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A direct link between Ca 2+ and lipid homeostasis has not been definitively demonstrated. In this study, we show that manipulation of ER Ca 2+ causes the re-distribution of a portion of the intracellular unesterified cholesterol to a pool that is not available to the SCAP-SREBP complex. The SREBP processing pathway in ER Ca 2+ depleted cells remained fully functional and responsive to changes in cellular cholesterol status but differed unexpectedly in basal activity. These findings establish the role of Ca 2+ in determining the reference set-point for controlling cellular lipid homeostasis. We propose that ER Ca 2+ status is an important determinant of the basal sensitivity of the sterol sensing mechanism inherent to the SREBP processing pathway.

We have shown earlier that overexpression of Calreticulin (CRT) contributed to a poor prognosis for patients with esophageal squamous cell carcinoma (ESCC). Here, we have shown an important role of CRT in tumorigenesis through enhancing cell motility and anoikis resistance. SiRNA-mediated knockdown of CRT caused impaired cell migration, invasion and resistance to anoikis. Notably, CRT downregulation decreased the expression of Cortactin (CTTN), which has been previously reported as a candidate oncogene associated with anoikis through the PI3K-Akt pathway. In addition, Akt phosphorylation was abolished after CRT downregulation and its activation can be refreshed by CRT upregulation, suggesting that CRT-enhanced cell resistance to anoikis through the CRT-CTTN-PI3K-Akt pathway. Moreover, the CTTN mRNA level was decreased in CRT-siRNA cells, coupled with the inactivation of STAT3. Expression of both CTTN and p-STAT3 was reduced in tumor cells following incubation with the JAK-specific inhibitor, AG490. Chromatin immunoprecipitation assay showed direct binding of p-STAT3 to the conservative STAT3-binding sequences in CTTN promoter. Furthermore, overexpression of CTTN in CRT-downregulated ESCC cells restored its motility and resistance to anoikis. This study not only reveals a role of CRT in motility promotion and anoikis resistance in ESCC cells, but also identifies CRT as an upstream regulator in the CRT-STAT3-CTTN-Akt pathway.