Page 172 - Vitamin D and Cancer

7 Induction of Differentiation in Cancer Cells by Vitamin D 159

complex. However, it is not clear if the lack of growth factors normally provided by
the serum contributes to the observed effects. The role of the PI3K pathway in
1,25-induced differentiation was further studied by Marcinkowska and colleagues
[162–164], who showed that the activation of PI3K by 1,25D can also be demon-
strated in HL60 cells, and that the signal is transmitted to AKT. This function of
AKT may contribute to the differentiation-related increase in 1,25D-induced cell
survival [139]. An additional role of PI3K, as well as of the Ras/Raf/ERK, pathway
in human leukemia cells is the stimulation of steroid sulfatase activity, an enzyme
that converts inactive estrogen and androgen precursors to the active sex hormones
[147]. If this is also operative in breast and/or prostate tissues, it could offer an
explanation for the mutual activation of VDR and the estrogen and androgen
nuclear receptors, as shown in Fig. 7.2.
The mechanisms of the upregulation of MAPK pathways in the initial phase of
1,25D action on leukemia cells are still unclear. The very rapid effects of 1,25D on
the MAPK pathway in intestinal cells that result in rapid calcium transport (“tran-
scaltachia”) have been attributed to a cell membrane receptor (“mVDR”) [165–167],
but whether direct, non-genomic action of such mVDR can initiate or enhance
MAPK pathways activity in leukemia cells has not been well documented. In non-
starved leukemia cells, 1,25D elicits less rapid (hours rather than minutes) activa-
tion of the MAPKs. One possibility is that this is achieved by the transcriptional
upregulation of Kinase Suppressor of Ras-1 (KSR-1), a membrane-associated kinase/
molecular scaffold, also known as ceramide-activated protein kinase [168, 169].
Although a kinase activity associated with KSR-1 has been reported [170–172], the
best established function of KSR-1 is to provide a platform for Raf-1 kinase to
phosphorylate and thus activate its downstream targets in the MAPK pathways
[173, 174]. Thus, since KSR-1 has been shown to have a functional DNA element
regulated by VDR (VDRE) [175], the activation of the MAPKs may be a direct,
“genomic” action of 1,25D, as depicted in Fig. 7.3, rather than signaling initiating
at the membrane and “non-genomic.”
Our studies [169, 176] combined with those of Marcinkowska and colleagues
[164, 177] suggest that leukemia cell differentiation is initiated when 1,25D promotes
nuclear translocation of liganded VDR, which dimerizes with RXR and transacti-
vates several VDRE-regulated genes, including KSR-1 and KSR-2. The latter
appears to have a role in increasing the survival potential of differentiating mono-
cytic cells [24], while KSR-1 acts as a scaffold, which by simultaneously binding
to Ras and Raf-1 (and perhaps other proteins) facilitates or redirects the signaling
cascade, at least initially, to MEK/ERK, and thus amplifies the signal that initiates
monocytic differentiation (Fig. 7.3).
Raf-1 participation has been shown to be required for the later stages of differ-
entiation, when animpairment in cell cycle progression becomes apparent, and at
this more advanced point of the differentiation process MEK/ERK signaling does
not appear to be involved [178, 179]. While this requires further study, the current
model, also supported by observations in other differentiation signaling systems
[180–182], suggests that Raf-1 can signal p90RSK activation independently of
MEK and ERK, as outlined in Fig. 7.4.