The inhibitory effect of anti-CD33 monoclonal antibodies on AML cell growth correlates with Syk and/or ZAP-70 expression.
OBJECTIVE
Acute myeloid leukemia (AML) cells express the cell surface antigen CD33 that can function as a downregulator of cell growth, mediating growth arrest and apoptosis. The protein kinase Syk is an essential element in several cascades coupling certain antigen receptors to cell responses. Recently we reported that CD33 recruits Syk for its signaling in AML cell lines. In this study, we further investigated the mechanism(s) of Syk engagement in CD33 signaling in primary AML samples.


METHODS
We investigated 25 primary AML samples for their proliferative response (3H-thymidine incorporation) and biochemical changes (Western blot analysis) to anti-CD33 mAb treatment.


RESULTS
Proliferation studies demonstrated that 14 (56%) of AML samples were responsive (R) while 11 (44%) were nonresponsive (n-R) to inhibitory antibody activity. Seven of 25 AML samples (28%) expressed undetectable levels of Syk. However, cells from two of these patients expressed the ZAP-70 protein kinase. In Syk/ZAP-70(+) samples, CD33 ligation inhibited proliferation in 70% of cases, while none of the Syk/ZAP-70(-) samples was responsive. There were significant biochemical differences between responder and nonresponder AML populations. In responder samples, CD33 ligation induced phosphorylation of CD33 andSyk and formation of the CD33/Syk complex. In nonresponder samples, CD33 was not phosphorylated, and Syk was in complex with the SHP-1 protein phosphatase constitutively.


CONCLUSIONS
Syk is an important component in the regulation of proliferation in AML cells. The differential response of AML cells to CD33 ligation is associated with the level of the Syk expression.
Balaian
Larisa
L
Department of Medicine and Cancer Center, University of California, San Diego School of Medicine, La Jolla, Calif., USA.
Zhong
Rui-kun
RK
Ball
Edward D
ED
D000911:Antibodies, Monoclonal / Q000494:pharmacology*; D015703:Antigens, CD / Q000502:physiology*; D015214:Antigens, Differentiation, Myelomonocytic / Q000502:physiology*; D002455:Cell Division; D004792:Enzyme Precursors / Q000032:analysis / Q000502:physiology*; D018922:HL-60 Cells; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D015470:Leukemia, Myeloid, Acute / Q000378:metabolism / Q000473:pathology*; D011505:Protein-Tyrosine Kinases / Q000032:analysis / Q000502:physiology*; D063268:Sialic Acid Binding Ig-like Lectin 3; D013267:Stilbenes / Q000494:pharmacology; D000072377:Syk Kinase; D051746:ZAP-70 Protein-Tyrosine Kinase
D000911:Antibodies, Monoclonal; D015703:Antigens, CD; D015214:Antigens, Differentiation, Myelomonocytic; C571163:CD33 protein, human; D004792:Enzyme Precursors; D047908:Intracellular Signaling Peptides and Proteins; D063268:Sialic Acid Binding Ig-like Lectin 3; D013267:Stilbenes; C041525:3,3',4,5'-tetrahydroxystilbene; D011505:Protein-Tyrosine Kinases; C000606896:SYK protein, human; D000072377:Syk Kinase; D051746:ZAP-70 Protein-Tyrosine Kinase; C497261:ZAP70 protein, human