4376] GROWTH INHIBITION AND CELL DEATH PATHWAYS IN NON-HODGKIN'S LYMPHOMA. Richard J. Ford, Lan Pham, Archito Tamayo, Irving Pass, Keyi Niu, Nicholas Donato. University of Texas M.D. Anderson Cancer Center, Houston, TX. The non-Hodgkin's lymphomas (NHL) are primarily of B cell lineage (NHL-B), that are currently treated with chemotherapy. While these regimens induce remissions in many cases, they are quite toxic, due in part to lack specificity for NHL-B cells. Our studies have been evaluating a role for biotherapeutic agents in vitro, in cell lines we have derived from patients with transformed follicular (DLCL) NHL-B. In vitro studies indicate that biotherapeutic agents (BTA), such as retinoids and interferons, are effective agents in inducing cell cycle arrest in G1 phase of the cell cycle, followed by cell death through various pathways involving bcl-2/bcl-x/bax modulation, and caspase activation in the lymphoma cells. In addition to modulating bcl-2/bax, retinoids such as liposomal ATRA (L-ATRA), also up-regulate the transforming growth factor beta (TGF-B) pathway (TGF-BR, SMAD2/4 activation, etc.), in NHL-B that leads to both G1 arrest through the CDKIs p21 and p27 and apoptosis through activation of caspase 3. Simultaneous treatment of NHL-B cells with increasing doses of L-ATRA and TGF-B, indicate that there is synergistic growth inhibitory activity. We have derived three NHL-B cell lines that are refractory to both the ATRA and TGF-B1 treatment through different mechanisms, including point mutations in TGF-RII promotor. Interestingly, these cell lines are also, refractory to IFN-a, that fails to G1 arrest the NHL-B cells, or induce cell death through caspase 3. Since our NHL-B cell lines all show constitutive activation the central transcription factor, NF-kB, as well as cell cycle arrest in response to several BTA, we have tested the response of our NHL-B cell lines to several proteasome inhibitors (lactacystin, MG-132, etc.) to determine if the pathways involved in BTA-mediated growth arrest and apoptosis, could be mediated through NF-kB, or proteasomal modulation of apoptotic pathways. Our studies show that proteasomal inhibition by selective blockade, closely mimics that action of BTAs such as retinoids and IFNs in blocking cell growth, and in inducing cell death in vitro. All of the proteasome inhibitors tested, down-regulated constitutively expressed NF-kB in the NHL-B cells, and simultaneously showed down-regulation of bcl-2, and up-regulation of bax and caspase 3, similar to that observed with L-ATRA on the lymphoma cells. Also, the NHL-B cell lines that were refractory to treatment with the various BTA, also showed a diminished response to the proteasome inhibitors. These studies suggest that the various BTA that show activity in mediating cellular growth arrest and cell death in aggressive NHL-B cells, apparently utilize similar inhibitory pathways, that interdict mitogenic pathways regulated through NF-kB. The molecular basis for the constitutive expression of NF-kB is unknown, but afferent up-stream mitogenic signals from growth factors (autocrine or paracrine) or other cytokines may provide the growth stimulus that is interdicted by various BTAs.