Targeting Ca2+ signaling: A new arsenal against cancer

The drug resistance of cancer cells is a major concern in medical oncology, resulting in the failure of chemotherapy. Ca²⁺ plays a pivotal role in inducing multidrug resistance in cancer cells. Calcium signaling is a critical regulator of many cancer hallmarks, such as angiogenesis, invasiveness, and migration. In this review, we describe the involvement of Ca²⁺ signaling and associated proteins in cancer progression and in the development of multidrug resistance in cancer cells. We also highlight the possibilities and challenges of targeting the Ca²⁺ channels, transporters, and pumps involved in Ca²⁺ signaling in cancer cells through structure-based drug design. This work will open a new therapeutic window to be used against cancer in upcoming years.

Calcium induces tobramycin resistance in Pseudomonas aeruginosa by regulating RND efflux pumps

Pseudomonas aeruginosa is an opportunistic multidrug resistant pathogen causing severe chronic infections. Our previous studies showed that elevated calcium (Ca²⁺) enhances production of several virulence factors and plant infectivity of the pathogen. Here we show that Ca²⁺ increases resistance of P. aeruginosa PAO1 to tobramycin, antibiotic commonly used to treat Pseudomonas infections. LC-MS/MS-based comparative analysis of the membrane proteomes of P aeruginosa grown at elevated versus not added Ca²⁺, determined that the abundances of two RND (resistance-nodulation-cell division) efflux pumps, MexAB-OprM and MexVW-OprM, were increased in the presence of elevated Ca²⁺. Analysis of twelve transposon mutants with disrupted RND efflux pumps showed that six of them (mexB, muxC, mexY, mexJ, czcB, and mexE) contribute to Ca²⁺-induced tobramycin resistance. Transcriptional analyses by promoter activity and RT-qPCR showed that the expression of mexAB, muxABC, mexXY, mexJK, czcCBA, and mexVW is increased by elevated Ca²⁺. Disruption of mexJ, mexC, mexI, and triA significantly decreased Ca²⁺-induced plant infectivity of the pathogen. Earlier, our group showed that PAO1 maintains intracellular Ca²⁺ (Ca²⁺_in) homeostasis, which mediates Ca²⁺ regulation of P. aeruginosa virulence, and identified four putative Ca²⁺ transporters involved in this process (Guragain et al., 2013). Here we show that three of these transporters (PA2435, PA2092, PA4614) play role in Ca²⁺-induced tobramycin resistance and one of them (PA2435) contributes to Ca²⁺ regulation of mexAB-oprM promoter activity. Furthermore, mexJ, czcB, and mexE contribute to the maintenance of Ca²⁺_in homeostasis. This provides the first evidence that Ca²⁺_in homeostasis mediates Ca²⁺ regulation of RND transport systems, which contribute to Ca²⁺-enhanced tobramycin resistance and plant infectivity in P. aeruginosa.

Is Ca2+ involved in the signal transduction pathway of boron deficiency? New hypotheses for sensing boron deprivation

Plants sense and transmit nutrient-deprivation signals to the nucleus. This increasingly interesting research field advances knowledge of signal transduction pathways for mineral deficiencies. The understanding of this topic for most micronutrients, especially boron (B), is more limited. Several hypotheses have been proposed to explain how a B deprivation signal would be conveyed to the nucleus, which are briefly summarized in this review. These hypotheses do not explain how so many metabolic and physiological processes quickly respond to B deficiency. Short-term B deficiency affects the cytosolic Ca(2+) levels as well as root expression of genes involved in Ca(2+) signaling. We propose and discuss that Ca(2+) and Ca(2+)-related proteins - channels/transporters, sensor relays, and sensor responders - might have major roles as intermediates in a transduction pathway triggered by B deprivation. This hypothesis may explain how plants sense and convey the B-deprivation signal to the nucleus and modulate physiological responses. The possible role of arabinogalactan-proteins in the B deficiency signaling pathway is also taken into account.

Calcium homeostasis in Pseudomonas aeruginosa requires multiple transporters and modulates swarming motility

Pseudomonas aeruginosa is an opportunistic human pathogen causing severe acute and chronic infections. Earlier we have shown that calcium (Ca(2+)) induces P. aeruginosa biofilm formation and production of virulence factors. To enable further studies of the regulatory role of Ca(2+), we characterized Ca(2+) homeostasis in P. aeruginosa PAO1 cells. By using Ca(2+)-binding photoprotein aequorin, we determined that the concentration of free intracellular Ca(2+) ([Ca(2+)]in) is 0.14±0.05μM. In response to external Ca(2+), the [Ca(2+)]in quickly increased at least 13-fold followed by a multi-phase decline by up to 73%. Growth at elevated Ca(2+) modulated this response. Treatment with inhibitors known to affect Ca(2+) channels, monovalent cations gradient, or P-type and F-type ATPases impaired [Ca(2+)]in response, suggesting the importance of the corresponding mechanisms in Ca(2+) homeostasis. To identify Ca(2+) transporters maintaining this homeostasis, bioinformatic and LC-MS/MS-based membrane proteomic analyses were used. [Ca(2+)]in homeostasis was monitored for seven Ca(2+)-affected and eleven bioinformatically predicted transporters by using transposon insertion mutants. Disruption of P-type ATPases PA2435, PA3920, and ion exchanger PA2092 significantly impaired Ca(2+) homeostasis. The lack of PA3920 and vanadate treatment abolished Ca(2+)-induced swarming, suggesting the role of the P-type ATPase in regulating P. aeruginosa response to Ca(2+).