In contrast, Vegfc, important in the growth of lymphatic vessels, showed down regulation in the skin at early time points, but was up regulated almost 4 fold in the pancreas at the later 32 hour time point. Vegfb showed a 2 fold increase from 16 hours in pancreas compared to no change in the skin. These results indicate a transcriptional response upon MYC activation for genes relating to neovascular growth. Activation of MYC leads to loss of differentiation Activation of MYC is often associated with loss of dif ferentiation of cells and has been found to block term inal differentiation in a variety of cell types. Activation of MYC in the pancreatic b cells resulted in down regulation of Ins1 and Ins2 by 5 fold at 4 hours, indicating acute loss of Insulin production within a short time period following MYC deregulation.

How ever, the expression levels of both subsequently Inhibitors,Modulators,Libraries increased dramatically Inhibitors,Modulators,Libraries showing almost 10 fold up regu lation from 16 hours. Since the islet GSK-3 area used for RNA extraction was roughly identical for each sam ple, this indicates acute increase in the levels of Insulin production within the b cells in response to continuous MYC activation, and not in response to an increase in b cell mass. Although perhaps a paradox at first glance, this response may be the result of a positive feedback loop due to increased Insulin release into the blood stream immediately following MYC activation as we have previously shown. Observation of transcript levels of both mRNAs at a later time point indicated that this period of high Insu lin production is limited, as gene expression subse quently returned to lower levels indicative of loss of b cell differentiation.

This indicates a short window within the first two days where a balance is struck between Inhibitors,Modulators,Libraries an increased rate of Insulin production and the simulta neous loss of cells due to MYC driven apoptosis. Members of the homeodomain transcription factor family, Pdx1, Pax4, Hb9, Nkx2. 2 and Nkx6. 1, are essen tial in pancreatic development. Probe sets for the pancreatic and duodenal homeobox gene Pdx1, whose product activates transcription of the Insulin gene as well as a number of genes involved in glucose sensing, showed a significant loss Inhibitors,Modulators,Libraries in expression at 8 hours following MYC activation, which correlated with the early reduction seen in Insulin production. The transcription factor gene Nkx6.

1, whose product is essential for b cell differentiation, also showed sig nificant down regulation in the early stages of MYC activation, although expression of this gene was shown to increase during later stages. Further Pdx1 regulated s Slc2a2 and Gck, both part of the glucose sensing machinery and involved in mem brane transport and phosphorylation of glucose respec tively, also followed similar expression profiles. Gu et al.