STEATOSIS, OR FATTY liver, is a common pathological feature of impaired hepatic metabolism resulting from chronic alcohol consumption, diet, toxin exposure, or metabolic disorders such as obesity and diabetes. Although often benign and reversible, it is widely believed that steatosis contributes to more advanced liver pathologies, including steatohepatitis and fibrosis. However, the precise role of steatosis in development of these pathologies remains poorly understood (Rubin and Farber, 1999). Histologically, steatosis is characterized by the accumulation of lipid droplets, composed chiefly of triacylglycerol (TAG), in the cytoplasm of hepatocytes (Cairns and Peters, 1983; Day and Yeaman, 1994). The size and lobular distribution of these cytoplasmic lipid droplets (CLD) are variable, reflecting the etiology, severity, and duration of steatosis. Large (macrovesicular) CLD are prominent in livers of individuals with chronic metabolic disorders or ethanol exposure, whereas numerous smaller (microvesicular) CLD have been associated with conditions of acute steatosis such as in Reye’s syndrome, in addition to being present in the early stages of some chronic disorders (Day and Yeaman, 1994). Although they have been hypothesized to reflect different stages of steatotic progression and represent different pathogenic risks (Day and Yeaman, 1994), it is unclear whether macrovesicular and microvesicular CLD are pathophysiologically distinct entities because information about their molecular compositions and biologic properties is limited, and the mechanisms regulating their formation and accumulation are incompletely understood. Steatosis associated with alcoholic, and many forms of nonalcoholic liver disease, is described as initiating in centrilobular areas (zone 3), with advancement to intermediate (zone 2) and periportal (zone 1) areas occurring with disease progression (Brunt, 2007; Lefkowitch, 2005). However, in certain malnutrition disorders, infections, and toxic insults, steatosis is described as originating in zone 1 (James and Day, 1998). At present, details about mechanisms underlying zone-dependent initiation and progression of steatosis are limited (Stewart et al., 2001). Activation of cell stress pathways, which can include the microsomal ethanol-oxidizing system (Cyp2E1), hypoxia, endoplasmic reticulum stress, mitochondrial dysfunction, or lipid peroxidation, appears to be a common feature of steatosis in most disorders (French, 1989). However, the individual, or collective, contributions of these processes to zone-specific promotion and progression of steatosis have not been fully established, and detailed knowledge of their specific effects on the cellular processes controlling CLD accumulation is still incomplete. Members of the perilipin (PAT) family of CLD-associated proteins, including perilipin, adipophilin (ADPH), TIP47, and S3–12, regulate TAG accumulation in eukaryotic cells, either through promotion of processes that enhance TAG synthesis and packaging in CLD (Gao and Serrero, 1999; Larigauderie et al., 2006; Robenek et al., 2006) or by controlling TAG lipolysis (Listenberger et al., 2007; Londos et al., 2005; Sztalryd et al., 2006; Wolins et al., 2006). Several lines of evidence suggest that perilipin family members may be important in the initiation and progression of steatosis. ADPH has been shown to be one of the major CLD-associated proteins in mouse livers (Wu et al., 2000), and its expression is required for induction of steatosis in mice fed high fat diets (Chang et al., 2006; Imai et al., 2007). ADPH immunofluorescence has also been shown to correlate with steatosis induced by alcohol consumption in rats (Mak et al., 2008), and ADPH, perilipin, and TIP47 have been detected on CLD in liver biopsies from patients with alcoholic and nonalcoholic liver disease by immunofluorescence (Straub et al., 2008). Evidence of functional linkage between hypoxia and ADPH transcript expression in HepG2 cells (Saarikoski et al., 2002) further suggest that genes of PAT family members may be direct targets of physiological mediators of steatosis. The possibility that PAT proteins differentially affect steatotic progression, and CLD growth is suggested by observations that PAT protein family members appear to differentially coat distinct populations of CLD in human liver biopsies (Straub et al., 2008) and by evidence from differentiating 3T3-L1 adipocytes indicating that CLD become sequentially coated by TIP47, ADPH, and perilipin as they undergo enlargement (Wolins et al., 2006). Importantly, alterations in CLD protein composition (including PAT family members) in response to lipolytic stimulation demonstrate that CLD are dynamic structures responsive to physiological stimuli (Brasaemle, 2007; Clifford et al., 2000; Gross et al., 2006). In this study, we used the well-established Lieber-DeCarli ethanol feeding model (Lieber and DeCarli, 1970) in mice to test the hypotheses that dietary ethanol induction of steatosis involves distinct alterations in CLD properties, including their protein composition, and is mediated by specific metabolic stress processes. We also investigated the relative abilities of perilipin, ADPH, and TIP47 to promote CLD accumulation in response to fat and ethanol in cell culture models in which these proteins were stably expressed. Our results demonstrate that feeding mice the Lieber-DeCarli ethanol diet (LD-Et) only modestly increased hepatic triglycerides over that of calorically matched Lieber-DeCarli control (LD-Co) diet-fed animals after 6 weeks. However, CLD in livers of LD-Et-fed mice differed markedly from those in LD-Co-fed mice in their larger size, their predominant zone 2 distribution, the presence of perilipin on their surface, and their association with ER stress and lipid peroxide adduction in hepatocytes. Our cell culture studies provide evidence that the presence of perilipin on CLD is sufficient to induce formation of large CLD. Together our data indicate that ethanol ingestion induces specific alterations in CLD protein properties that contribute to large CLD formation during hepatosteatosis.