant erythroid progenitors from MPN patients. Together, our results identify activation of the PKC-ULK1-p38 MAPK cascade as a key and essential component for the IFN-response, regulated by a feedback loop involving ROCK1/2. These findings suggest the clinical potential of pharmacological inhibition of ROCK1/2 in combination with IFN-therapy for the treatment of MPN patients.