Cyclin D1 regulation of mitochondrial function in breast cancer

4472 Cyclin D1 is overexpressed in human breast cancers and is sufficient for the development of murine mammary tumors. The cyclin D1 gene encodes a regulatory subunit of the holoenzyme that phosphorylates and inactivates the pRb tumor suppressor protein to promote nuclear DNA synthesis. In addition to the ability of cyclin D1 to form a holoenzyme with cdk4/6, we and others have previously shown that cyclin D1 regulates distinct cellular functions through physical association with nuclear receptors and transcriptional coregulators. In knockdown experiments using siRNA against cyclin D1 we observed that both mitochondial size and activity were increased, this observation was validated using cyclin D1 anti-sense transgenic mice. Mammary epithelial cells obtained from cyclin D1 anti-sense transgenic mice demonstrated induction of genes governing mitochondrial function and glycolysis. Reciprocal expression of these genes was observed in mammary tumors induced by mammary gland targeted cyclin D1 overexpression. Global gene expression profiling and functional analysis of mammary epithelial cell-targeted cyclin D1 anti-sense transgenics demonstrated cyclin D1 inhibits mitochondrial activity, and aerobic glycolysis in vivo . We further examined the mitochondrial components including mitochondrial transcriptional factor A (mtTFA) and mitochondrial nuclear respiratory factor 1 (NRF-1) that are key regulators of mitochondrial DNA synthesis and function. Cyclin D1 repressed expression of mtTFA and inhibited D-loop transcriptional activity. NRF-1, which induces nuclear-encoded mitochondrial genes, was transcriptionally repressed and its activity was inhibited by cyclin D1. Cyclin D1 levels and NRF-1 expression were inversely correlated during cell cycle progression. In addition, NRF-1- and cyclin D1-regulated genes were inversely correlated by microarray expression profiling. Cyclin D1 associated with NRF-1 in vivo by immunoprecipitation and in mammalian two-hybrid assays. Screening for the potential phosphorylation site of NRF-1 demonstrated that cyclin D1-dependent kinase phosphorylated NRF-1 at Serine 47. In summary, in addition to regulating nuclear DNA synthesis, cyclin D1 regulates mitochondrial function in vivo , coordinating metabolic substrate utilization within the cell.