The Role of Sphingosine Kinase 2 in Apoptosis of Human Breast Cancer Cells
Annual summary rept. 5 Apr 2004-4 Apr 2007
VIRGINIA COMMONWEALTH UNIV RICHMOND
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Two isoforms of sphingosine kinase SphK catalyze the formation of sphingosine-1-phosphate S1P. Whereas, SphK1 stimulates cell growth and survival, it was found that when overexpressed in mouse NIH 3T3 fibroblasts SphK2 enhances caspase-dependent apoptosis in response to serum deprivation, independently of S1P receptors. Sequence analysis revealed that SphK2 contains a 9 amino acid motif similar to that present in BH3-only proteins. Studies showed that the BH3-only domain and catalytic activity contribute to the apoptotic effects of overexpressed SphK2. Further studies in human carcinoma cells showed that overexpression of SphK2 increased the expression of the cyclin dependent kinase cdk inhibitor p21, but interestingly had no effect on p53 or its phosphorylation. Correspondingly,down regulation of endogenous SphK2 with small interfering RNA siRNA targeted to unique mRNA sequences decreased basal and doxorubicin-induced expression of p21 without affecting p53. In addition, down regulation of SphK2 decreased G2M arrest in response to doxorubicin. Surprisingly however, siSphK2 markedly enhanced apoptosis induced by doxorubicin in MCF7 cells. This result raises the question of how overexpression of SphK2 decreases cell growth and enhances apoptosis while its down regulation sensitizes cells to apoptosis. A partial answer may come from the possibility that when SphK2 is overexpressed it does not always have the same subcellular distribution as the endogenous protein. It may also be possible that proteolysis of overexpressed SphK2 might induce apoptosis due to liberation of its BH3 peptide domain, which does not occur at the levels at which endogenous SphK2 is expressed. Collectively, these results demonstrate that endogenous SphK2 is important for p53-independent induction of p21 expression by doxorubicin and suggest that SphK2 expression may influence the balance between cytostasis and apoptosis.
- Medicine and Medical Research