This contrasts with b cells, in which both Ccnb1 and Cdc2a showed significant up regulation. Although speculation here, these findings may suggest SBK are able to enter the G1 S phase of the cell cycle, which is a Inhibitors,Modulators,Libraries known func tion of MYC, but not progress through the G2 M phase. Interestingly, it was Inhibitors,Modulators,Libraries previously shown that over expres sion of MYC causes a P53 dependent G2 arrest in nor mal fibroblasts. Such cells may then be enforced by MYC to reinitiate DNA replication, resulting in aneuploidy. Of these cell cycle related genes, Ccna2 and Ccnd1 have been previously designated as puta tive MYC targets through high throughput screening, and Ccnb1 and Ccnd2 have been previously confirmed as direct transcriptional targets of MYC through the use of chromatin Immunoprecipitation analysis.

The cyclin Cilengitide D2 related kinase Cdk4, also a previously Inhibitors,Modulators,Libraries characterized direct MYC target, showed increased expression after 4 hours of MYC acti vation in the pancreas, with a 6 fold increase detected subsequently at 16 hours. Cdk4 was also found to be highly up regulated at 8 hours in the skin, with a fold change of almost 12. No significant change was detected for the cyclin E associated CDK gene Cdk2 in either the skin or the pancreas, However Cdk7, which has a role in both activating cyclin complexes and regu lating transcription, was up regulated at 8 hours in the skin. Down regulation of another known MYC target gene, the cyclin dependent kinase inhibitor Cdkn1b, which inhibits G1 S phase transition by asso ciation with the cyclin E Cdk2 complex, was detected for both the skin and the pancreas.

Also, the expression of Cks2, a MYC target gene whose Inhibitors,Modulators,Libraries product is involved in degradation of p27Kip1, increased from 8 hours following MYC activation in the pancreas. Inter estingly, the Cdc2a gene, whose product Cdk1 is essen tial for mammalian cell division, was also found to be highly up regulated in b cells. Cdk1 has been found to substitute for other CDKs to drive cell cycle progression, and is particularly associated with Cdk4 in G1 S phase progression. This may indicate a significant role for Cdk1 in the pro motion of cell cycle progression following MYC activa tion in b cells. Alternatively, it has been shown that premature activation of Cdk1 can lead to mitotic cata strophe in G2 M phase and apoptosis in neurons.

Given that this CDK was also detected at later time points, this may indicate a possi ble role for Cdk1 in the MYC induced apoptosis path ways. In addition to this, the CDK inhibitor Cdkn2c, which inhibits G1 S phase transition via inter actions with Cdk4 and Cdk6, was down regu lated early in the pancreas. However, by 16 hours expression levels had risen dramatically, which may be indicative of cell cycle arrest prior to apoptosis. In addi tion, the CDK inhibitor Cdkn1a a down stream target of the tumour suppressor p53 was up regulated at 8 hours.