Among the first observations of increased cell growth by a death receptor was that of TNF-α costimulation of T and B cell growth by Lipsky and coworkers (24
). Many other reports confirm that in cell types as diverse as vascular smooth muscle (26
) and dendritic (27
), TNF-α can induce differentiation or activate cell function, if not promote cell growth. That TNF-α might confer opposing functions of cell growth and differentiation and death has met with less resistance than similar claims for other death receptor ligands, since two TNFR types have long been known, one (p55 TNFR1) bearing a death domain, and the other (p75 TNFR2) lacking this sequence. However, it does not appear that these two receptor types specialize in the manner that was originally supposed, since recent studies show that TNFR2 can confer signals for both death and growth in T cells (28
). The levels of the TNFR-associating kinase receptor interacting protein (RIP) seem to be pivotal in this switch in T cells. RIP levels are low in resting T cells, which allows TNF-α signals to promote growth, whereas RIP levels increase with cell cycling and confer sensitivity to cell death (28
). In the case of Fas signaling, despite the existence of only a single receptor (Fas), there are several instances of signals by this classically proapoptotic molecule promoting increased growth of T cells (29
), fibroblasts (28
), certain tumors (31
), hepatocytes (32
), and increased differentiation of dendritic cells (33
). Fas may also induce the physiological and morphological changes in cardiomyocytes seen in cardiac hypertrophy. Thus, Badorff et al. recently observed that Fas ligation of cardiac myocytes leads to phosphorylation of glycogen synthase kinase-3β, which results in its inactivation and increased cardiac protein synthesis (34
). This process is necessary for cardiac hypertrophy, and the authors found that in a model of cardiac overload, which ordinarily leads to hypertrophy, the hearts of Fas-deficient lpr
mice fail to adapt and instead undergo dilatation. Each of these cases is worth close scrutiny to determine whether alternate interpretations of the findings are possible.
The initial suggestion that Fas might promote growth signals was inspired by work of David Lynch’s group at Immunex Research and Development Corp. (Seattle, Washington, USA), showing that antibodies to Fas that were cytolytic toward tumor cells and cycling T cells were powerfully costimulatory for proliferation and cytokine production with CD3 activation of resting T cells (29
). While there remained the possibility that this might result from the antibodies blocking ligation by endogenous FasL, many of the costimulatory anti-Fas antibodies used were cytolytic on other cells. More recently, this concern was resolved by similar findings using soluble FasL in place of antibodies. Kennedy et al. (35
) showed that proliferation of CD3-activated primary human T cells can be augmented threefold by cross-linked, but not by uncross-linked, FasL, suggesting that oligomerization is necessary to augment proliferation, much as it is with cell death. While both studies used purified T cells, it is still possible that this effect might depend on an accessory cell, for example, on a minor contamination with dendritic cells. Recently, researchers in two groups observed not only that both mouse and human dendritic cells resist FasL-induced cell death, but also that, in these cells, FasL actually induces upregulation of surface B7.1, B7.2, and MHC class II (33
). Later studies have shown that dendritic cells express high levels of the Fas inhibitor FLIP (36
), which may help explain the diversion of signals from cell death and toward a growth signal pathway.
In related T cell work, immobilized anti-Fas or soluble FasL alone has been reported to promote proliferation of T cells from patients with systemic lupus erythematosus (37
). Furthermore, caspase inhibitors partially blocked this augmented growth. The observations are consistent with those of Suzuki and coworkers (38
), who observed that Fas-Fc can attenuate the proliferation of murine T cells. These latter findings might indicate that blocking FasL with Fas-Fc inhibits Fas costimulation, or, as favored by the authors, that Fas-Fc can induce FasL to exert retrograde positive signals. Another report observed that anti-Fas can stimulate a nearly tenfold increase in proliferation of normal human skin fibroblasts, in a manner similar to that of TNF-α (39
), whereas only TNF-α stimulates IL-6 production. As the cultures in this study appear to have been confluent, it is unclear whether some of this effect might result from the death of some fibroblasts by anti-Fas, which would then make room for the remaining cells to re-enter cell cycling.
Two reports indicate that Fas can induce growth of tumor cells. In one study of various B cell lymphoma cell lines, whereas most were sensitized to undergo apoptosis by anti-Fas after priming with Staphylococcus aureus
Cowan I (SAC) plus IL-2, one patient’s lymphoma cells consistently manifested increased proliferation to anti-Fas (40
). This lymphoma was also the only one studied in which Bcl-2 levels did not drop following treatment with SAC and IL-2. It is not clear whether Bcl-2 levels contribute to this unusual feature, since Bcl-2 does not activate cell proliferation, and it generally does not block Fas-induced death in lymphocytes. A second study examined a variety of tumors and observed that only 4 of the 11 Fas-positive nonhematopoietic tumors were sensitive to killing by anti-Fas (31
). Moreover, anti-Fas enhanced growth of 3 of the 11 tumors, including one epidermoid carcinoma, one melanoma, and one pancreatic carcinoma. In these studies, resistance to Fas and enhanced proliferation did not correlate with levels of Bcl-2 expression.
Two further situations, somewhat more complex given their in vivo settings, also raise the possibility of Fas-induced cell growth. Biancone et al. (41
) found that the subcutaneous slow release of agonistic Fas antibody from a matrix gel in mice can promote angiogenesis, activating endothelial cell infiltration and canalization, as well as a subsequent inflammatory infiltration of neutrophils in the new blood vessels. This phenomenon was dose-dependent and required interaction with Fas, as Fas-deficient lpr
mice did not respond in this manner. In addition, apoptotic cells were not observed at any time inside the implant or in the surrounding tissue. In another study, examining liver regeneration in mice after partial hepatectomy (32
), whereas anti-Fas in vivo induced rapid hepatocyte apoptosis as observed earlier (42
), the same Fas antibody increased cell cycling of hepatocytes during liver regeneration. This change in the response to anti-Fas between resting and regenerating hepatocytes is reminiscent of the differential effects of TNFR1, which also signals cell death in resting hepatocytes (43
) but stimulates proliferation during liver regeneration (44
). Desbarats and Newell (32
) found that the change correlates with higher levels of FLIP in regenerating versus resting hepatocytes after treatment with anti-Fas, and they noted that regeneration is significantly delayed in lpr
mice. The source of FasL during liver regeneration was not addressed in this study, but it may be significant that athymic nude mice, bearing few T cells, manifest delayed liver regeneration (45
), and that activated T cells are resident in the liver (46
Some of these studies leave open the possibility that anti-Fas or FasL could cause the appearance of increased cell growth by actually causing cell death of some cell types, which then might allow the enhanced proliferation of the remaining cells. However, because this alternative explanation is not compelling in all these cases, the question arises as to how Fas might actually promote a cell growth signal.