This is a multi-investigator proposal to study a novel aspect of the mechanism by which retinoic acid (RA) regulates cell growth and differentiation through induced expression and signaling by the CD38 receptor. RA is a cancer chemopreventive agent. It is a prohormone provided in the diet and is a common dietary insufficiency. RA can convert immature neoplastically transformed cells to a mature differentiated phenotype. The proposed studies utilize an immature, uncommitted precursor cell, HL-60 human myeloblastic leukemia cells, which undergo G0 arrest and either myeloid or monocytic differentiation when treated with RA or vitamin D3 respectively. RA causes activation of MAPK signaling in the process of inducing terminal myeloid differentiation/G0 arrest. Surveys of RA-induced gene expression from our and other laboratories revealed that CD38 is the earliest known receptor induced by RA. Crippling RA-induced CD38 expression cripples RA-induced differentiation, and ectopic over expression of CD38 enhances RA-induced differentiation. CD38 has 3 ways to originate signaling, due to either ectoenzyme activity or receptor signaling. (1) The ectoenzyme activity catalyzes production of ADPR from NAD; and cADPR is able to mobilize calcium, a known regulator of differentiation and proliferation. (2) CD38 is also capable of clustering, and receptor clustering is another well-known initiator of signaling. (3) CD38 is capable of MAPK signaling, reflecting the action of its short cytosolic tail binding to signaling molecules. The proposed studies focus on the role of these 3 signaling modalities in effecting the cellular outcome attributed to RA, namely differentiation and G0 cell cycle arrest. The proposed goals are to use small molecule probes that bind CD38, CD38 mutants, and X-ray crystallographic structures to characterize the mechanism by which CD38 promotes cell differentiation and arrest. The specific aims are to (1) determine the contribution to RA-induced differentiation and G0 arrest of the ectoenzyme activity and the receptor structure responsible for it, and (2) determine the contribution of receptor signaling, namely clustering and the cytosolic tail MAPK signaling, and the receptor structural characteristics needed for that. In sum we intend to identify the CD38 activity needed to propel RA-induced differentiation and show the structural features necessary to do that. The impact of these studies is a potential paradigm shift in understanding how RA works that will point to means of enhancing its mechanism of action. The significance of the studies is that they could provide the basic mechanistic insight into a novel means of enhancing the chemopreventive, chemotherapeutic effects of RA. The innovation is that a new paradigm based on receptor expression and signaling activities is proposed as a mechanism of action for RA in reverting neoplastic transformation. The basic hypothesis is thus that RA induces the early expression of CD38 which has different generating capabilities due to different structural attributes of the receptor and its associated signaling complex, and that these different capabilities control different components of the differentiation/arrest program elicited by RA.