Interferon resistance is common to the majority of cancer cell lines and primary tumors, and confers increased tumorigenicity and evasion of immune surveillance. HPP1, a recently characterized tumor suppressor gene associated with STAT1 signaling, is silenced in the majority of colorectal neoplasms. We have sought to determine if HPP1's tumor suppressive effects are mediated by interferon‐associated pathways and whether HPP1 is capable of restoring sensitivity to interferon. We have overexpressed HPP1 in the HCT 116 colorectal cancer cell line and, by microarray analysis, have demonstrated that ectopic expression of HPP1 resulted in a dramatic upregulation of STAT1 as well as a number of associated interferon‐inducible genes. We have previously shown that downregulation of STAT1 by siRNA in HPP1 transfectants results in a restoration of growth potential. Given that STAT1 homodimers bind specifically to promoter GAS (gamma activating sites) sites causing transcriptional upregulation of target genes, we transfected luciferase GAS reporter (pLucGAS) and control plasmids into HPP1 and control transfectants. Overexpression of HPP1 was associated with a significant increase in GAS site binding (p=0.007), suggesting that HPP1 mediates tumor suppression via its association with Type II‐like (gamma) interferon pathways. To investigate the importance of Type II‐like pathways, we treated HCT 116 cells, with and without overexpression of HPP1, with increasing doses of interferon gamma. Induction of activated STAT1 (phosphoSTAT1), was demonstrable in HPP1‐overexpressing transfectants with a progressive rise concordant with interferon dose escalation. Interferon gamma was unable to induce any augmentation of activated STAT1 in controls. Despite progressive induction of phosphoSTAT1, no significant alterations in proliferation, apoptosis or cell cycle distribution were observed with interferon gamma treatment of HPP1 transfectants. We conclude that activated STAT1 and Type II‐like interferon‐associated pathways are important in mediating the tumor suppressive effects of HPP1. The lack of additional anti‐proliferative effects of interferon on HPP1 transfectants may be due to saturation of the pathway. However, further investigation of the role of other STAT1‐associated mechanisms in HPP1 tumor suppression, such as the Type I‐like interferon (alpha) pathway is warranted.
Citation Information: Cancer Prev Res 2010;3(1 Suppl):B60.