Role of STIM1- and Orai1-mediated Ca2+ entry in Ca2+-induced epidermal keratinocyte differentiation

Abstract

Summary The uppermost thin layer on the surface of the skin, called the epidermis, is responsible for the barrier function of the skin. The epidermis has a multilayered structure in which each layer consists of keratinocytes (KCs) of different differentiation status. The integrity of KC differentiation is crucial for the function of skin and its loss causes or is accompanied by skin diseases. Intracellular and extracellular Ca2+ is known to play important roles in KC differentiation. However, the molecular mechanisms underlying Ca2+ regulation of KC differentiation are still largely unknown. Store-operated Ca2+ entry (SOCE) is a major Ca2+ influx pathway in most non-excitable cells. SOCE is evoked in response to a fall in Ca2+ concentration in the endoplasmic reticulum. Two proteins have been identified as essential components of SOCE: STIM1, a Ca2+ sensor in the ER, and Orai1, a subunit of Ca2+ channels in the plasma membrane. In this study, we analyzed the contribution of SOCE to KC growth and differentiation using RNAi knockdown of STIM1 and Orai1 in the human keratinocyte cell line, HaCaT. KC differentiation was induced by a switch in extracellular Ca2+ concentration from low (0.03 mM; undifferentiated KCs) to high (1.8 mM; differentiated KCs). This Ca2+ switch triggers phospholipase-C-mediated intracellular Ca2+ signals (Ca2+-switch-induced Ca2+ response), which would probably involve the activation of SOCE. Knockdown of either STIM1 or Orai1 strongly suppressed SOCE and almost completely abolished the Ca2+-switch-induced Ca2+ responses, resulting in impaired expression of keratin1, an early KC differentiation marker. Furthermore, loss of either STIM1 or Orai1 suppressed normal growth of HaCaT cells in low Ca2+ and inhibited the growth arrest in response to a Ca2+ switch. These results demonstrate that SOCE plays multiple crucial roles in KC differentiation and function.