Objectives: 

The pocket protein family members, pRb, p107 and p130, are negative regulators of the cell cycle. While the role of pRB as a tumor suppressor has been extensively established, genetic inactivation of p107 or p130 is uncommon in tumors. Previous studies from our group have shown that p130, but not p107, cooperates with the CDK inhibitor p27Kip1 restricting proliferation of mouse intermediate lobe pituitary cells.

Materials: 

Because of extensive functional redundancy between p130 and p107, to study their contributions as tumor suppressors we have generated both cell lines and animal models carrying mutations in these proteins. We employed the mouse pituitary cell line AtT20 to, by using lentiviral-mediated shRNA delivery, establish cell populations lacking p107 and p130, alone or in combination. We have studied the proliferative and transformation properties of these mutant cell lines. To further confirm our observations, we have generated p107+/­; p130-/- mice and combined them with p27 inactivation (5/6KO strain).

Results: 

We found that reduction of p107 gene dosage on a p130-p27 double deficient background caused a significative reduction in tumor-free survival time. Microscopic examination of pituitary tumors revealed that 5/6KO tumors were clearly invasive and showed features of aggressive carcinomas. To kwon how the pocket proteins perform their antioncogenic functions we performed a genome-wide expression analysis to identified those genes whose expression changed in function of the genotype. We focused in one of the gene overexpressed in 5/6KO tumors, Mad2L1, which deregulation causes aneuploidy. Finally, to determine the contribution of Mad2L1 to the aggressive phenotype, we established different AtT20 cell derivatives with combination of knockdowns. Then, we investigated their different transformation and proliferative abilities, as well as metastatic dissemination capacities.

Conclusions: 

Our results support an Rb-independent tumor suppressor role for pocket proteins p130Rbl2 and p107Rbl1 in mouse pituitary through regulation of E2F target genes.