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Brown University, Providence, Rhode Island 02912 [I. K. K.]; Mayo Clinic, Scottsdale, Arizona 85259 [S. R. R., S. J. G.]; and Laboratory Medicine and Pathology [L. J. B., S. C. Z., P. C. R.] and Department of Medicine, Division of Gastroenterology [W. E. K.], Mayo Clinic, Rochester, Minnesota 55905
| ABSTRACT |
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, ß1, and
, as well as dysplasia-specific punctate nuclear
staining of PKC µ. We conclude that reduced protein expression of PKC
, ß1, and
, and nuclear localization of PKC µ are markers of,
and are perhaps involved in, adenomatous transformation induced by APC
inactivation in ApcMIN mice. | Introduction |
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, PKC ß1, PKC
, and PKC
are frequently lower
in colorectal cancers of humans and carcinogen-treated rodents than in
surrounding normal mucosa (2, 3, 4, 5, 6)
. In colorectal cancer
cell lines, forced expression of PKC
or PKC ß1 inhibits growth
and tumorigenicity (7
, 8)
, whereas activation of PKC
induces apoptosis (9)
. In contrast, activation or
overexpression of PKC
stimulates proliferation in colon cancer
cells (9)
and induces transformation of rat colonic
epithelial cells and other cell types (10
, 11)
. Similarly,
expression of a PKC ß2 transgene in murine intestine enhances
formation of carcinogen-induced preneoplastic lesions and appears to
activate the Apc/ß-catenin pathway in vivo
(12)
. Thus, PKC isozymes exhibit features of tumor
suppressor genes (PKCs
, ß1, and
) and proto-oncogenes (PKCs
ß2 and
) in intestinal epithelium. However, the potential links
between these activities and the molecular events involved in
triggering progression of colorectal carcinogenesis remain unclear. Among the earliest and most common molecular changes during colorectal carcinogenesis is inactivation of the APC tumor suppressor gene product (13) . To determine whether adenomatous transformation induced by APC loss is associated with altered expression and/or localization of PKC isozymes, we examined the levels and distributions of PKC isozymes by immunohistochemistry of fresh frozen normal ileal mucosa and adenomas from ApcMIN mice.
| Materials and Methods |
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), 1:500; sc-209 (PKC ß1), 1:100; sc-210-G (PKC ß2), 1:100,
sc-211 (PKC
), 1:100; sc-213 (PKC
), 1:200; sc-214-G (PKC
),
1:50; sc-215-G (PKC
), 1:50; sc-935 (PKC µ), 1:200; sc 212-G (PKC
), 1:300; and sc-7262 (PKC
), 1:500.
Immunohistochemistry.
All animal procedures were approved by the Mayo Clinic (Scottsdale, AZ)
Institutional Animal Care and Use Committee. Immediately after
CO2 asphyxiation, ileal tissues from eight 60- to
80-day-old ApcMIN and eight 60- to
80-day-old wild-type C57BL/6J mice were removed and photographed.
Representative 1-cm pieces of normal and adenoma-containing tissues
were immediately embedded in Tissue-Tek II cryogenic embedding medium
(Scientific Products, McGaw Park, IL) and snap frozen. Sections (5
µm) were thaw-mounted onto silanized glass slides, air dried, fixed
in acetone, and postfixed in 1% paraformaldehyde. After inhibition of
endogenous peroxidase activity with 0.01% azide/3% hydrogen peroxide,
slides were blocked with NGS/PBST for 10 min followed by incubation at
22°C for 30 min with primary antibody diluted in NGS/PBST with or
without preabsorption by respective epitope competitory peptide
(10-fold excess by weight). After rinsing with water, slides were
incubated 20 min at 22°C with biotinylated swine antirabbit or swine
antigoat IgG F(ab')2 (Dako, Carpinteria, CA)
diluted 1:200 in NGS/PBST, rinsed with water, and then incubated with
Dako peroxidase-labeled streptavidin (P397) diluted 1:300 in 1%
NGS/PBST for 20 min at 22°C. After thorough rinsing, slides were
incubated for 30 s in 0.1 M sodium acetate
buffer, pH 5.2, followed by 15 min of incubation with 0.02%
3-amino-9-ethylcarbazole in 50 mM sodium acetate,
pH 5.2, and 3% hydrogen peroxide. After a final rinse, slides were
counterstained with Mayers hematoxylin, and coverslips were secured
with Kaisers glycerin jelly.
Intensities of PKC isozyme immunoreactivity in each of eight areas of the ileum were blindly and subjectively scored relative to intensities after preabsorption with competitory epitope peptide on an arbitrary 03 scale (0, none; 1, weak; 2, moderate; 3, strong). The eight areas examined included: crypt; lower, middle, and upper third of the villi; adenomas; lamina propria; smooth muscle; and myenteric plexus. For each isozyme, at least three mice were evaluated for each of the eight areas to assure reproducible results. Digital photographs were taken using a Zeiss Axiovert S100TV microscope and a Spot digital camera (Diagnostic Instruments, Inc., Sterling Heights, MI). Images were batch-processed for consistent color balance and contrast using Adobe Photoshop, version 4.0.
| Results |
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,
, and
(Figs. 1
(Fig. 3)
|
|
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, ß1,
ß2,
, and µ each showed higher staining intensity in villi
compared with crypts. Staining was diffusely cytoplasmic for PKCs
,
ß1, and
but predominantly localized to the apical cytoplasm or
brush border for PKCs ß2 and µ (Figs. 1
was primarily nuclear within the proliferative zone of
crypts and predominantly cytoplasmic toward the villus tips (Figs. 1
immunostaining was moderately strong along the apical
cytoplasm of villus enterocytes (Figs. 2
(Figs. 2
,
, or
were low or undetectable in the epithelial layer of the ileum (not
shown).
