Synergistic and antagonistic effects of TNF-[alpha] and IGF-I in heart failure : "in vitro" and "in vivo" study of cardiac and skeletal muscle

Cardiovascular diseases are a major cause of morbidity and mortality in
industrialized countries. All forms of myocardial injury first lead to compensatory
hypertrophy, which eventually progresses to heart failure. The pathophysiologic
mechanisms underlying this process are not fully understood. Nevertheless, cellular
remodeling is considered as a prime contributor to the pathogenesis of heart failure.
The remodeling process involves cardiomyocyte hypertrophy, alterations in gene
expression and myocyte shape as well as changes in the extracellular matrix. The
same factors that induce cardiac hypertrophy during early compensatory changes,
can also lead to apoptosis and secondary detrimental events associated with the
development of heart failure. In the present thesis I will focus on a three of these
factors, namely tumor necrosis factor-alpha (TNF-α), insulin-like growth factor (IGF)-I
and angiotensin II (Ang II).
TNF-α is a pro-inflammatory cytokine produced in the myocardium in response to
various types of injury. Studies using experimental animals demonstrated the
important role of TNF-α in the development of heart failure, however the use of TNF-
α blockers in clinical trials did not demonstrate beneficial effects. A frequent
consequence of catabolic conditions, including chronic heart failure, is muscle mass
loss. TNF-α is considered to play a major role in muscle catabolism. With evidence
of beneficial next to detrimental effects in both cardiac and skeletal muscle, the role
of TNF-α remains controversial.
IGF-I is involved in maintaining cardiac function in post-infarct events. This growth
factor has also been shown to induce survival and hypertrophy in many cells,
including skeletal and cardiac muscle cells. Important modulators of IGF-I activity are
the IGF-binding proteins (IGFBPs). Interactions between TNF-α and IGF-I have been
reported. Most of the studies were undertaken in skeletal muscle and showed
essentially an inhibitory effect of TNF-α either on IGF-I-induced responses or on IGFI
and/or IGFBPs expression.
The neurohormone Ang II plays a central role in hypertension and cardiovascular
diseases, and is also involved in the myocardial remodeling process. Functional
crosstalk between Ang II and TNF-α exists in cardiac hypertrophy, and is believed to
promote tissue damage.
The present work was undertaken in order to gain more insight into the mechanisms
of regulation involved in cardiac remodeling and muscle atrophy through multiple
factor interactions. To this end, we used two cell culture models of cardiac and
skeletal muscle cells, as well as animal models.
In primary cultures of adult rat cardiomyocytes, we show that TNF-α acts on the IGFI
system by downregulating mRNA expression of IGFBP-4, by interfering with IGF-Iinduced
Akt signaling, and by potentiating IGF-I-induced activation of the ERK1/2
signaling pathway. The latter effect may present a synergistic role for TNF-α and
IGF-I in cardiomyocyte hypertrophy. In this model we also show that TNF-α has
immediate positive effects by increasing cardiomyocyte viability, however longer-term
incubation resulted in decreased viability and enhanced expression of apoptotic
markers.
To determine the in vivo relevance of the IGF-I system regulation by factors involved
in cardiac remodeling, we analyzed the expression pattern of cardiac IGFBPs in two
animal models of hypertension. We show up-regulation of IGFBP-4 mRNA
expression in both models, increased IGFBP-5 in salt-fed Dahl salt sensitive rats,
and decreased IGFBP-3 in Ang II-infused rats. Specific down-regulation of IGFBP-3
by Ang II may play an important role in pressor-independent cardiac effects of this
neurohormone.
We also analyzed protein content regulation in the skeletal muscle cellular model.
Using C2C12 mouse myotubes, we show that TNF-α and IGF-I both enhance protein
synthesis by activating different signaling pathways. TNF-α acts mainly via PI3K-Akt
and to a lesser extent via MEK-ERK1/2, while IGF-I acts independently of PI3K.
Mechanisms which activate protein degradation through the ubiquitin proteasome
pathway were analyzed by measuring Atrogin-1 mRNA expression. Levels of this
marker of atrophy were transiently increased by TNF-α via the p38 MAPK signaling
pathway, and this effect was inhibited by IGF-I. However, longer-term incubations
with TNF-α decreased Atrogin-1 mRNA levels suggesting inhibition of protein
breakdown.
To conclude, this work demonstrates regulation of cardiac IGFBPs expression by
TNF-α and Ang II at the cellular and tissue level, respectively. In the models studied
here, we show that factors involved in the remodeling process can modulate IGF-I,
which is important for cardiac function maintenance, through regulation of the
IGFBPs. These mechanisms highlight the important role of multiple factor
interactions in the development of heart failure. Furthermore, by studying the
regulation of skeletal muscle protein content, TNF-α proved to increase protein
synthesis and to inhibit protein degradation mechanisms by decreasing Atrogin-1
expression. These results propose a novel beneficial role for TNF-α in the prevention
of muscle wasting.