Page 548 - Withrow and MacEwen's Small Animal Clinical Oncology, 6th Edition

526 PART IV Specific Malignancies in the Small Animal Patient

this breed could be explained by a single genetic risk factor resid- been investigated in canine OSA cell lines and pet dogs, respec-
ing within an autosome. 71 tively. In cell lines, molecular studies demonstrated that IGF-1R is
expressed and functionally activated by IGF-1 in a paracrine man-
Although the putative contribution of specific genes in
VetBooks.ir canine OSA development and biology has been more thoroughly ner and leads to the activation of both MAPK and AKT signaling
nodes. Importantly, the protein expression of IGF-1R in canine
explored, the role of microRNAs (miR) and their differential
expression, which might contribute to OSA behavior, has recently OSA tissue samples showed that most (71%) naturally occurring
been studied. In a comparative investigation with human and tumors express this receptor and that higher expression levels of
83
canine OSA tissue samples with clinically linked data, decreased IGF-1R correlated with a decreased survival time (ST).
expression of orthologous miR-134 and miR-544 in canine OSA
samples imparted a reduced likelihood of survival and suggest that Epidermal Growth Factor
miR-134 and miR-544 might transcriptionally regulate target The protooncogene erbB-2 encodes human epidermal growth fac-
genes involved in aggressive behavior. Similarly, the overexpres- tor receptor 2 (HER2), which is a tyrosine kinase receptor capable
72
sion of miR-9 in canine OSA cell lines was shown to promote a of promoting cell transformation and growth. In both dogs and
metastatic phenotype through the upregulation of gelsolin, a pro- people, the overexpression of HER2 protein as a result of gene
tein involved in cytoskeletal remodeling. Additionally, the loss amplification is a negative prognostic factor in mammary carci-
73
of miR-34a in canine OSA cell lines resulted in the enrichment of noma; however, less clarity exists for the role of HER2 overex-
genes responsible for cell invasion and motility, whereas the forced pression in OSA. 84,85 To better characterize HER2 expressions in
expression of miR-34a exerted potential anticancer effects through canine OSA, one study evaluated HER2 mRNA transcript and
74
reduced expressions of KLF4, SEM3A, and VEGFA transcripts. protein expression in seven cell lines and 10 OSA tumor speci-
86
Collectively, these initial findings exploring the regulatory role of mens. Based on RT-PCR, six of the OSA cell lines and 40% of
miRNAs demonstrate their likely contribution to canine OSA primary OSA tumor samples overexpressed HER2; results of the
biology and pathogenesis. study suggested the possibility of HER2 overexpression as a nega-
tive prognostic factor for survival.
Molecular Factors
Given the heterogeneous and chaotic nature of OSA, it has been Mammalian Target of Rapamycin
difficult to definitively ascribe specific molecular derangements The mammalian target of rapamycin (mTOR) is an evolutionary
75
responsible for the etiopathogenesis of OSA. Nonetheless, sub- conserved protein kinase downstream of AKT, which acts as a cen-
stantive progress has been achieved through experimental, pre- tral hub for the integration of cellular signals induced by growth
clinical, and comparative investigations to identify dysregulated factors, nutrients, energy, and stress for the purposes of regulat-
intracellular signaling and cell survival pathways likely to partici- ing cell cycle progression and growth. As such, aberrant signaling
pate in the pathogenesis of OSA. Significantly, the identification through the mTOR pathway contributes to growth, survival, and
and tumorigenic consequences of several putative pathways have chemotherapy resistance in multiple tumor types. To characterize
been characterized in immortalized OSA cell lines and spontane- the functionality of the mTOR pathway in canine OSA, one study
ously arising OSA tissue samples. using three canine OSA cell lines investigated the expressions of
mTOR and p70S6K, a downstream effector protein of mTOR,
Tyrosine Protein Kinase MET and demonstrated pathway activity and capacity to inhibit signal-
The MET protooncogene encodes a tyrosine kinase receptor which, ing with rapamycin. In a complementary phase I dose-escalation
87
upon ligation with hepatocyte growth factor (HGF), mediates study to assess the feasibility of mTOR inhibition as a treatment
multiple cellular functions including cell scattering, motility, and strategy, the pharmacokinetics and pharmacodynamics of rapamy-
proliferation. Given its biologic activities, excessive or dysregulated cin were investigated in dogs with primary OSAs and demon-
MET signaling in canine OSA has been demonstrated to promote strated that modulation of mTOR target proteins by rapamycin
tumorigenic phenotypes in cell lines. 76–78 Furthermore, in a small was achievable within the bone tumor microenvironment.
88
pilot study with spontaneously arising OSA samples, the expression
of MET protooncogene was identified in the majority (71%) of Hedgehog and Notch
79
tumor specimens by northern blot analysis. Additionally, a novel Hedgehog-GLI and Notch-HES1 signaling pathways have been
germline mutation resulting in enhanced receptor phosphorylation linked to the growth, survival, and metastases of various human
and aberrant MET signaling has been identified primarily in the tumor histologies and recently have been investigated for their
Rottweiler breed. In a larger study of 59 primary OSA samples, potential role in canine OSA biology. In one investigation, over-
80
mRNA expression of MET and HGF was detected by real-time expression and functionality of the Hedgehog (HHG) pathway
PCR (RT-PCR) in all specimens; this suggested the existence of a was confirmed in canine OSA cell lines by demonstrating elevated
81
putative MET/HGF autocrine or paracrine feedback loop. GLI1 and GLI2 transcription factors with correlative downstream
target gene upregulation (PTCH1 and PAX6). Additionally, the
Insulin-like Growth Factor functionality and biologic consequences of HHG signaling path-
The cellular effects of growth hormone (GH) are mediated through way blockade was assessed whereby inhibition of GLI transcrip-
the hepatic production of insulin-like growth factor 1 (IGF-1). In tion factors by GANT61 resulted in reduced proliferation and
osteoblasts, IGF-1 induces cell mitogenesis and protection from colony-forming capacity in canine OSA cell lines. Analogous to
89
apoptosis, as well as promotes angiogenesis. Derived from experi- HHG signaling, the involvement of the Notch pathway in canine
mental and preclinical investigations, aberrant or excessive IGF-1 OSA biology has been evaluated through the inclusion of a dichot-
signaling likely participates in OSA pathogenesis. In three canine omous outcome linked array of canine OSA tissues whereby dogs
OSA cell lines, the expression and functionality of the IGF-1/ with OSA were categorized into unfavorable (disease-free interval
82
IGF-1 receptor signaling cascade has been reported. Recently, [DFI] <100 days) versus favorable (DFI >300 days) responders.
the activation and prognostic significance of the IGF pathway has Gene array analysis of Notch/HES1 associated genes suggested