Cyclin D1 and pancreatic carcinoma: a proliferative agonist and chemotherapeutic antagonist.

Cyclin D1, the regulatory partner of the G1 cyclin-dependent kinases, CDK4/6, plays a critical role in promoting cell proliferation by virtue of its capacity to trigger cell cycle progression through the first gap phase, G1. Given the central role for this enzyme in regulating growth factor–dependent cell cycle progression, it is not surprising that overexpression of cyclin D1 is frequently associated with neoplasia where it is thought to play critical role in tumor initiation and progression. By analogy with the c-myc oncoprotein, which can induce proliferation or apoptosis depending upon the cellular environment, cyclin D1 expression has been paradoxically associated with either induction of apoptosis or in some instances resistance to apoptosis. If cyclin D1 indeed plays an integral role in promoting tumor cell proliferation, elucidating its function in either promoting or inhibiting commitment of these cells toward apoptosis will critically effect the response of tumors to therapeutic intervention. In this issue of Clinical Cancer Research, Biliran et al. address this notion by investigating the influence of cyclin D1 overexpression on pancreatic tumor cell sensitivity to cisplatin (1). All three D-type cyclins are responsive to growth factor– mediated signaling; growth factors induce transcription, translation, and subsequent assembly of the penultimate D-type cyclin-CDK4/6 complex during the G1 phase of the cell cycle (2). This kinase complex is subsequently transported into the nucleus where it initiates the phosphorylation-dependent inactivation of the retinoblastoma family of proteins, Rb, p107, and p130 defining a critical step in the irreversible commitment of a cell to a single round of cell division (3). Functional inactivation of Rb and related proteins relieves repression of the E2F family of transcription activators, thereby permitting expression of genes whose products determine S-phase entry and progression. Cyclin D1 is also negatively regulated via proteasomedependent degradation and nucleocytoplasmic shuttling. Upon entry into the S phase, cyclin D1 is phosphorylated at a single threonine residue, Thr, which in turn triggers association with the CRM1 nuclear exportin, an event essential for nuclear export of the cyclin D1/CDK complex during the S phase (4). Once cyclin D1 is localized to the cytoplasm, it is a substrate for an as yet unidentified E3 ligase that triggers cyclin polyubiquitylation and proteasomal degradation (5). Whereas residues that direct cyclin D1 nuclear export are conserved in cyclins D2 and D3, it has not yet been established whether these D-type cyclins are similarly regulated. Although cyclin D1 is frequently overexpressed in cancer, the precise role it plays in cancer genesis remains unclear. Despite accumulating data spanning 15 years of extensive research describing the overexpression of cyclin D1 in cancer, the cyclin D1 protein does not fit the classic definition for an oncogene. For example, overexpression of cyclin D1 will decrease cell cycle transit times, and in some cell types reduce growth factor requirements; however, overexpression of wild-type cyclin D1 by itself is not sufficient to induce cellular transformation (6). Recently, however, it was shown that cyclin D1 mutants that are refractory to nuclear export are in fact capable of driving cellular transformation (3, 7). These findings suggest that in cancers wherein cyclin D1 is the initiating oncogene, mere overexpression of the wild-type protein is unlikely to be the initiating event. In addition to its function in cancer initiation, cyclin D1 overexpression can be associated with induction of either programmed cell death (8, 9) or cell survival due to insensitivity to cytotoxic drugs (10, 11). In the latter scenario, cyclin D1 is thought to provide a critical but undefined prosurvival function. Specifically in the context of pancreatic cancer, overexpression of cyclin D1 is associated with poor prognosis and insensitivity to chemotherapy (12). Thus, its expression might contribute to cancer cell survival following treatment with chemotherapeutic agents. Indeed, previous work from the Korc laboratory revealed that antisense ablation of cyclin D1 in cells derived from a pancreatic carcinoma (Panc-1) increased cellular sensitivity to cisplatin (10). To directly assess the contribution of cyclin D1 in the sensitivity of pancreatic cancer cells to chemotherapeutic treatment, Biliran et al. used a pancreatic tumor cell line derived from elastase-myc transgenic mice (Ela-myc). The established cell line was previously shown to contain low levels of endogenous cyclin D1 protein, making it a suitable model for the direct manipulation of cyclin D1 levels. To determine the consequence of cyclin D1 overexpression in these cells, the authors engineered additional cell lines following transfection of cyclin D1–encoding constructs into parental Ela-myc cells. As expected, overexpression of cyclin D1 endowed these cells with a growth advantage characterized by a contracted G1 interval and reduced but not abrogated growth factor requirements relative to parental controls. Overexpression of cyclin D1 also conferred increased propensity for growth in semisolid medium. Biliran et al. subsequently assessed the sensitivity of parental versus cyclin D1–overexpressing cells to the chemotherapeutic drugs such as cisplatin and gemcitabine. Strikingly, significant resistance to the cytotoxic effects of either cisplatin The Biology Behind