Store-operated calcium entry depicts the movement of extracellular Ca2+ into cells through plasma membrane Ca2+ channels activated by depletion of intracellular Ca2+ stores. The members of the canonical subfamily of transient receptor potential channels (TRPC) have been implicated as the molecular bases for store-operated channels (SOC). Here we investigate the role of phospholipase C (PLC) in regulation of native SOC and the expression of endogenous TRPC in human epidermal keratinocytes. Calcium entry in response to store depletion with thapsigargin was reversibly blocked by 2-aminoethoxydiphenyl borane, an effective SOC inhibitor, and suppressed by the diacylglycerol analoge, 1-oleoyl-2-acetyl-sn-glycerol. Inhibition of PLC with U73122 or transfection of a PLCgamma1 antisense cDNA construct completely blocked SOC activity, indicating a requirement for PLC, especially PLCgamma1, in the activation of SOC. RT-PCR and immunoblotting analyses showed that TRPC1, TRPC3, TRPC4, TRPC5, and TRPC6 are expressed in keratinocytes. Knockdown of the level of endogenous TRPC1 or TRPC4 inhibited store-operated calcium entry, indicating they are part of the native SOC. Co-immunoprecipitation studies demonstrated that TRPC1, but not TRPC4, interacts with PLCgamma1 and the inositol 1,4,5-trisphosphate receptor (IP3R). The association of TRPC1 with PLCgamma1 and IP3R decreased in keratinocytes with higher intracellular Ca2+, coinciding with a downregulation in SOC activity. Our results indicate that the activation of SOC in keratinocytes depends, at least partly, on the interaction of TRPC with PLCgamma1 and IP3R.

Calcium regulates the proliferation and differentiation of keratinocytes both in vivo and in vitro. Elevated extracellular Ca2+ concentration ([Ca2+](o)) raises the intracellular free calcium ([Ca2+](i)) and activates differentiation-related genes. Cells lacking the calcium-sensing receptor (CaR) fail to respond to [Ca2+](o) and to differentiate, indicating a role for CaR in keratinocyte differentiation. These concepts derived from in vitro experiments have been tested and confirmed in two mouse models. (C) 2003 Elsevier Ltd. All rights reserved.

Hailey-Hailey disease (MIM16960) is a blistering skin disease caused by mutations in the Ca2+ ATPase ATP2C1. We found that the abnormal Ca2+ signaling seen in Hailey-Hailey disease keratinocytes correlates with decreased protein levels of ATP2C1. Human ATP2C1 protein approximated 115 kDa in size. The ATP2C1 is localized to the Golgi apparatus in human keratinocytes, similar to its localization in yeast and Caenorhabditis elegans. To test whether the ATP2C1 controls Golgi Ca2+ stores, we measured intraorganelle Ca2+ concentrations using specifically targeted aequorins. Whereas normal keratinocytes display Golgi Ca2+ levels comparable to other epithelial cells, Hailey-Hailey disease keratinocyte Golgi Ca2+ refill is slower, and the maximum Ca2+ concentration reached is significantly lower. These findings were replicated in vivo, because clinically normal Hailey-Hailey disease epidermis contained lower Ca2+ stores and displayed an abnormal Ca2+ gradient. In this report we localize the ATP2C1, demonstrate its physiologic relevance in mammalian cells, and measure intraorganelle Golgi Ca2+ in keratinocytes.