Cyclin D1 Mediates Resistance towards p53-Independent and -Dependent Apoptosis through Anti-Apoptotic Factors and Molecular Chaperones.

Cyclin D1 is a key regulator of the G1 to S transition of the cell cycle, physiologically absent in normal lymphocytes, but overexpressed in some lymphoproliferative disorders such as mantle cell lymphoma or multiple myeloma. While cyclin D1 has also been shown to control other aspects of the cellular fate, such as cellular senescence, apoptosis or tumorigenesis, its precise role in the pathogenesis of B-cell lymphomas and leukemias is still unclear. In this context, using various cell lines derived from the pre-B murine cell line BaF3 and conditionally expressing cyclin D1, we have previously reported that when expressed at a level comparable to the one detected in tumoral cells, cyclin D1 impairs cell proliferation mainly by inducing apoptosis. We now report that the BaF3-derived cell line BD1-A21, which constitutively expresses a low but notable level of cyclin D1, presents a recurrent resistance towards various apoptotic stimuli involving different signaling pathways, such as cytokine withdrawal, etoposide treatment and staurosporine-mediated kinases inhibition, when compared to the parental BaF3 cells. This lack of sensitivity towards apoptosis was correlated in cyclin D1-expressing cells with an increase in the basal levels of several antiapoptotic factors such as IAPs (c-IAP1, c-IAP2, XIAP and survivin) or the death receptors pathway inhibitor cFLIP, and the relative maintenance of these factors in IL-3-starved cells. 2D-PAGE analysis also pointed out in cyclin D1 expressing cells the accumulation of a series of molecular chaperones, which the most overexpressed is hsp70, concording with previous works describing the cyclin D1-dependent synthesis of hsp70 in lymphoid cells. Consistently with a protective role of hsp70 in cyclin D1-expressing cells, its pharmacological inhibition by the mean of the flavonoid quercetin allowed to re-sensitize BD1-A21 cells to the apoptotic stimulus evoked by IL-3 starvation. Hsp70 protective efect appeared to act upstream of mitochondrion, as determined by mesurement of mitochondrial membrane potential and western-blot or FACS analysis of the main Bcl-2 family members. The relationship between cyclin D1 constitutive expression, antiapoptotic factors and chaperones accumulation, and resistance to apoptosis is currently under investigation.