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Introduction: The novel tumor suppressor gene, HPP1 is epigenetically silenced in over 80% of colorectal cancers. Despite growing interest in HPP1 as a potential serum and stool-based biomarker, very little is known about its biologic mechanisms of action. We have previously reported that the growth suppressive effects of HPP1 require the activation of STAT1 and STAT2 and are associated with the concomitant downregulation of oncogenic STAT3. Janus Associated Kinases (JAK) are the upstream regulators of STAT proteins and can demonstrate cross reactivity. In this study, we sought to elucidate the roles of the different JAKs in mediating HPP1’s biologic functions.Methods: Full-length HPP1 was amplified and cloned into the pcDNA3.1 expression vector which was then stably transfected into the HCT116 colorectal cancer cell line. We have previously demonstrated that this results in substantial in vivo and in-vitro growth suppression. Western Blot analyses were performed to assess both expression and phosphotyrosine activation of JAK1, JAK2, JAK3 and TYK2 in HPP1 transfectants and empty vector controls. Transient knockdown was achieved using RNA interference with subsequent analysis of effects on STAT1 and STAT2 as well as on cell behavior by MTT and soft agar assays.Results: We have demonstrated that overexpression of HPP1 results in a substantial increase in both the expression and activation of JAK1 with corresponding reductions for TYK2 (the dominant kinase for STAT3). JAK1 siRNA treatment of HPP1 transfectants resulted in a 90% reduction of JAK1 expression. The inhibition of JAK1 resulted in a dramatic reduction in the levels of activated STAT1 and STAT2. JAK1 knockdown in HPP1 transfectants also resulted in a significant 2-fold increase in proliferation (p<0.04) and an 8-fold increase in anchorage-independent colony formation (p<0.01) as compared to controls.Conclusions: Suppression of JAK1 activity in the setting of HPP1 overexpression results in an attenuation of STAT1/STAT2 activity and a restoration of proliferiative potential. JAK1 appears to be the critical upstream regulator of HPP1-mediated growth suppression via activation of STAT1/STAT2 pathways.