Imprinted genes are up-regulated by growth arrest in embryonic fibroblasts

It was previously reported that the imprinted insulin-like growth factor II and H19 genes are up-regulated in embryonic fibroblasts upon growth arrest. We have examined six other imprinted genes and found that all were up-regulated upon confluency and at least four of them by serum starvation. The significance of the results was confirmed by demonstrating that only one-third of randomly selected genes are up-regulated upon confluency. Our findings suggest that the cell cycle should be carefully controlled when imprinted genes are studied in cell cultures. Moreover, the unique property could have significance in the mechanistic or the evolutionary aspect of imprinting or both.