The degradation of troponin (Tn) subunits by calpain was studied by incubating either isolated cardiac Tns or myocardial cryosections with two different calpain isoenzymes isolated from rat skeletal muscle. Western-blot analysis with monoclonal antibodies against TnI and TnT showed that mu-calpain was at least ten times more active than m-calpain in degrading TnI and TnT both in vitro and in situ. TnC was completely resistant to both proteinase forms. Phosphorylation by cyclic AMP-dependent protein kinase (PKA) isolated from rat skeletal muscle reduced the sensitivity of TnI to degradation. This effect in combination with an increased efficiency of the endogenous inhibitor [Salamino, De Tullio, Michetti, Mengotti, Melloni and Pontremoli (1994) Biochem. Biophys. Res. Commun. 199, 1326-1332] probably reduces the proteolytic activity of calpain in cells on PKA stimulation. Conversely, phosphorylation by protein kinase C (PKC) resulted in a twofold increase in the degradation of TnI. Degradation by m-calpain was not modified by Tn phosphorylation. The different sensitivity to mu-calpain might be related to changes in TnI oligomeric structure. Indeed, on PKC phosphorylation, the apparent molecular mass of TnI calculated from the distribution coefficient of Tn complex in Sephadex G-100 matrix was reduced from 90 to 30 kDa suggesting dissociation of the Tn complex.

Calpains, the thiol proteinases of the calciumdependent proteolytic system, are regulated by a natural inhibitor, calpastatin, which is present in brain tissue in two forms. Although both calpastatins are highly active on human erythrocyte calpain, only one form shows a high inhibitory efficiency with both rat brain calpain isozymes. The second calpastatin form is almost completely inactive against homologous proteinases and can be converted into an active one by exposure to a phosphoprotein phosphatase, also isolated from rat brain. Phosphorylation of the active calpastatin by protein kinase C and protein kinase A promotes a decrease in its inhibitory efficiency. The interconversion between the two inhibitor forms seems involved in the adjustment of the level of intracellular calpastatin activity on specific cell requirements.

Cachexia is a syndrome characterized by profound tissue wasting that frequently complicates malignancies. In a cancer cachexia model we have shown that protein depletion in the skeletal muscle, which is a prominent feature of the syndrome, is mostly due to enhanced proteolysis. There is consensus on the views that the ubiquitin/proteasome pathway plays an important role in such metabolic response and that cytotoxic cytokines such as TNFα are involved in its triggering (Costelli and Baccino, 2000), yet the mechanisms by which the relevant extracellular signals are transduced into protein hypercatabolism are largely unknown. Moreover, little information is presently available as to the possible involvement in muscle protein waste of the Ca 2+ -dependent proteolysis, which may provide a rapidly activated system in response to the extracellular signals. In the present work we have evaluated the status of the Ca 2+ -dependent proteolytic system in the gastrocnemius muscle of AH-130 tumour-bearing rats by assaying the activity of calpain as well as the levels of calpastatin, the natural calpain inhibitor, and of the 130 kDa Ca 2+ -ATPase, both of which are known calpain substrates. After tumour transplantation, total calpastatin activity progressively declined, while total calpain activity remained unchanged, resulting in a progressively increasing unbalance in the calpain/calpastatin ratio. A decrease was also observed for the 130 kDa plasma membrane form of Ca 2+ -ATPase, while there was no change in the level of the 90 kDa sarcoplasmic Ca 2+ -ATPase, which is resistant to the action of calpain. Decreased levels of both calpastatin and 130 kDa Ca 2+ -ATPase have been also detected in the heart of the tumour-bearers. These observations strongly suggest that Ca 2+ -dependent proteolysis was activated in the skeletal muscle and heart of tumour-bearing animals and raise the possibility that such activation may play a role in sparking off the muscle protein hypercatabolic response that characterizes cancer cachexia. © 2001 Cancer Research Campaign http://www.bjcancer.com

Localization of the two main components of the Ca2+-dependent proteolytic system has been investigated in human neuroblastoma LAN-5 cells. Using a monoclonal antibody which recognizes the N-terminal calpastatin domain, it has been shown that this inhibitory protein is almost completely confined in two granule-like structures not surrounded by membranes. Similar calpastatin distribution has been found in other human and in murine cell types, indicating that aggregation of calpastatin is a general property and not an exclusive characteristic of neuronal-like cells. The existence of such calpastatin aggregates is confirmed by the kinetics of calpastatin-activity release during rat liver homogenization, which does not correspond to the rate of appearance of cytosolic proteins or to the disruption of membrane-surrounded organelles. Calpastatin distribution is affected by the intracellular increase in free Ca2+, which results in calpastatin progressively becoming a soluble protein. However, calpain is distributed in the soluble cell fraction and, in activating conditions, partially accumulates on the plasma membrane. Similar behaviour has been observed in calpastatin localization in LAN-5 cells induced with retinoic acid, suggesting that the proteolytic system is activated during the differentiation process of these cells. The involvement of calpastatin in controlling calpain activity, rather than its activation process, and the utilization of changes in calpastatin localization as a marker of activation of the system is discussed.

We have previously reported that, in neuroblastoma LAN-5 cells, calpastatin is in an aggregated state, close to the cell nucleus [de Tullio, Passalacqua, Averna, Salamino, Melloni and Pontremoli (1999) Biochem. J. 343, 467-472]. In the present paper, we demonstrate that aggregated calpastatin is predominantly in a phosphorylated state. An increase in intracellular free [Ca2+] induces both dephosphorylation of calpastatin, through the action of a phosphoprotein phosphatase, and its redistribution as a soluble inhibitor species. cAMP, but not PMA-induced phosphorylation, reverses calpastatin distribution favouring its aggregation. This intracellular reversible mechanism, regulating the level of cytosolic calpastatin, could be considered a strategy through which calpain can escape calpastatin inhibition, especially during earlier steps of its activation process.