CD44 is a cell adhesion
glycoprotein that also governs cell signaling. Dysregulated CD44 expression characterizes most human
cancers, including
prostate cancer (PCa). PCa loses expression of CD44 standard (CD44s) that is present in benign epithelium, and overexpresses the novel splice variant (v)
isoform, CD44v7-10. We studied CD44 in PCa for more than a decade, and in a series of papers, established its functional significance. Using retrovi-ral gene delivery to PC-3M PCa cells, we expressed
luciferase-only, enforced CD44s re-expression as a fusion
protein with
luciferase at its C-terminus or as a
protein separate from
luciferase, or knocked down CD44v7-10 by RNAi. Invasion, migration, proliferation, soft
agar colony formation, adhesion,
Docetaxel sensitivity, and xenograft growth assays were carried out. Compared to
luciferase-only PC-3M cells, all 3 treatments reduced invasion and migration. Growth and soft
agar colony formation were reduced only by re-expression of CD44s as a separate or fusion
protein but not CD44v7-10 RNAi.
Hyaluronan and
osteopontin binding were greatly strengthened by CD44s expression as a separate
protein, but not a fusion
protein. CD44v7-10 RNAi in PC-3M cells caused marked sensitization to
Docetaxel; the 2 CD44s re-expression approaches caused minimal sensitization. In limited numbers of mouse subcutaneous xeno-grafts, all 3 alterations produced only nonsignificant trends toward slower growth compared with
luciferase-only controls. In further work, we tested the effects of the anti-growth compound
silibinin, a milk thistle derivative. Using a
luciferase promoter construct to test for CD44 promoter activity,
silibinin significantly and dose-dependently inhibited promoter activity at physiologic doses. Total CD44
RNA and CD44v7-10
RNA were significantly decreased; both were also decreased at the
protein level. Phenyl-methylene
hydantoins (PMH),
guanidine alkaloids derived from Red Sea sponges, have the ability to increase cell-cell adhesion in
prostate cancer cells and reduce invasion. Expression of CD44 total
mRNA and CD44v7-10 were markedly decreased by PMH and its S-ethyl derivative. The oncogenic mi-croRNAs, miR-373 and miR-520c, which interact with CD44, were studied in
prostate cancer cells and human tissues. We found that they bound the
3' untranslated region of the CD44
RNA, and suppressed CD44 in
prostate cancer, by preventing the translation of CD44
RNA, rather than by degrading the
RNA. Thus, stable re-expression of CD44s reduces PCa growth and invasion in vitro, and possibly in vivo, suggesting CD44's potential as gene therapy. Finally, CD44v7-10 may be a target for chemosensitization, and plays a role in nutraceutical abrogation of
tumor development. In vivo effects of CD44 alteration still need to be investigated by use of orthotopic or renal
capsule xenografts, which confer a different stromal microenvironment than that of the subcutaneous grafts.