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Unscheduled expression of cyclins D1 and D3 in human tumour cell lines
1Cancer Research Institute, New York Medical College, Valhalla 10593, USA.
Abstract:
D-type cyclins are involved in regulation of cell traverse through G1 primarily by activating the cyclin-dependent kinase 4 (CDK4) and targeting it to the retinoblastoma tumour suppressor protein. There is a vast body of evidence that defective expression of D-type cyclins is associated with tumour development and/or progression. Immunocytochemical detection of D cyclins combined with multiparameter flow cytometry makes it possible to measure the expression of these proteins in individual cells in relation to their cell cycle position without the need for cell synchronization. This approach was used in the present study to compare the cell cycle phase specific expression of cyclins D3 and D1 in human normal proliferating lymphocytes and fibroblasts, respectively, with nine tumour cell lines of different lineage. During exponential, unperturbed growth, expression of cyclin D1 in fibroblasts from donors of different age, or cyclin D3 in lymphocytes, was limited to mid-G1 cells: Less than 7% of the cells entering S phase or progressing through S and G2 were cyclin D positive. In contrast, expression of either cyclin D1 or cyclin D3 in tumour cell lines of different lineage was not limited to G1 phase. Namely, over 80% of the cells in S and G2+M were cyclin D positive in eight of the nine cell lines studied. The data indicate that while expression of cyclin D1 or D3 in normal cells is discontinuous, occurring transiently in G1, these proteins are expressed in some tumour lines persistently throughout the cell cycle. This suggests that the partner kinase CDK4 is perpetually active throughout the cell cycle in these tumour lines.
Insights
D-type cyclins regulate cell cycle progression. In normal cells, cyclins D1 and D3 are transiently expressed in G1, but in many tumor cells, they persist throughout the cell cycle, suggesting continuous CDK4 activity.
Area of Science:
- Cell Biology
- Molecular Oncology
- Biochemistry
Background:
- D-type cyclins (D1, D3) are crucial regulators of the G1 phase of the cell cycle.
- Their activity is primarily mediated through the activation of cyclin-dependent kinase 4 (CDK4).
- Aberrant expression of D-type cyclins is frequently observed in various cancers, implicating them in tumor development and progression.
Purpose of the Study:
- To investigate and compare the cell cycle phase-specific expression patterns of cyclins D1 and D3.
- To analyze these patterns in normal human lymphocytes and fibroblasts versus a panel of nine diverse tumor cell lines.
- To assess the implications of observed expression differences for CDK4 activity in cancer.
Main Methods:
- Utilized immunocytochemical detection of D-type cyclins.
- Employed multiparameter flow cytometry to analyze protein expression relative to cell cycle position.
- Examined expression in synchronized and unsynchronized normal cells and tumor cell lines.
Main Results:
- In normal fibroblasts (cyclin D1) and lymphocytes (cyclin D3), expression was confined to mid-G1 phase, with minimal presence in S or G2+M phases (<7%).
- In contrast, eight out of nine tumor cell lines exhibited persistent cyclin D1 or D3 expression throughout the cell cycle (S and G2+M phases, >80% positivity).
- This sustained expression in tumor cells suggests continuous activation of their partner kinase, CDK4.
Conclusions:
- Normal cell expression of D-type cyclins is transient and restricted to the G1 phase.
- Tumor cells frequently display persistent expression of D-type cyclins across all cell cycle phases.
- Persistent cyclin D expression in tumors indicates continuous CDK4 activity, potentially contributing to uncontrolled cell proliferation and cancer progression.