LOSS OF THE P16 MTS1 TUMOR-SUPPRESSOR GENE CAUSES E2F-MEDIATED DEREGULATION OF ESSENTIAL ENZYMES OF THE DNA PRECURSOR METABOLISM/

Homozygous deletions of the tumor suppressor gene p16/MTS1 were report ed in a wide variety of tumors and tumor cell lines, Its product inhib its the phosphorylation of the retinoblastoma protein (pRb) by CDK4 an d CDK6. Because phosphorylation of pRb is a major regulatory event in the activation of the transcription factor E2F, a role for p16 in the regulation of E2F-dependent transcription was presumed. We investigate d the effect of the loss of p16 on E2F-mediated transcription in a tum or progression model consisting of three cell lines originating from a common precursor cell-one p16-positive cell line established from the primary biopsy and two lines derived from more advanced stages of the tumor representing the same cell clone after loss of p16. We observed up- and deregulation of E2F-dependent transcription during the cell c ycle of the p16-negative cell clones, which returned to normal after t ransient expression of p16. This p16-dependent regulation affects a se t of enzymes necessary for the activation of all four DNA precursors; it is paralleled by the interconversion of transcriptionally active fr ee E2F and transcriptionally inactive higher molecular complexes of E2 F and is dependent on the existence of endogenous pRb. Furthermore, we show that p16-negative cell clones exhibit a growth advantage compare d to their p16-positive counterparts. One might speculate that one fea ture of tumor progression could be deregulation of E2F-dependent trans cription caused by loss of p16.