Mounting evidence suggests that dynamic interactions between tumor and its microenvironment play a critical role in tumor development, cell cycle progression and response to therapy. In this study we used mantle cell lymphoma (MCL) as a model to characterize the mechanisms by which stroma regulate cell-cycle progression. We demonstrated adhesion of MCL and other non-Hodgkin lymphomas (NHL) cells to bone marrow stromal cells resulted in a reversible G1 arrest associated with elevated p27(Kip1) and p21(WAF1) proteins. The adhesion mediated p27(Kip1) and p21 increases were posttranslationally regulated via the down-regulation of Skp2, a subunit of SCF(Skp2) ubiquitin ligase. Overexpression of Skp2 in MCL decreased p27(Kip1), whereas inhibition of Skp2 by siRNA increased p27(Kip1) and p21 levels. Furthermore, we found cell adhesion up-regulated Cdh1 (an activating subunit of anaphase-promoting complex (APC) ubiquitin ligase), and reduction of Cdh1 by siRNA induced Skp2 accumulation, and hence p27(Kip1) degradation, thus implicating Cdh1 as an upstream effector of Skp2/p27(Kip1) signaling pathway. Overall, this report, for the first time, demonstrates that cell-cell contact controls tumor cell cycle via ubiquitin-proteasome proteolytic pathways in MCL and other NHL. The understanding of this novel molecular pathway may prove valuable in designing new therapeutic approaches for modifying tumor cell growth and response to therapy.