Impact of Pretreatment Weight Loss on Radiotherapy Utilization and Clinical Outcomes in Non-Small Cell Lung Cancer

BACKGROUND

Cancer cachexia is a syndrome of unintentional weight loss resulting in progressive functional impairment. Knowledge of radiation therapy utilization in patients with cancer cachexia is limited. We evaluated the use of curative and palliative-intent radiation for the management of patients with non-small cell lung cancer (NSCLC) with cachexia to determine whether tumor-directed therapy affected cachexia-associated outcomes.

METHODS

Using an Institutional Tumor Registry, we evaluated all patients with stages of NSCLC treated at a tertiary care system from 2006 to 2013. We adopted the international consensus definition for cachexia, with staging designated by the registry and positron emission tomography. Radiotherapy delivery and intent were retrospectively assessed.

RESULTS

In total, 1330 patients with NSCLC were analyzed. Curative-intent radiotherapy was utilized equally between patients with cachexia and non-cachexia with stages I to III NSCLC. Conversely, significantly more patients with stage IV disease and cachexia received palliative radiotherapy versus those without (74% vs 63%, P = 0.006). Cachexia-associated survival was unchanged irrespective of tumor-directed radiation therapy with curative or palliative intent. In fact, pretreatment cachexia was associated with reduced survival for patients with stage III NSCLC receiving curative-intent radiotherapy (median survival = 23.9 vs 15.0 mo, P = 0.009). Finally, multivariate analysis identified pretreatment cachexia as an independent variable associated with worsened survival (hazard ratio = 1.31, CI: 1.14,1.52).

CONCLUSION

Patients with advanced NSCLC with cachexia received more palliative-intent radiation than those without weight loss. Tumor-directed therapy in either a curative or palliative approach failed to alter cachexia patient survival across all stages of the disease. These findings offer critical information on the appropriate utilization of radiation in the management of patients with NSCLC with cachexia.

Tumor loss-of-function mutations in STK11/LKB1 induce cachexia

Cancer cachexia (CC), a wasting syndrome of muscle and adipose tissue resulting in weight loss, is observed in 50% of patients with solid tumors. Management of CC is limited by the absence of biomarkers and knowledge of molecules that drive its phenotype. To identify such molecules, we injected 54 human non-small cell lung cancer (NSCLC) lines into immunodeficient mice, 17 of which produced an unambiguous phenotype of cachexia or non-cachexia. Whole-exome sequencing revealed that 8 of 10 cachexia lines, but none of the non-cachexia lines, possessed mutations in serine/threonine kinase 11 (STK11/LKB1), a regulator of nutrient sensor AMPK. Silencing of STK11/LKB1 in human NSCLC and murine colorectal carcinoma lines conferred a cachexia phenotype after cell transplantation into immunodeficient (human NSCLC) and immunocompetent (murine colorectal carcinoma) models. This host wasting was associated with an alteration in the immune cell repertoire of the tumor microenvironments that led to increases in local mRNA expression and serum levels of CC-associated cytokines. Mutational analysis of circulating tumor DNA from patients with NSCLC identified 89% concordance between STK11/LKB1 mutations and weight loss at cancer diagnosis. The current data provide evidence that tumor STK11/LKB1 loss of function is a driver of CC, simultaneously serving as a genetic biomarker for this wasting syndrome.

A preponderance of gastrointestinal cancer patients transition into cachexia syndrome

BACKGROUND

Cancer cachexia is frequently documented by self-reported, single time-point weight histories. This approach lacks the granularity needed to fully elucidate the progression of cachexia syndrome. This study aimed to longitudinally assess body weight changes pre- and post-cancer diagnosis in gastrointestinal (GI) cancer patients.

METHODS

Body weights and relevant clinical data recorded in the electronic health record 12 months pre- and post-GI cancer (colorectal, gastroesophageal, hepatobiliary and pancreatic) diagnosis were extracted. Weight loss was categorized by the International Consensus Definition for cachexia.

RESULTS

A total of 879 patients were included in the final cohort including patients diagnosed with colorectal (n = 317), hepatocellular (n = 185), biliary (n = 72), pancreatic (n = 186) or gastroesophageal (n = 119) cancer. Stage of disease was equally distributed. Patients without cachexia at diagnosis (n = 608) remained weight stable during the 12 months pre-diagnosis (+0.5 ± 0.5% body weight; P = 0.99). Patients with cachexia at diagnosis (n = 271) remained weight stable 6 to 12 months prior to diagnosis (+0.4 ± 0.8%; P > 0.9999) and lost 8.7 ± 0.6% (P < 0.0001) within the 6 months pre-diagnosis. Patients without cachexia at diagnosis lost more weight post-diagnosis (6.3 ± 0.6%) than patients with cachexia at diagnosis (4.7 ± 1.0%; P = 0.01). Pre-diagnosis weight trajectories did not differ between primary malignancies or stage of disease in patients without or with cachexia at diagnosis (all P ≥ 0.05). Post-diagnosis weight trajectories did differ by primary malignancy (P ≤ 0.0002) and stage (P < 0.0001). In both patients without and with cachexia at diagnosis, colorectal patients lost the least amount of weight post-diagnosis and gastroesophageal patients lost the most amount of weight post-diagnosis. Stage 4 patients without or with cachexia at diagnosis lost the most weight post-diagnosis (P ≤ 0.0003). Regardless of cachexia status at diagnosis, patients lost more weight when treated with systemic therapy (7.1 ± 0.7%; P < 0.0001; n = 419) or radiation therapy (8.4 ± 1.4%; P = 0.02; n = 116) compared to those who did not. Patients who did not have surgery lost more weight post-diagnosis (7.6 ± 1.1%; P < 0.0001; n = 355) compared to those who did have surgery. By 12 months post-diagnosis, 83% of the surviving GI cancer patients in this cohort had transitioned into cachexia syndrome.

