Filamin A C-terminal fragment modulates Orai1 expression by inhibition of protein degradation

Filamin A (FLNA) is an actin-binding protein that has been reported to interact with STIM1 modulating the activation of Orai1 channels. Cleaving of FLNA by calpain leads to a C-terminal fragment that is involved in a variety of functional and pathological events, including pro-oncogenic activity in different types of cancer. Here we show that full-length FLNA is downregulated in samples from colon cancer patients as well as in the adenocarcinoma cell line HT-29. This is consistent with an increased calpain-dependent FLNA cleaving with enhanced expression of the C-terminal FLNA fragment accompanied by enhanced expression of Orai1 and STIM1, as well as store-operated Ca²⁺ entry (SOCE). To further explore the mechanism underlying the enhancement of SOCE by the C-terminal FLNA fragment we expressed in HEK-293 cells the C-terminal FLNA region encompassing repeats 16-24 (FLNA^16-24 fragment), which enhanced both Orai1 and STIM1 as well as SOCE. Transfection of the FLNA^16-24 fragment attenuates Orai1 and STIM1 protein degradation, and, specifically, abrogates Orai1α lysosomal degradation and retains this channel in the plasma membrane. However, the C-terminal FLNA fragment did not induce a detectable modification in Orai1β degradation. Due to the relevance of SOCE in cell physiology, our results provide evidence of a novel mechanism for the regulation of Ca²⁺ influx with relevant pathophysiological implications.