Background: The endoplasmic reticulum (ER) Ca2+ sensor, stromal interaction molecule1 (STIM1) activates the plasma membrane (PM) channel Orai1 in order to mediate store-operated Ca2+ entry (SOCE) in response to ER store depletion. Enhanced expression of STIM1 in cancer tissue has been associated with poor patient prognosis. Therefore, this study investigated the functional consequences of enhanced expression of STIM1 and Orai1 in a tumor-initiating subpopulation of Huh-7 hepatocellular carcinoma (HCC) cells that express epithelial cell adhesion molecule (EpCAM) and Prominin 1 (CD133).
Methods: We performed qRT-PCR, intracellular Ca2+ monitoring, protein analyses, and real-time cell proliferation assays on EpCAM(+)CD133(+) subpopulation of tumor-initiating Huh-7 HCC cells expressing high levels of STIM1 and/or Orai1. Statistical significance between the means of two groups was evaluated using unpaired Student's t-test.
Results: Enhanced STIM1 expression significantly increased ER Ca2+ release and proliferation rate of EpCAM(+)CD133(+) cells.
Conclusion: STIM1 overexpression may facilitate cancer cell survival by increasing ER Ca2+-buffering capacity, which makes more Ca2+ available for the cytosolic events, on the other hand, possibly preventing Ca2+-dependent enzymatic activity in mitochondria whose Ca2+ uniporter requires much higher cytosolic Ca2+ levels.
Keywords: Ca2+; HCC; Orai1; SOCE; STIM1; TIC.