Effects of rituximab on lipids and rafts
Lipid components seem to play an important role in rituximab signaling through CD20. Binding of rituximab results in redistribution of CD20 to lipid microdomains or “rafts” in the membranes of cells from a Burkitt lymphoma cell line (
11). In addition, deletion analyses have identified a membrane-proximal sequence in the cytoplasmic carboxyl tail of CD20 as being responsible for CD20 relocalization into rafts (
12). This redistribution modifies raft stability and organization, and it modulates the associated signaling pathways (
11). The clustering of CD20 into raft microdomains results in an apoptotic response mediated through an
src kinase-dependent pathway (
13). The integrity of lipid rafts and their association with CD20 and consequent activation of
src kinases are all dependent on cholesterol, with cholesterol depletion inhibiting rituximab-induced apoptosis (
14). In follicular lymphoma (FL) cell lines, we found that rituximab inhibits B-cell–receptor signaling by preventing B-cell–receptor relocalization into rafts and decreasing raft-associated cholesterol content (
15).
CD20 translocation into rafts seems to be necessary for calcium mobilization and for rituximab-induced apoptosis in Burkitt lymphoma cell lines (
16). However, it has also been shown that anti-CD20 antibodies induce cell death through a caspase- and raft-independent mechanism (
17), and a recent study showed that, at supersaturating doses, calcium release induced by rituximab (without crosslinking) is independent of both rafts and
src kinases (
18).
In the absence of crosslinking, rituximab binding to CD20 can also induce a rapid, transient increase in acid-sphingomyelinase activity and ceramide generation in raft microdomains in Burkitt lymphoma and FL cell lines (
19). The consequent increase in intracellular ceramide concentration can result, through protein kinase C ζ (PKCζ)-mTOR module inhibition, in a decrease in growth and survival (
20).