Interference with Ca2+-Dependent Proteolysis Does Not Alter the Course of Muscle Wasting in Experimental Cancer Cachexia

Abstract

Protein hypercatabolism significantly contributes to the onset and progression of muscle wasting in cancer cachexia. In this regard, a major role is played by the ATP-ubiquitin-proteasome-dependent pathway and by autophagy. However, little is known about the relevance of the Ca2+-dependent proteolytic system. Since previous results suggested that this pathway is activated in the skeletal muscle of tumor hosts, the present study was aimed to investigate whether inhibition of Ca2+-dependent proteases (calpains) may improve cancer-induced muscle wasting. Two experimental models of cancer cachexia were used, namely the AH-130 Yoshida hepatoma and the C26 colon carcinoma. The Ca2+-dependent proteolytic system was inhibited by treating the animals with dantrolene or by overexpressing in the muscle calpastatin, the physiologic inhibitor of Ca2+-dependent proteases. The results confirm that calpain-1 is overexpressed and calpastatin is reduced in the muscle of rats implanted with the AH-130 hepatoma, and show for the first time that the Ca2+-dependent proteolytic system is overactivated also in the C26-bearing mice. Yet, administration of dantrolene, an inhibitor of the Ca2+-dependent proteases, did not modify tumor-induced body weight loss and muscle wasting in the AH-130 hosts. Dantrolene was also unable to reduce the enhancement of protein degradation rates occurring in rats bearing the AH-130 hepatoma. Similarly, overexpression of calpastatin in the tibialis muscle of the C26 hosts did not improve muscle wasting at all. These observations suggest that inhibiting a single proteolytic system is not a good strategy to contrast cancer-induced muscle wasting. In this regard, a more general and integrated approach aimed at targeting the catabolic stimuli rather than the proteolytic activity of a single pathway would likely be the most appropriate therapeutic intervention.

Autophagy Exacerbates Muscle Wasting in Cancer Cachexia and Impairs Mitochondrial Function

“…Such a hypothesis was previously demonstrated in colon cancer cells [21], although convincing evidence in the skeletal muscle is lacking. In this regard, previous data reported that either proteasome [4] or calpain [22] inhibition do not improve cancer-induced muscle wasting, in agreement with the idea that targeting a specific proteolytic system likely leads to compensatory activation of the others, vanishing treatment effectiveness. The other way round, simultaneously blocking more than one proteolytic system likely results in potential cumulative side effects, considering the reported accumulation of p62 upon beclin-1 knock-down, although a phase 1 clinical trial using both proteasome and autophagy inhibitors was reported in patients with relapsed/refractory myeloma [23].…”

The Redox Balance: A Target for Interventions Against Muscle Wasting in Cancer Cachexia?

“…Despite lysosomes and proteasome appear overactivated in conditions associated with muscle wasting, they are unable to directly break down myofibrillar proteins. To become available for degradation, the latter should undergo a preventive cleavage likely operated by calpains and/or caspases (91), whose activity in the muscle is indeed upregulated in both tumor-bearing animals (29,71,101) and cancer patients (124).…”

Modulating Metabolism to Improve Cancer‐Induced Muscle Wasting

“…Increased calpain expression was reported in the muscle of tumor-bearing animals [ 19 ], while rats transplanted with the Yoshida AH-130 hepatoma showed a progressive reduction of calpastatin levels and increased in vitro cleavage of specific fluorogenic substrates [ 20 ]. More recently, calpain activation was also demonstrated in mice bearing the C26 colon carcinoma [ 21 ]. Both increased or unchanged muscle calpain expression were reported in cancer patients [ 12 , 22 ].…”

“…Despite the literature report data supporting the involvement of Ca 2+ -dependent proteolysis in the pathogenesis of cancer-induced muscle wasting, protein hypercatabolism was not downregulated in preparations of isolated muscles obtained from tumor-bearing animals and incubated in the presence of calpain inhibitors [ 19 , 33 , 34 ]. More recently, both pharmacological and genetic approaches aimed at inhibiting the Ca 2+ -dependent proteolytic system were not able to prevent or delay cancer-induced muscle wasting [ 21 ], although contrasting results were reported in this regard [ 35 ].…”