Histone acetyltransferases and histone deacetylases (HDACs) control the acetylation state of histones and other proteins regulating transcription and protein function. A number of structurally diverse HDAC inhibitors have been developed as cancer therapeutic agents and have been shown to cause differentiation, cell cycle arrest or apoptosis. Depsipeptide is a natural tetrapeptide that was isolated from Chromobacterium violaceum No. 968. We have evaluated depsipeptide against 39 independently derived childhood tumors (9 brain tumors, 11 kidney cancers, 9 rhabdomyosarcomas, 3 neuroblastomas, and 7 osteosarcomas) growing in scid mice as xenografts. Western Blot analysis was performed to determine the acetylation state of histones (H2A, H2B, H3 and H4), protein levels of p53, pp53 (ser15), p21cip1 and cleaved PARP following administration of depsipeptide. The MTD was 4.4 mg/kg administered q7dx3iv (repeated q21dx2). Of 39 tumors evaluated, three were considered sensitive (2 stable disease, IRS56 and BT35, and 1 complete response, BT27). We have compared pharmacodynamic changes in these sensitive tumors and contrasted the pattern of histone acetylation (H2A, H2B, H3 and H4) with that in three resistant tumors (GBM2, SKNEP and WT10). Sensitive and resistant tumors followed a similar pattern of histone acetylation; maximal increases in histone acetylation occurred at 8hr and were elevated for 96hr. BT35 tumors showed the highest levels of histone acetylation and therefore more genes involved in growth arrest may be transcribed. To investigate the putative role of p53 in sensitivity to depsipeptide, total p53 and phospho-p53 (Ser15) were determined in control and treated tumors. In two sensitive tumor lines, IRS56 and BT27, there was no detectable p53 in the controls but an increase was observed maximal at 8 hr associated with induction of p21cip1 after treatment; no pp53 (Ser15) was detected. BT35, GBM2, SKNEP and WT10 tumors had stable p53, GBM2 and SKNEP tumors also had pp53, throughout the time course studied suggesting that these tumors have mutant p53. Cleaved PARP was detected in all of the tumors; the highest levels were detected 24 hours after treatment. Our results demonstrate that depsipeptide does inhibit its target in vivo i.e. causes increased histone acetylation, however this does not correlate with drug sensitivity. The magnitude and duration of acetylation may be an important factor in whether a tumor responds, as may be the p53 status of the tumor. Altogether, determining whether depsipeptide would work on a given tumor seems to be multifactorial, which is understandable as depsipeptide can have multiple effects in a cell. Supported by CA23099 and ALSAC.
[Proc Amer Assoc Cancer Res, Volume 46, 2005]