In addition to its roles in acute stress responses, nSMase2 has also been implicated in cell growth inhibition and tumorigenesis. Three years prior to the cloning of nSMase2, a cDNA fragment CCA1- later identified as nSMase2 - was found to be upregulated in growth-arrested confluent cells. A role for nSMase2 in the regulation of cell growth was confirmed when it was found that nSMase2-overexpressing cells exhibited slower cell growth at the late exponential phase (Marchesini et al., 2003). Subsequently, it was found that upregulation of nSMase2 mRNA and protein caused a cell cycle arrest in the G0/G1 phase with consequent increases in ceramide levels, especially in the very long chain C(24:1) and C(24:0) ceramides (Marchesini et al., 2004). Conversely, siRNA downregulation of nSMase2 prevented both ceramide increases and the cell cycle arrest. Mechanistically, nSMase2 downregulation prevented dephosphorylation of the retinoblastoma protein and induction of p21/WAF1 in confluent cells, thus providing a link between nSMase2 and key regulators of cell cycle. Further, PP1, a lipid-regulated protein phosphatase, was implicated as acting downstream of nSMase2 in regulating cell motility during cell confluence
