Page 167 - Vitamin D and Cancer

154 E. Gocek and G.P. Studzinski

Transformed keratinocytes which give rise to SCC tend to be resistant to the
differentiation-inducing action of 1,25D [107, 108], even though apoptosis and cell
cycle arrest induced by 1,25D have been demonstrated in models of SCC [109, 110].
While VDR expression is required for 1,25D-induced differentiation, the resistance
of SCCs to 1,25D is not due to the lack of functional VDR [111]. The possible
explanations for the 1,25D resistance include the finding that the VDRE in the
human PLCg-1 gene is not functional [111]. Another explanation for the resistance
is that increased serine phosphorylation of retinoid X receptor alpha (RXRa) by the
Ras/MAPK pathway leads to its degradation, and thus VDR loses its heterodimeric
partner for gene transactivation [112]. Yet another possibility is that VDR coactiva-
tors in SCCs are not appropriate for transactivation of differentiation-inducing genes
[95]. Specifically, it was suggested that the expression of differentiation markers
required a complex of VDR with the Src family of coactivators [113], but in SCC
the DRIP coactivator complex is overexpressed, and there is a failure of SCCs to
switch from DRIP to Src, resulting in the inability to express genes required for dif-
ferentiation. It would be interesting to learn if this model has a wider applicability.

7.2.5 Osteosarcoma and Osteoblasts

Differentiation, as well as growth inhibition, has been documented in 1,25D-treated
human and rat osteosarcoma cells [114, 115]. The differentiation was recognized by a
morphological change to the chondrocyte phenotype, and by increased Alk Pase stain-
ing. The presence of Alk Pase or osteocalcin could also be detected at the mRNA
level [115]. In human fetal osteoblastic cell line responsive to 1,25D, mineralized
nodules were detected [116], demonstrating that an advanced degree of differentia-
tion can be achieved in this cell system. Interestingly, 1,25D-induced differentiation
in osteoblasts and osteocytes is accompanied by an increase in the potential for cell
survival through enhanced anti-apoptotic signaling [117]. It is possible that this is
mediated by EGFR-relayed signals, as in contrast to other cell types [32, 62, 118],
1,25D-treated osteoblastic cells show increased levels of EGFR mRNA [119].
Recent studies suggest that the anti-apoptotic effects of 1, 25D on osteoblasts and
osteocytes are mediated by Src, PI3K, and JNK kinases [117]. The suggested
mechanisms include an association of Src with VDR, though transcriptional mecha-
nisms were required, as shown by an inhibition of the biological effect by exposure
to actinomycin D or cycloheximide. The association of VDR with other proteins may
be particularly important in osteoblast cells induced to differentiate by 1,25D, as another
group reported that IGF-binding protein-5 (IBP-5) interacts with VDR, and blocks
the RXR/VDR heterodimerization in the nuclei of MG-63 and U2-OS cells, thus
attenuating the expression of bone differentiation markers [120]. Also, in ROS 17/28
cells the NFkB p65 subunit integrates into the VDR transcription complex and dis-
rupts VDR binding to its coactivator Src-1 [121]. Although protein–protein binding
between VDR and p65 has not been demonstrated, this remains a possibility, further
highlighting the importance of this mode of control of VDR activity.