|
, and µ;
slightly reduced staining for PKC
; and markedly reduced staining
for PKCs
, ß1, and
(Table 1
in adenomas was
primarily localized within nuclei as seen in normal cryptal cells (Fig. 4)| Discussion |
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, ß1, and
, as well
as dysplasia-specific nuclear localization of PKC µ, among adenomas
compared with normal mucosa. Given that the common denominator of
adenomas in ApcMIN mice is absent
expression of the remaining wild-type APC allele (14)
,
these adenoma-specific alterations should reflect downstream effects of
abrogated APC function. Transgenic overexpression of PKC ß2 in murine colon increases colonic proliferation, accelerates formation of carcinogen-induced colon tumors, and appears to activate the Apc/ß-catenin pathway (12) . We found similar levels and patterns of expression of PKC ß2 in adenomas and normal ileal mucosa of ApcMIN mice, suggesting that although PKC ß2 may be an upstream activator of the Apc/ß-catenin pathway, it appears not to be a downstream target of this pathway and may not play a role in adenoma formation induced by Apc loss.
Reduced protein levels of PKCs
, ß1, and
are reported in
colorectal cancers of humans and carcinogen-treated rodents
(2, 3, 4, 5
, 15)
. Our data indicate that reduced expression of
these isoforms occurs early during premalignant adenomatous stages
after APC loss. All three of these isozymes exhibit properties of tumor
suppressor genes when overexpressed in transformed cells (7
, 8
, 16)
, suggesting that the tumor suppressor functions of APC and
PKCs
, ß1, and/or
could be mechanistically linked. Future
studies are needed to determine whether the suppressed levels of these
isozymes are caused by transcriptional, translational, or
posttranslational events and how they may be linked to APC loss. If
abrogated expression of one or more of these isoforms proves to be
important for adenomatous transformation induced by APC loss, then
pharmacological induction of their expression or activity may prevent
adenomas in ApcMIN mice, as well as
adenomas associated with familial adenomatous polyposis and most
sporadic adenomas in humans.
Among our most interesting and novel observations was the
dysplasia-specific localization of PKC µ to discrete sites within
nuclei (Figs. 2
3
4)
. Among PKC isozymes, PKC µ is the least well
characterized. It is calcium independent and is activated by
diacyl-glycerol, so it shares features of the "novel" PKC
subgroup (
,
,
,
) (17)
. However, several
features are unique to PKC µ, including the presence of a pleckstrin
homology domain, lack of a pseudosubstrate domain, activation by
heparin sulfate, and inhibition by basic proteins that ordinarily serve
as substrates for PKCs including p53, myelin basic protein, and histone
H1 (18
, 19)
. PKC µ is overexpressed in many malignancies
(18)
. The apparent translocation of PKC µ from the
cytoplasm in normal ileal epithelial cells to the nucleus in adenomas
suggests that this isozyme is activated during adenomatous
transformation. Further studies are required to determine whether
dysplasia-specific nuclear localization of PKC µ occurs in human
adenomas, whether this isozyme plays a role in adenomatous
transformation, and how it may be regulated by APC loss.
PKC
, which is generally regarded to be neuron specific, was
detected in nuclei of cells predominantly within the proliferative zone
of normal crypts and in all nuclei of adenomas (Figs. 1
2
3
4)
. PKC
mRNA is expressed in some human colorectal cancers (20)
and is activated by insulin-like growth factor 1 in HT-29 colorectal
cancer cells (21)
. Together, these observations support
the hypothesis that PKC
plays a role in growth factor-mediated
signaling in normal and adenomatous intestinal epithelium.
Our observation of increased expression of PKCs
, ß2,
, and
toward the villus tips in normal ileal mucosa of
ApcMIN mice is consistent with previous
studies in normal rat ileum and supports the notion that these isozymes
play roles in postmitotic processes (22)
. We additionally
observed strong specific staining for PKC
in isolated cells within
the crypts. The cell type and significance of PKC
expression in this
distinct epithelial compartment remain unknown.
Our analysis of PKC isozymes in ApcMIN mice revealed several novel observations related to their distributions of expression in normal ileum and adenomas in APCMIN mice. We speculate that the adenoma-specific changes we observed are downstream effects of APC loss that may participate in adenomatous transformation. Future work will help determine the importance of these alterations as early events of colorectal carcinogenesis and as potential targets of preventative and therapeutic interventions.
| FOOTNOTES |
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1 Supported by NIH Grants R29 CA71974
(W. E. K., Jr.), K11 CA01674 (W. E. K., Jr.), and RO1 CA64389
(S. J. G.); the Mayo Foundation; and the Summer Undergraduate
Research Fellowship (SURF) (I. K. K.). ![]()
2 To whom requests for reprints should be
addressed, at Alfred GI Research Unit, 2435 SMH, Mayo Clinic, 200
First St. SW, Rochester, MN 55905. Phone: (507) 284-6635; Fax: (507)
255-6318; E-mail: arnes.william{at}mayo.edu ![]()
3 The abbreviations used are: PKC, protein kinase
C; APC, adenomatous polyposis coli; NGS/PBST, 5% normal goat
serum/PBS/0.05% Tween 20, pH 7.4. ![]()
Received 2/10/00. Accepted 3/ 3/00.
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expression increases cellular proliferation, decreases differentiation, and enhances the transformed phenotype of CaCo-2 cells. Cancer Res., 58: 1074-1081, 1998.
and
in NIH 3T3 cells induces opposite effects on growth, morphology, anchorage dependence, and tumorigenicity. J. Biol. Chem., 268: 6090-6096, 1993.
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