Recent studies have shown that certain endoplasmic reticulum (ER) membrane proteins modulate cell survival by controlling the migration of calcium from ER to mitochondria. We have identified and characterized a cDNA that corresponds to a novel gene which encodes a putative transmembrane protein that we have named SPOC (Survival Promoter Overexpressed in Cancers). Our preliminary results indicate that the SPOC mRNA is highly expressed in primary colon tumors when compared to their matching normal tissues. Interestingly, SPOC mRNA expression is further increased in tumor metastases examined. Exogenous GFP-tagged SPOC co-localizes with ER-specific protein markers suggesting that SPOC appears to reside in the ER. Thapsigargin and sulindac sulfide are known to perturb intracellular Ca2+ homeostasis by promoting ER Ca2+ pool depletion. Both agents induced apoptosis in HCT116 human colon cancer cells that was coupled with down-regulation of SPOC mRNA expression. Interestingly, constitutive expression of exogenous SPOC in HCT116 cells countered the apoptotic effects of these agents suggesting that SPOC appears to promote cell survival. We are now proposing studies to further investigate the role of SPOC in digestive diseases such as colon cancer. We will analyze a larger pool of fresh-frozen and paraffin-embedded tissue specimens to evaluate the expression of SPOC at mRNA and protein levels. To explore the molecular mechanisms by which SPOC promotes cell survival, we will study its effect on PI3-K/Akt-dependent growth and survival signaling pathways. We will also investigate whether SPOC regulates intracellular Ca2+ homeostasis and modulates the Ca2+-dependent signaling pathway(s) to promote cell survival. To determine whether ER transmembrane localization is important for its survival promoting function, we will mutate and/or delete its putative N-terminal ER-retrieval motif as well as the transmembrane regions and study the subcellular localization of mutant SPOC and its effect on cell survival. These are exploratory/developmental studies that, upon completion, will generate sufficient new data and reagents that will form the basis of future in-depth studies investigating the molecular mechanism(s) of action of this novel ER transmembrane protein in regulation of cell survival in general and digestive diseases in particular.
