Growth inhibition of astrocytoma cells by farnesyl transferase inhibitors is mediated by a combination of anti-proliferative, pro-apoptotic and anti-angiogenic effects

Abstract

While 25% of human cancers harbor oncogenic Ras mutations, such mutations are not found in astrocytomas. We have previously demonstrated that the activation of receptor tyrosine kinases expressed by malignant human astrocytoma cells and specimens results in functional upregulation of the Ras signalling pathway and increased levels of activated Ras.GTP. Farnesyl transferase inhibitors (FTIs) are promising anti-cancer agents in early clinical trials, which may exert their e ect through pharmacological inhibition of the Ras signalling pathway. In this study we establish the anti-tumorigenic properties of the FTI L-744,832 against a panel of malignant human astrocytoma cell lines. Furthermore, we demonstrate the multiple mechanisms by which L-744,832 exerts its e ect. L-744,832 demonstrates both cytostatic and cytotoxic e ects on astrocytoma cells, and cells expressing a truncated constitutively phosphorylated Epidermal Growth Factor Receptor common in highgrade astrocytomas (EGFRvIII/p140 EGF-R ) demonstrate increased sensitivity to the agent. L-744,832 is capable of inducing apoptosis in astrocytoma cells under anchoragedependent conditions; this process occurs in a p53-independent manner and is associated with increased expression of Bax and Bak. L-744,832 also induces cell cycle arrest at both the G 1 /M and G 2 /S checkpoints; this process is also independent of p53 mutational status. Cell cycle arrest in drug-treated cells can be accompanied by induction of p21 WAF1/CIP1 , but this induction is not necessary for the cell cycle inhibitory e ects, nor is it dependent on functional p53. Finally, angiogenesis in astrocytomas has been shown to be dependent on secretion of Vascular Endothelial Growth Factor (VEGF) by tumour cells, particularly under hypoxic conditions. L-744,832 potently inhibits the secretion of VEGF under hypoxic conditions. These combinations of mechanisms suggest that these tumours, despite the absence of oncogenic Ras mutations, will be amenable to growth inhibition by FTIs, through a combination of anti-proliferative, pro-apoptotic, and anti-angiogenic e ects.

RhoB and the Cellular Response to Farnesyltransferase Inhibitors: The Search for Clues to Cell Death Mechanisms

“…12,13,20,21 However in many of these studies the presence of mutant Ras was not required for FTI activity. 9,11,13 In other studies where FTIs showed efficacy, K-Ras, which is resistant to FTI inhibition due to alternative prenylation, 22 was the activated species. 13,16,21 These observations have given rise to two hypotheses.…”

“…FTIs have shown potent anti-tumor activity against rodent and human tumors in vitro. [9][10][11][12][13] They have also shown potent activity in transgenic onco-mouse studies [14][15][16][17] and spontaneous tumor induction studies, 18,19 and inhibit human tumor xenograft growth. 12,13,20,21 However in many of these studies the presence of mutant Ras was not required for FTI activity.…”

Inactivation of farnesyltransferase and geranylgeranyltransferase I by caspase-3: Cleavage of the common α subunit during apoptosis

“…Inhibition of prenylation with inhibitors including FTI, GGTI-298, or lovastatin has recently shown to cause growth arrest and promote apoptosis Jansen et al, 1997;Suzuki et al, 1998;Stark et al, 1998;Sun et al, 1999;Ghosh et al, 1999;Feldkamp et al, 1999), suggesting that prenylation is an important step in the survival signaling cascade. Here we show that FTase/GGTasea, a common subunit of heterodimeric FTase (a/b FTase ) and GGTase I (a/b GGTase ), is cleaved by caspase-3 in vitro and in vivo to be inactivated during apoptosis, which may help us to understand how the prenylation signaling responds to apoptosis.…”

Farnesyltransferase inhibition: Who are the aktors?

“…43 There is some controversy over the role of p53 status as a factor in determining whether arrest occurs in G 1 or G 2 /M. 22,44,45 Although AKT activation contributes to cell cycle regulation through E2F, cyclin D 46 and mdm2/p53, 47 a role for AKT in determining the nature of cell cycle arrest after FTI treatment has not been established, and the current report would argue against such a link. Alternative candidates for mediators of cell cycle arrest after FTI treatment include CENP-E and CENP-F 43,48 and the PDK phosphatases 49,50 all of which are farnesylated.…”