Boric acid inhibits stored Ca²⁺ release in DU-145 prostate cancer cells

Boron (B) is a developmental and reproductive toxin. It is also essential for some organisms. Plants use uptake and efflux transport proteins to maintain homeostasis, and in humans, boron has been reported to reduce prostate cancer. Ca²⁺ signaling is one of the primary mechanisms used by cells to respond to their environment. In this paper, we report that boric acid (BA) inhibits NAD⁺ and NADP⁺ as well as mechanically induced release of stored Ca²⁺ in growing DU-145 prostate cancer cells. Cell proliferation was inhibited by 30% at 100μM, 60% at 250μM, and 97% at 1,000μM BA. NAD⁺-induced Ca²⁺ transients were partly inhibited at 250μM BA and completely at 1,000μM BA, whereas both NADP ⁺ and mechanically induced transients were inhibited by 1,000μM BA. Expression of CD38 protein increased in proportion to BA exposure (0-1,000μM). In vitro mass spectrometry analysis showed that BA formed adducts with the CD38 products and Ca²⁺ channel agonists cyclic adenosine diphosphate ribose (cADPR) and nicotinic acid adenine dinucleotide phosphate (NAADP). Vesicles positive for the Ca²⁺ fluorophore fluo-3 acetoxymethyl ester accumulated in cells exposed to 250 and 1,000μM BA. The BA analog, methylboronic acid (MBA; 250 and 1,000μM), did not inhibit cell proliferation or NAD⁺, NADP⁺, or mechanically stimulated Ca²⁺ store release. Nor did MBA increase CD38 expression or cause the formation of intracellular vesicles. Thus, mammalian cells can distinguish between BA and its synthetic analog MBA and exhibit graded concentration- dependent responses. Based on these observations, we hypothesize that toxicity of BA stems from the ability of high concentrations to impair Ca²⁺ signaling.