Syk is a 72kDa non-
receptor tyrosine kinase that is best characterized in hematopoietic cells. While Syk is pro-tumorigenic in some
cancer cell types, it also has been reported as a negative regulator of metastatic cell growth in others. An examination of the RelA (p65) subunit of NF-κB expressed in MCF7
breast cancer cells indicated that either treatment with
pervanadate or stable expression of Syk protected RelA from
calpain-mediated proteolysis. Similar results were observed with the
tyrosine phosphatase, PTP1B, another sensitive
calpain substrate. The activity of
calpain in MCF7 cell lysates was inhibited by both treatment with
hydrogen peroxide and expression of Syk, the former due to oxidative inactivation of
calpain and the latter to enhanced expression of
calpastatin (CAST), the endogenous
calpain inhibitor. The level of CAST was elevated in the cytosolic fraction of Syk-positive
breast cancer cells resulting in more CAST present in complex with
calpain in cell lysates. The high levels of CAST coincided with elevated basal levels of
calcium-and of intracellular
calpain activity-in Syk-expressing cells resulting from decreased levels of Bcl-2, an inhibitor of IP3-receptor-mediated
calcium release. The inhibition of cellular
calpain stimulated the Syk-mediated enhancement of NF-κB induced by TNF-α, enhanced
tyrosine phosphorylation resulting from
integrin crosslinking, and increased the localization of Syk to the plasma membrane.