Introduction
Cancer cachexia is a progressive and involuntary wasting condition of
lean body mass that afflicts approximately 80 % of all cancer patients
and can be attributed to 20-40% of cancer-associated deaths1, 2. Cancer cachexia is defined as losses in body
mass >5 %, primarily due to loss of muscle mass which can
be accompanied with or without losses in adipose tissue3, 4. To date, no singular method of treatment has
shown effective at eliminating cancer cachexia development due to the
systemic and complex nature of its underlying mechanisms5-7. Further complicating research into effective
treatments for cancer cachexia is the emerging role of sexual dimorphism
on cancer cachexia progression, as recent studies have shown that males
and females can differentially respond to cancer cachexia8-10. Metabolic and inflammatory instabilities are
known contributors to cancer cachexia progression and have been greatly
examined 11, 12, however the role of fibrosis on the
development of cancer cachexia remains incompletely understood.
Excessive fibrosis of healthy skeletal muscle worsens overall outcomes
in cancer patients 13. Moreover, unlike metabolic and
inflammatory abnormalities that can improve following cancer cessation,
this fibrosis of vital and functional lean tissue is largely
irreversible and can affect patients throughout their lifespan. The
extracellular matrix (ECM) is composed primarily of collagen 1 and 314. The ECM is critical for force transduction, growth
factor secretion, etc. The matrix metalloproteinases (MMPs) are
responsible for the degradation of collagens, and in skeletal muscle
MMP2 and MMP9 are typically the most abundant15.
Skeletal muscle MMP expression is altered in cancer cachexia16 which may contribute to the excessive fibrosis
associated with cancer cachexia. Transforming growth factor-β (TGF-β) is
a transcriptional activator of many cellular processes including the
activation of satellite cells as well as synthesis, degradation, and
remodeling of the ECM that surrounds skeletal muscle17, 18. Upregulation of TGF-β can result in excessive
ECM dysregulation and deposition of ECM components ultimately resulting
in the replacement of healthy skeletal muscle with non-contractile
proteins such as collagens 19, 20. This upregulation
of TGF-β can be induced via its upstream regulator in Interleukin-6
(IL-6), an inflammatory cytokine which is commonly elevated in cachectic
cancer patients 21. Taken together, these data suggest
fibrosis in cancer patients is altered via TGF-β, and evaluation of
TGF-β and its downstream effectors could provide insights on fibrosis as
it pertains to cancer cachexia progression.
Males and females exhibit sexual dimorphism in skeletal muscle
properties such as fiber type composition within the same muscle,
mitochondrial content, and number of satellite cells innately, and this
dimorphism is documented in some muscle conditions such as disuse
atrophy 22-24. Differences in the relative amounts of
circulating hormones like testosterone and estrogen only adds to the
complexity of the male and female phenotypic makeup within skeletal
muscle. For example, testosterone is typically higher in males and
promotes muscle protein synthesis and regeneration, while estrogen is
typically higher in females and has a greater impact on reducing
inflammation 25, 26. Therefore, female mice could
possess a greater protective effect from inflammatory-based muscle
wasting conditions such as cancer cachexia when compared to males10. Innate differences between sexes can therefore
greatly affect overall skeletal muscle health and even skeletal muscle
health in pathologies such as cancer cachexia 8, 10.
This study is, to our knowledge, the first to evaluate the role of
fibrosis during the development of cancer cachexia across both sexes.
Implantation of Lewis Lung Carcinoma (LLC) cells allows for the
controlled development of cancer cachexia, and thorough analysis of
fibrotic pathways during the progression of cachexia. Evaluation of
these fibrotic markers could elucidate potential avenues for therapeutic
interventions aimed towards the prevention of fibrosis and its
contributions to cancer cachexia development. Therefore, the purpose of
this study was to utilize the LLC allograft model of cachexia to
evaluate key regulators of the ECM during the controlled development of
cancer cachexia. We hypothesized the ECM and signaling pathways involved
in its modulation would be altered prior to cachexia. Herein we provide
novel evidence of altered fibrotic signaling coinciding with cancer
cachexia development, however this varied between sexes. These data
highlight the emerging role of fibrosis on cancer cachexia and further
strengthens the growing need for research in both males and females as
sex continues to create divergent responses to cancer cachexia and its
underlying mechanisms.