Importantly, such protein up-regulation in response to Hsp90 inhibition has thus far only been reported for certain other heat-shock proteins such as HSF1 and Hsp70. growth and in a model of peritoneal carcinomatosis. Human colon cancer tissues were analyzed for ATF3 expression. == Results == The results show that therapeutic Hsp90 inhibition substantially up-regulates the expression of ATF3 in various cancer cells, including colon, gastric and pancreatic cancer. This effect was evident bothin vitroandin vivo. RNAi mediated knock-down of ATF3 in HCT116 colon cancer cells significantly increased cancer cell migrationin vitro. Moreover, in xenogenic mouse models, ATF3 knock-down promoted subcutaneous tumor growth and hepatic metastasis, as well as peritoneal carcinomatosis. Importantly, ATF3 expression was lower in human colon cancer specimens, as compared to corresponding normal surrounding tissues, suggesting that ATF3 may represent a down-regulated tumor suppressor in colon cancer. == Conclusion == In conclusion, ATF3 down-regulation in colon cancer promotes tumor growth and metastasis. Considering that blocking Hsp90 4-Hydroxytamoxifen induces ATF3 expression, Hsp90 inhibition may represent a valid strategy to treat metastatic colon cancer by up-regulating this anti-metastatic transcription factor. == Background == Heat shock protein 90 (Hsp90) targeting has emerged as a valuable strategy for cancer therapy [1,2], because these proteins are being up-regulated in malignant and non-malignant cells types upon exposure to a variety of stressors [3]. At constitutive levels, heat-shock proteins regulate proper folding and stabilization of abundant intracellular proteins, and their stress-associated induction improves cell survival. Hsp90, one of the most studied molecular chaperons, is overexpressed in tumor cells and is essential for the stability and function of a wide range of oncogenic client proteins [4]. These Hsp90 clients comprise kinases such as ERBB2, EGFR, CDK4, RAF, AKT, cMET and BCR-ABL, and transcription factors such as HIF-1, STAT3, and STAT5 [2,5,6]. Thus, Hsp90 is a promising target for cancer therapy, as demonstrated by the expanding armamentarium of Hsp90 inhibitors and by new clinical studies incorporating the use of these inhibitors [7]. Nevertheless, due to the broad and complex inhibition of multiple signaling pathways affected by Hsp90, the biological effects remain poorly defined and incompletely understood. We recently demonstrated that therapeutic inhibition of Hsp90 not only elicits antineoplastic efficacy through blocking oncogenic signaling, but also up-regulates certain signaling molecules in human colon carcinoma cell lines. One of these molecules is activating transcription factor-3 (ATF3), which is Hsp90-inhibitor inducible in HCT116, SW620 and HT29 colon cancer cells [8]. Importantly, such protein up-regulation in response to Hsp90 inhibition has thus far only been reported for certain other heat-shock proteins such as HSF1 and Hsp70. This response may counteract the anti-neoplastic potential of Hsp90 inhibitors for the following reasons [9,10]. ATF3 belongs to the ATF/cyclic AMP response element binding (CREB) family of transcription factors and most cells have very weak or absent ATF3 expression under steady-state conditions. A significant increase in ATF3 can be observed when cell-stress is induced [11], making ATF3 an universal adaptive response gene” [12,13]. Importantly, different roles for ATF3 have been proposed. In normal tissues, ATF3 may promote both apoptosis and cell proliferation [13], while in neoplasms it has been identified as either an oncogene or as tumor suppressor, depending on tumor entity and grade [13-15]. For instance, ATF3 can mediate pro-apoptotic effects in human mammary epithelial cells, whereas in breast cancer cells (MCF10A) it may promote cell survival, motility and invasiveness [15]. Transgenic mice that overexpress MGMT ATF3 in basal epithelial cells develop epidermal hyperplasia, dysplastic lesions and oral squamous cell carcinoma [16]. Also in favor of oncogenicity, the tumor suppressor gene Drg-1 mediates its anti-metastatic properties through ATF3 down-regulation in prostate cancer [17]. In colon cancer, the effects of ATF3 expression are particularly perplexing. 4-Hydroxytamoxifen In one respect, ATF3 was shown to be overexpressed in human colon cancer specimens and appears to promote tumor growth and migration in an experimental HT29 colon cancer model [18,19]. In another respect, ATF3 has been described to mediate anti-neoplastic and anti-invasive effects of nonsteroidal 4-Hydroxytamoxifen anti-inflammatory drugs (i.e. COX-2 inhibitors) in colorectal cancer [14]. In the present study, we sought to clarify ATF3 regulation and its role in human colon cancer using xenogenic mouse models. We hypothesized that Hsp90 inhibitor-mediated induction of ATF3 expression does not counteract the anti-neoplastic and anti-metastatic potential of Hsp90 targeting agents. == Methods == == Cell culture == The human colorectal cancer cell lines HCT116, SW620 and HT29 were.
