Alternative splicing disrupts a nuclear localization signal in spleen tyrosine kinase that is required for invasion suppression in breast cancer.
Spleen tyrosine kinase (Syk) is a candidate tumor (metastasis) suppressor that is highly expressed in mammary epithelial cells. Loss of Syk expression through promoter hypermethylation is associated with increased invasiveness in a subset of breast cancer. Here, we show that in addition to full-length Syk [Syk(L)], an alternatively spliced variant, Syk(S), is frequently expressed in breast cancer cells. Syk(S) is identical to Syk(L), except that it lacks 23 amino acid residues (deletion) within the interdomain B (IDB) of Syk. We also show that the aberrant expression of Syk(S) occurs frequently in primary breast tumors but never in matched normal mammary tissues, suggesting a contribution of Syk(S) to mammary tumor progression. Expression of Syk(L) suppressed breast cancer cell invasiveness. In contrast, Syk(S) expression did not affect the cell invasion potential. This differential phenotypic response is accompanied by their different subcellular localization. Immunocytochemical studies and nuclear and cytoplasmic fractionation experiments indicated that Syk(L) could enter the nucleus, whereas Syk(S) was located exclusively in the cytoplasm. Five basic residues in deletion were found to be critical in determining Syk(L) nuclear transport and invasion suppression activity; mutations completely excluded Syk(L) from the nucleus and blocked Syk(L)-inducible invasion suppression. Moreover, IDB acted as an autonomous nuclear localization signal to facilitate nuclear transport of a heterologous protein. Thus, the IDB of Syk(L) contains a nuclear localization signal that is responsible for Syk(L) nuclear translocation. The correlation of the nuclear localization and invasion suppression function of Syk(L) indicated that nuclear Syk possesses biological activities associated with tumor suppression in mammary epithelial cells.
Wang
Lei
L
Department of Molecular Pathology, The University of Texas M. D. Anderson Cancer Center, 1515 Holcombe Boulevard, Houston, TX 77030, USA.
Duke
Lindsay
L
Zhang
Peter S
PS
Arlinghaus
Ralph B
RB
Symmans
W Fraser
WF
Sahin
Aysegul
A
Mendez
Richard
R
Dai
Jia Le
JL
D017398:Alternative Splicing; D000595:Amino Acid Sequence; D001943:Breast Neoplasms / Q000201:enzymology* / Q000235:genetics / Q000473:pathology; D002467:Cell Nucleus / Q000378:metabolism / Q000502:physiology; D004792:Enzyme Precursors / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D006801:Humans; D047908:Intracellular Signaling Peptides and Proteins; D008969:Molecular Sequence Data; D009361:Neoplasm Invasiveness; D019913:Nuclear Localization Signals / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D020033:Protein Isoforms; D017434:Protein Structure, Tertiary; D011505:Protein-Tyrosine Kinases / Q000096:biosynthesis / Q000235:genetics / Q000378:metabolism / Q000502:physiology*; D000072377:Syk Kinase; D014162:Transfection; D014407:Tumor Cells, Cultured
D004792:Enzyme Precursors; D047908:Intracellular Signaling Peptides and Proteins; D019913:Nuclear Localization Signals; D020033:Protein Isoforms; D011505:Protein-Tyrosine Kinases; C000606896:SYK protein, human; D000072377:Syk Kinase