Regulation of lipid biosynthesis and expansion of the endoplasmic reticulum by the unfolded protein response

Plasma cells are terminally differentiated B lymphocytes that produce and secrete large quantities of antibodies. The differentiation of these specialized secretory cells involves an increase in exocytic pathway components and expansion of the endoplasmic reticulum (ER) in order to accommodate high-rate synthesis of immunoglobulin molecules. ER expansion requires membrane biogenesis, a process requiring enhanced phospholipid biosynthesis. However, the transcription factors and mechanisms that regulate lipid biosynthesis and membrane biogenesis during the differentiation process are poorly understood.;Studies demonstrate a link between membrane biogenesis and the unfolded protein response (UPR), a signaling pathway emanating from the ER that is triggered by increased demand upon the secretory pathway. Enforced expression of XBP-1(S), a transcription factor regulated by the UPR, results in enhanced phospholipid biosynthesis and ER expansion. Since XBP-1(S) can regulate lipid metabolism changes, I reasoned that another UPR transcriptional activator, ATF6alpha might also regulate such events to accommodate ER stress. Therefore, I hypothesized that ATF6alpha like XBP-1(S) induces phospholipid biosynthesis and ER expansion. First, I found that enforced expression of ATF6alpha results in enhanced phosphatidylcholine (PtdCho) biosynthesis. PtdCho is the predominant phospholipid in the ER membrane. Increased PtdCho synthesis in ATF6alpha overexpressing cells correlates with an increase in choline kinase (CK) activity and choline phosphotransferase/choline ethanolamine phosphotransferase (CPT/CEPT) activity. However, I found no change in phosphocholine cytidylyltransferase (CCT) activity. CK, CCT, and CEPT are enzymes in the CDP-choline pathway, the major pathway for PtdCho biosynthesis. These results are quite distinct from what is found in XBP-1(S) overexpressing cells where there is no change in CK activity and increased CCT and CPT/CEPT activity. Additionally, I found that overexpression of ATF6alpha results in expansion of the rough endoplasmic reticulum. Interestingly, I found that enforced expression of ATF6alpha increases secretion of stably expressed secreted alkaline phosphatase (SEAP), demonstrating that ATF6alpha has the capacity to enhance secretory output.;In summary, my research demonstrates that the UPR transcriptional activator ATF6a has the capacity to upregulate phospholipid biosynthesis, expand the ER, and augment secretory capacity. Based on these results, ATF6alpha might play a critical role in the development of specialized secretory cells like antibody-secreting cells.