Cyclin D1 Splice Variants

Abstract

Cyclin D1 is a proto-oncogene that functions by inactivation of the retinoblastoma tumor suppressor protein, RB. A common polymorphism in the cyclin D1 gene is associated with the production of an alternate transcript of cyclin D1, termed cyclin D1b. Both the polymorphism and the variant transcript are associated with increased risk for multiple cancers and the severity of a given cancer; however, the underlying activities of cyclin D1b have not been elucidated relative to the canonical cyclin D1a. Because cyclin D1b does not possess the threonine 286 phosphorylation site required for nuclear export and regulated degradation, it has been hypothesized to encode a stable nuclear protein that would constitutively inactivate the RB pathway. Surprisingly, we find that cyclin D1b protein does not inappropriately accumulate in cells and exhibits stability comparable to cyclin D1a. As expected, the cyclin D1b protein was constitutively localized in the nucleus, whereas cyclin D1a was exported to the cytoplasm in S-phase. Despite enhanced nuclear localization, we find that cyclin D1b is a poor catalyst of RB phosphorylation/inactivation. However, cyclin D1b potently induced cellular transformation in contrast to cyclin D1a. In summary, we demonstrate that cyclin D1b specifically disrupts contact inhibition in a manner distinct from cyclin D1a. These data reveal novel roles for D-type cyclins in tumorigenesis.Cyclin D1 is an essential regulator of cell cycle progression, and aberrant induction of cyclin D1 activity is well established in human tumorigenesis (1-7). Initially, cyclin D1 was identified as the PRAD1 oncogene by mapping the sites of amplification in parathyroid adenomas to 11q13 (1,8). Subsequently, deregulation of cyclin D1 has been observed to occur in a variety of cancer types (9). For example, the BCL-1 translocation in centrocytic lymphoma deregulates the expression of cyclin D1 (8). In animal models, cyclin D1 has also been shown to exhibit oncogenic activity when overexpressed in specific tissues (10). Lastly, cyclin D1 activity is critical for tumor formation induced by other oncogenes (e.g. Ras), as mice deficient in cyclin D1 are resistant to tumorigenesis (11,12). Because of the intimate connections between cyclin D1 and oncogenesis, extensive analyses have focused on the mechanisms through which cyclin D1 contributes to human cancer.Cyclin D1 exerts its effects on cellular proliferation by integrating external signals (e.g. mitogens) with the cell cycle machinery (3-7, 13-15). Given this critical role, cyclin D1 action is highly regulated. Cyclin D1 transcription is stimulated as a delayed early response to mitogenic signaling cascades (4, 16). Additionally, protein stability is regulated through the glycogen synthase kinase 3␤ signal transduction pathway to coordinately enhance accumulation of cyclin D1 protein (17, 18). Once synthesized, cyclin D1 interacts with and activates the G 1 cyclin-dependent kinases (CDK), 1 CDK4 and CDK6 (19,20). This interaction is assisted through the action of both m...

Cyclin D1: polymorphism, aberrant splicing and cancer risk

“…Rather, the half-lives and relative accumulation of cyclin D1 and cyclin D1b were indistinguishable in most model systems (Lu et al, 2003;Solomon et al, 2003). By contrast, these same studies validated the prediction that cyclin D1b is constitutively nuclear in localization (Lu et al, 2003;Solomon et al, 2003). This was a critical observation, as constitutive nuclear localization of cyclin D1 (as achieved through mutation of T286) is known to promote oncogenic transformation (Alt et al, 2000).…”

“…The expectation that cyclin D1b would exhibit increased protein stability (as compared to cyclin D1a) was not significantly observed (Lu et al, 2003;Solomon et al, 2003). Rather, the half-lives and relative accumulation of cyclin D1 and cyclin D1b were indistinguishable in most model systems (Lu et al, 2003;Solomon et al, 2003). By contrast, these same studies validated the prediction that cyclin D1b is constitutively nuclear in localization (Lu et al, 2003;Solomon et al, 2003).…”

“…Several groups have recently assessed the action of cyclin D1b on these parameters. The expectation that cyclin D1b would exhibit increased protein stability (as compared to cyclin D1a) was not significantly observed (Lu et al, 2003;Solomon et al, 2003). Rather, the half-lives and relative accumulation of cyclin D1 and cyclin D1b were indistinguishable in most model systems (Lu et al, 2003;Solomon et al, 2003).…”

“…This was a critical observation, as constitutive nuclear localization of cyclin D1 (as achieved through mutation of T286) is known to promote oncogenic transformation (Alt et al, 2000). Strikingly, it was demonstrated that cyclin D1b harbors increased transforming capability (as compared to cyclin D1) (Lu et al, 2003;Solomon et al, 2003), thus indicating that cyclin D1b may serve as a more potent oncogene in human cancers.…”

“…Cyclin D1b maintains its capacity to associate with CDK4 (Lu et al, 2003;Solomon et al, 2003), yet may have an altered capacity to drive CDK4 activity. In one set of reported experiments, cyclin D1b/CDK4 complexes immunopurified from cells phosphorylated recombinant retinoblastoma substrate as efficiently as cyclin D1/CDK4 complexes (Lu et al, 2003).…”