Identification of novel inhibitors of human SPCA2

Inhibition of the human secretory pathway Ca2+, Mn2+-ATPase1a by 1,3-thiazole derivatives

The human Golgi/secretory pathway Ca²⁺,Mn²⁺-ATPase 1 (hSPCA1) transports Ca²⁺ and Mn²⁺ into the Golgi lumen. Studies of the biological functions of hSPCA1 are limited by a lack of selective pharmacological tools for SPCA1 inhibition. The aim of this study was therefore to identify compounds that specifically inhibit hSPCA1 activity. We found that five 1,3-thiazole derivatives exhibited inhibitory action towards the ATP hydrolysis activity of hSPCA1a in a concentration-dependent manner. Among the derivatives tested, compound 1 was the most potent, completely inhibiting hSPCA1a activity with a half-maximal inhibition (IC₅₀) value of 0.8 μM. Compound 1 also partially inhibited the activity of another Ca²⁺,Mn²⁺-ATPase (hSPCA2) and Ca²⁺-ATPase (rSERCA1a), but had no effect on Na⁺,K⁺-ATPase or H⁺,K⁺-ATPase. Treatment of HeLa cells with compound 1 led to fragmentation of the Golgi ribbon into smaller stacks. In addition, compound 1 mobilized intracellular Ca²⁺ in HeLa cells that had been pre-treated with thapsigargin. Therefore, based on its selectivity and potency, compound 1 may be a valuable tool with which to further explore the role of SPCA1 in cellular processes.

The pancreas-specific form of secretory pathway calcium ATPase 2 regulates multiple pathways involved in calcium homeostasis

Acinar cell exocytosis requires spatiotemporal Ca²⁺ signals regulated through endoplasmic reticulum (ER) stores, Ca²⁺ATPases, and store-operated Ca²⁺ entry (SOCE). The secretory pathway Ca²⁺ATPase 2 (SPCA2) interacts with Orai1, which is involved in SOCE and store independent Ca²⁺ entry (SICE). However, in the pancreas, only a C-terminally truncated form of SPCA2 (termed SPAC2C) exists. The goal of this study was to determine if SPCA2C effects Ca²⁺ homeostasis in a similar fashion to the full-length SPCA2. Using epitope-tagged SPCA2C (SPCA2C^FLAG) expressed in HEK293A cells and Fura2 imaging, cytosolic [Ca²⁺] was examined during SICE, SOCE and secretagogue-stimulated signaling. Exogenous SPCA2C expression increased resting cytosolic [Ca²⁺], Ca²⁺ release in response to carbachol, ER Ca²⁺ stores, and store-mediated and independent Ca²⁺ influx. Co-IP detected Orai1-SPCA2C interaction, which was altered by co-expression of STIM1. Importantly, SPCA2C's effects on store-mediated Ca²⁺ entry were independent of Orai1. These findings indicate SPCA2C influences Ca²⁺ homeostasis through multiple mechanisms, some of which are independent of Orai1, suggesting novel and possibly cell-specific Ca²⁺ regulation.

An N-terminal Ca2+-binding motif regulates the secretory pathway Ca2+/Mn2+-transport ATPase SPCA1

The Ca²⁺/Mn²⁺ transport ATPases 1a and 2 (SPCA1a/2) are closely related to the sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) and are implicated in breast cancer and Hailey-Hailey skin disease. Here, we purified the human SPCA1a/2 isoforms from a yeast recombinant expression system and compared their biochemical properties after reconstitution. We observed that the purified SPCA1a displays a lower Ca²⁺ affinity and slightly lower Mn²⁺ affinity than SPCA2. Remarkably, the turnover rates of SPCA1a in the presence of Mn²⁺ and SPCA2 incubated with Ca²⁺ and Mn²⁺ were comparable, whereas the turnover rate of SPCA1a in Ca²⁺ was 2-fold higher. Moreover, we noted an unusual biphasic activation curve for the SPCA1a ATPase and autophosphorylation activity, not observed with SPCA2. We also found that the biphasic pattern and low apparent ion affinity of SPCA1a critically depends on ATP concentration. We further show that the specific properties of SPCA1a at least partially depend on an N-terminal EF-hand-like motif, which is present only in the SPCA1a isoform and absent in SPCA2. This motif binds Ca²⁺, and its mutation lowered the Ca²⁺ turnover rate relative to that of Mn²⁺, increased substrate affinity, and reduced the level of biphasic activation of SPCA1a. A biochemical analysis indicated that Ca²⁺ binding to the N-terminal EF-hand-like motif promotes the activity of SPCA1a by facilitating autophosphorylation. We propose that this regulation may be physiologically relevant in cells with a high Ca²⁺ load, such as mammary gland cells during lactation, or in cells with a low ATP content, such as keratinocytes.

Structure/activity relationship of thapsigargin inhibition on the purified Golgi/secretory pathway Ca2+/Mn2+-transport ATPase (SPCA1a)

The Golgi/secretory pathway Ca²⁺/Mn²⁺-transport ATPase (SPCA1a) is implicated in breast cancer and Hailey-Hailey disease. Here, we purified recombinant human SPCA1a from Saccharomyces cerevisiae and measured Ca²⁺-dependent ATPase activity following reconstitution in proteoliposomes. The purified SPCA1a displays a higher apparent Ca²⁺ affinity and a lower maximal turnover rate than the purified sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA1a). The lipids cholesteryl hemisuccinate, linoleamide/oleamide, and phosphatidylethanolamine inhibit and phosphatidic acid and sphingomyelin enhance SPCA1a activity. Moreover, SPCA1a is blocked by micromolar concentrations of the commonly used SERCA1a inhibitors thapsigargin (Tg), cyclopiazonic acid, and 2,5-di-tert-butylhydroquinone. Because tissue-specific targeting of SERCA2b by Tg analogues is considered for prostate cancer therapy, the inhibition of SPCA1a by Tg might represent an off-target risk. We assessed the structure-activity relationship (SAR) of Tg for SPCA1a by in silico modeling, site-directed mutagenesis, and measuring the potency of a series of Tg analogues. These indicate that Tg and the analogues are bound via the Tg scaffold but with lower affinity to the same homologous cavity as on the membrane surface of SERCA1a. The lower Tg affinity may depend on a more flexible binding cavity in SPCA1a, with low contributions of the Tg O-3, O-8, and O-10 chains to the binding energy. Conversely, the protein interaction of the Tg O-2 side chain with SPCA1a appears comparable with that of SERCA1a. These differences define a SAR of Tg for SPCA1a distinct from that of SERCA1a, indicating that Tg analogues with a higher specificity for SPCA1a can probably be developed.