ID2 SPECIFICALLY ALTERS REGULATION OF THE CELL-CYCLE BY TUMOR-SUPPRESSOR PROTEINS

Cells which are highly proliferative typically lack expression of diff erentiated, lineage-specific characteristics. Id2, a member of the hel ix-loop-helix (HLH) protein family known to inhibit cell differentiati on, binds to the retinoblastoma protein (pRb) and abolishes its growth -suppressing activity. We found that Id2 but not Idl or Id3 was able t o bind in vitro not only pRb but also the related proteins p107 and p1 30. Also, an association between Id2 and p107 or p130 was observed in vivo in transiently transfected Saos-2 cells. In agreement with these results, expression of Idl or Id3 did not affect the block of cell cyc le progression mediated by pRb. Conversely, expression of Id2 specific ally reversed the cell cycle arrest induced by each of the three membe rs of the pRb family. Furthermore, the growth-suppressive activities o f cyclin-dependent kinase inhibitors p16 and p21 were efficiently anta gonized by high levels of Id2 but not by Idl or Id3. Consistent with t he role of p16 as a selective inhibitor of pRb and pRb-related protein kinase activity, p16-imposed cell cycle arrest was completely abolish ed by Id2. Only a partial reversal of p21-induced growth suppression w as observed, which correlated with the presence of a functional pRb. W e also documented decreased levels of cyclin D1 protein and mRNA and t he loss of cyclin D1-cdk4 complexes in cells constitutively expressing Id2. These data provide evidence for important Id2-mediated alteratio ns in cell cycle components normally involved in the regulatory events of cell cycle progression, and they highlight a specific role for Id2 as an antagonist of multiple tumor suppressor proteins.