SPARC, also known as osteonectin, is a highly conserved between species phosphoprotein that has been recently identified as a tumor suppressor gene. Besides its involvement in bone metabolism, where it mediates collagen deposition, it has been shown that SPARC plays several roles, ranging from cell adhesion to inhibition of cell-cycle progression and VEGF activation. In mice, it has been reported that implanted tumors grow faster in SPARC-null animals. In humans, patients affected by MDS 5q-syndrome responsive to lenalidomide, have an increase of SPARC expression.
Chronic myeloid leukemia (CML) is a disease characterized by uncontrolled proliferation and adhesion defect. In our study we investigated the expression of SPARC mRNA by quantitative PCR in 22 CML patients. mRNA SPARC relative expression in mononuclear cells isolated from peripheral blood at diagnosis was significantly lower (range 0.004-0.98, mean 0.3, median 0.21, SD 0.3) than in normal control subjects assumed as a reference (range 0.31-2.5, mean 1, median 0.8, SD 0.8).
During imatinib treatment, SPARC mRNA increased of 2 logs at 3 months and of 3 logs at 1 year. In one patient that had to stop imatinib treatment a decrease of expression to levels comparable to normal subjects, around 1 log higher than pretreatment levels, was noticed; when the patient restarted treatment with the tyrosine kinase inhibitor, SPARC level raised of 2 logs again.
Bcr/Abl+ K562 cell line has a low level of SPARC mRNA (0.4 relative expression compared to pooled normal controls), probably due to promoter hypermethylation. Treatment of K562 with an hypomethylating agent, such as 5-azacytidine, for 72 h resulted in an increased expression of SPARC mRNA of 1 log. Moreover, K562 proliferation at 72 h was inhibited by adding esogenous SPARC to colture medium with an IC50 of 50 mcg/ml. In the presence of imatinib 1 μM, SPARC IC50 was lowered to 30 mcg/ml.
Taken all together these data suggest that SPARC deficiency in CML could play a role in excess growth, cell adhesion and angiogenesis impairment. Furthermore, its upregulation after imatinib treatment may be related to the alteration of bone metabolism that is observed in CML patients treated with this tyrosine kinase inhibitor.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.