There is also evidence that high cyclin A expression is an unfavorable prognostic factor in patients with RCC. Resistance development caused by an HDAC-inhibitor based regimen is, at least partially, characterized by a distinct accumulation of cdk/cyclin proteins, which may re-activate the cell cycle CHIR-99021 supply machinery. RCC cells chronically treated with VPA for 12 weeks in vitro have been shown to increase cyclin A and cyclin D3 expression and to simultaneously regain the capacity to grow. However, quantitative modification of cdk proteins was not observed in this model. Possibly, the in vitro conditions differ from the in vivo one presented here and identical results cannot be expected. Studies investigating the relevance of cdk-cyclin complexes in drug resistant RCC cells are necessary. The VPA-induced resistance could be due to increased levels of HDAC accompanied by reduced histone acetylation. However, neither HDAC3/HDAC4, nor H3/H4 acetylation was altered in the drug resistant mice, compared to the untreated control. This is important, since the HDAC system would be the specific target of an HDAC-inhibitor. Obviously, a feedback mechanism in the course of resistance development has not been established, leading to an up-regulation of HDAC and down-regulation of histone acetylation. Recently, resistance to the HDAC-inhibitor SAHA has been reported not to be accompanied by elevated expression of HDAC1 and HDAC3 in human colorectal adenocarcinoma cells. However, this does not mean that HDAC is irrelevant during the process of resistance induction. The following aspect must also be considered: VPA enhances histone H3 and H4 acetylation in RCC cells at a very early time point. This effect is lost following long-term exposure. The H3 and H4 acetylation levels are then similar to the expression level of untreated control cells. Hypothetically, resistance to VPA might be defined by the failure to up-regulate histone acetylation. The most prominent effect of VPA was a massive amplification of Akt expression and activity in the non-responders as demonstrated by western blotting, which did not occur in the untreated mice. Akt plays a central role in the control of cell growth, survival and MDV3100 clinical trial angiogenesis, whereby aberrant activation and dysfunction becomes evident in progressive RCC. Due to this relationship, blocking Akt and Akt downstream molecules by mammalian target of rapamycin inhibitors has been considered an effective strategy in fighting this disease. Indeed, mTOR-suppression has produced robust clinical effects in RCC, particularly in the early treatment phase. However, compensatory Akt activation seems to be a critical event under long-term application, which may limit the antitumor effect of mTORinhibitors. The data presented here demonstrates that upregulation of Akt is not a resistance phenomenon exclusively restricted to the use of mTOR-inhibitors, but may also occur in the presence of HDAC-inhibitors. This property could label Akt as a ubiquitous prognostic and therapeutic parameter for patients subjected to targeted drugs.