Inhibition of the SERCA Ca²⁺ pumps by curcumin

Abstract

Curcumin is a compound derived from the spice, tumeric. It is a potent inhibitor of the SERCA Ca 2þ pumps (all isoforms), inhibiting Ca 2þ -dependent ATPase activity with IC 50 values of between 7 and 15 mM. It also inhibits ATPdependent Ca 2þ -uptake in a variety of microsomal membranes, although for cerebellar and platelet microsomes, a stimulation in Ca 2þ uptake is observed at low curcumin concentrations (,10 mM). For the skeletal muscle isoform of the Ca 2þ pump (SERCA1), the inhibition of curcumin is noncompetitive with respect to Ca 2þ , and competitive with respect to ATP at high curcumin concentrations (< 10-25 mM). This was confirmed by ATP binding studies that showed inhibition in the presence of curcumin: ATP-dependent phosphorylation was also reduced. Experiments with fluorescein 5 0 -isothiocyanate (FITC)-labelled ATPase also suggest that curcumin stabilizes the E1 conformational state. The fact that FITC labels the nucleotide binding site of the ATPase (precluding ATP from binding), and the fact that curcumin affects FITC fluorescence indicate that curcumin must be binding to another site within the ATPase that induces a conformational change to prevent ATP from binding. This observation is interpreted, with the aid of recent structural information, as curcumin stabilizing the interaction between the nucleotidebinding and phosphorylation domains, precluding ATP binding.Keywords: SERCA; ATP binding; curcumin; phosphorylation; fluorescence.Tumeric is extensively used as a spice in Asian cooking and as a colouring agent in both the food and cosmetic industries [1]. Curcumin (diferuoylmethane or 1,7-bis(4-hydroxy-3-methoxyphenol)-1,6-heptadiene-3,5-dione) is a compound found in tumeric that gives it its distinctive yellow colour [2]. Recently it has been shown that curcumin has anticarcinogenic effects [3] that may be linked to its antioxidant properties [4]. Studies have shown that curcumin can affect a number of cellular processes including: activation of apoptosis in Jurkat T-cells [5], inhibition of platelet aggregation [6,7] and inhibition of inflammatory cytokine production in macrophages [8]. Curcumin has also been shown to affect the activity of a number of key enzymes such as cyclooxygenase [9], protein kinase C [10], protein tyrosine kinases [11] and a Ca 2þ -dependent endonuclease [12]. Many of these processes/enzymes are also known to be regulated by Ca 2þ .Cytosolic free Ca 2þ concentration ([Ca 2þ ] cyt ) is tightly controlled, due its importance in the regulation of many cellular processes. The sarco/endoplasmic reticulum Ca 2þ ATPase (SERCA) is one of the major mechanisms by which the low levels of [Ca 2þ ] cyt are maintained within cells. Three isoforms of the SERCA family of Ca 2þ pumps have so far been identified [13,14] and these are expressed in a tissuespecific manner [14]. SERCA1 is found predominantly in fast-twitch skeletal muscle while SERCA2a is found within cardiac and slow-twitch muscle. The splice variant form of SERCA2 (SERCA2b), which has an extended C-terminus is foun...

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