Attenuation of proteolysis‐mediated cyclin E regulation by alternatively spliced Parkin in human colorectal cancers

Abstract

Parkin has a critical role in the ubiquitin-proteasome system as an E3-ligase targeting several substrates. Our recent finding that Parkin-deficient mice are susceptible to tumorigenesis provided evidence that Parkin is a tumor suppressor gene. Dysfunction of the Parkin gene is frequently observed in various human cancers, but the mechanism underlying the cell cycle disruption induced by Parkin dysfunction that leads to carcinogenesis is not known. Here, we demonstrated that Parkin expression in colonic epithelial cells is regulated in a cell cycle-associated manner. Epidermal growth factor (EGF) stimulation upregulated Parkin gene expression in human colon cells. Inhibition of the phosphoinositide 3-kinase [PI(3)K]-Akt-dependent pathways suppressed growth factorinduced Parkin expression. The expression of alternatively spliced Parkin isoforms with various deletions spanning exons 3-6 was detected in 18 of 43 (42%) human colorectal cancer tissues. Wildtype Parkin induced the degradation of cyclin E protein, but the alternatively spliced Parkin identified in colon cancers showed defective proteolysis of cyclin E. These findings indicate that Parkin expression is induced by growth factor stimulation and is involved in the cell cycle regulation of colon cells. Tumor-specific expression of alternatively spliced Parkin isoforms might contribute to enhanced cell proliferation through the attenuation of proteolysismediated cyclin E regulation in human colorectal cancers. ' UICCKey words: Parkin; colorectal cancer; cyclin E; cell cycle; EGF Parkin functions in the ubiquitin-proteasome system as an E3 ligase, facilitating the ubiquitination of target proteins. [1][2][3] Parkin is developmentally regulated with induced expression during cell differentiation, and little to no expression in immature cells. 4 Several putative candidate substrates of Parkin-mediated proteolysis have been identified, including a-synphilin, a-synuclein interacting protein, synphilin-1 and Pael-R. 5,6 Recently, it was revealed that deficiency of Parkin potentiates the accumulation of cyclin E protein in cultured postmitotic neurons exposed to the glutamatergic excitotoxin kainate, suggesting that the cell cycle regulator, cyclin E, is a putative substrate of the Parkin ubiquitin ligase complex in neurons. 7 Since Parkin was first identified as a gene implicated in autosomal recessive juvenile Parkinsonism, one form of familial Parkinson disease, 8 attention has been focused on unveiling the role of Parkin in neurons. The expression and regulation of Parkin in epithelial cells as well as the role of Parkin in cell cycle regulation, however, has not been clarified.A loss of heterozygosity within chromosomal region 6q25-q27 containing the Parkin gene is frequently observed in various human tumors, including ovarian, 9-11 breast, 12 renal 13 and lung cancers. 14,15 In addition, the Parkin gene is frequently deleted in breast, ovarian and liver cancers. [16][17][18] We recently demonstrated that Parkin-deficient mice lacking exon 3 of the Parkin ge...

Parkin Enhances the Expression of Cyclin-dependent Kinase 6 and Negatively Regulates the Proliferation of Breast Cancer Cells*

“…Surprisingly, the growth promoting PI3/Akt signaling pathway appears to be up-regulated in MCF7 cells stably expressing parkin. This is intriguing given our observation that parkin negatively regulates MCF7 cell proliferation, although a correlative relationship between Akt activation and parkin expression has been previously reported by others (18,26). Conceivably, a network of other players might be involved in parkin-mediated suppression of breast cancer cell proliferation.…”

“…Parkin apparently ubiquitinates cyclin E and promotes its turnover. Moreover, recent studies demonstrated defects in cyclin E proteolysis in colon and brain cancer cells harboring dysfunctional parkin (12,18). We were therefore interested to examine whether cyclin E expression is affected in parkin-expressing MCF7 cells.…”

Alternative Spliced Variants as Biomarkers of Colorectal Cancer

“…Parkin spliced isoforms might cause cyclin E protein accumulation, leading to the progression of colorectal cancers [97].…”

Parkin Pathway Activation Mitigates Glioma Cell Proliferation and Predicts Patient Survival

“…It is therefore attractive to suggest that altered cyclin E proteolysis in parkin-deficient cancer and neuronal cells may underlie their respective susceptibility to undergo cell division and degeneration. Indeed, cyclin E accumulation is associated with parkin deficiency in several cancer cell lines as well as in cultured postmitotic neurons (8,11,12), although this phenomenon is not universally observed (9,13). Interestingly, parkin also exhibits an apparent E3-independent function in the control of gene transcription.…”