CONCLUSIONS

Significant weight loss in patients with GI cancer cachexia at diagnosis initiates at least 6 months prior to diagnosis, and most patients will transition into cachexia syndrome post-diagnosis, regardless of pre-diagnosis weight change and stage of disease. These findings punctuate the importance of weight surveillance in cancer detection and earlier palliative interventions post-diagnosis in the GI cancer patient population.

Association of race, ethnicity, and socioeconomic characteristics as predictors of cachexia risk and survival in stage IV non–small cell lung cancer (NSCLC)

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Background: Cachexia, seen in more than a third of patients with non-small cell lung cancer (NSCLC), directly leads to functional and survival detriments. As screening and interventions for cachexia and NSCLC improve, deficits of healthcare access and quality among patients disadvantaged by race and socioeconomic factors must be addressed. Methods: Through retrospective and prospective data collection, we established a cohort of 957 patients diagnosed with stage IV NSCLC between 2014-2020 in Dallas, Texas. Presence of cachexia at diagnosis was determined using consensus criteria for substantial (5% for BMI≥20 and 2% for BMI < 20) unintentional weight loss in the 6 months leading to diagnosis. Analyses including multivariate logistic regression and log-rank testing evaluated for significant associations of variables with cachexia incidence and survival. Results: Black race and Hispanic ethnicity independently associated with more than a 70% increased risk of cachexia at NSCLC diagnosis (Table; P < 0.05). Inclusion of private insurance status as a covariate diminished this finding for Hispanic patients only. Black patients presented at younger ages than White patients (Kruskal-Wallis P =0.0012; T-test P = 0.0002), with an average difference of 3 years. Cachexia at diagnosis was a major predictor of survival, highlighting the importance of addressing differential cachexia risk across race/ethnicity. Conclusions: The elevated cachexia risk observed for Black patients independent of insurance status reveals broader causative factors of disparity. A lack of private insurance may be a primary contributor in Hispanic cachexia prevalence. Targeting these differences could mitigate survival detriments linked to cachexia in minority groups. Supported contributors in broader NSCLC outcome inequities include poor representation in trials substantiating USPSTF guidelines as well as preventable barriers to primary and preventative care. Our interpretation of NSCLC outcome disparity in the context of cachexia demonstrates further consequences of these deficits. Moreover, our findings suggest novel targets for health policy by highlighting the prevalence of undiagnosed unintentional weight loss in minority populations with late-stage NSCLC.[Table: see text]

Cytokine-Mediated STAT3 Transcription Supports ATGL/CGI-58-Dependent Adipocyte Lipolysis in Cancer Cachexia

Adipose tissue inflammation is observed in multiple metabolically-altered states including cancer-associated cachexia and obesity. Although cachexia is a syndrome of adipose loss and obesity is a disease of adipose excess, both pathologies demonstrate increases in circulating levels of IL-6 family cytokines, β-adrenergic signaling, and adipocyte lipolysis. While β-adrenergic-stimulated adipocyte lipolysis is well described, there is limited mechanistic insight into how cancer cachexia-associated inflammatory cytokines contribute to adipocyte lipolysis under pathologic conditions. Here, we set out to compare adipocyte lipolysis signaling by cancer cachexia-associated IL-6 family cytokines (IL-6 and LIF) to that of the β-adrenergic agonist isoproterenol. Unlike isoproterenol, the IL-6 family of cytokines required JAK/STAT3-dependent transcriptional changes to induce adipocyte lipolysis. Furthermore, cachexia-associated cytokines that used STAT3 to induce lipolysis were primarily dependent on the lipase ATGL and its cofactor CGI-58 rather than lipases HSL and MAGL. Finally, administration of JAK but not β-adrenergic inhibitors suppressed adipose STAT3 phosphorylation and associated adipose wasting in a murine model of cancer cachexia characterized by increased systemic IL-6 family cytokine levels. Combined, our results demonstrate how the IL-6 family of cytokines diverge from β-adrenergic signals by employing JAK/STAT3-driven transcriptional changes to promote adipocyte ATGL/CGI-58-dependent lipolysis contributing to adipose wasting in cancer cachexia.

The impact of NSCLC disease evolution and treatment on pre- and post-diagnosis weight loss and cachexia designation

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Background: While cancer cachexia is associated with increased morbidity/mortality, little is known about longitudinal weight changes and opportunities for intervention pre- and post-diagnosis in this patient population. We therefore aimed to characterize body weight trajectories pre- and post- non-small cell lung cancer (NSCLC) diagnosis in patients without and with cachexia. Methods: Data were retrospectively extracted for each patient pre- and post-diagnosis of NSCLC from inpatient and outpatient visits within the UT Southwestern Medical Center Healthcare System. 3830 participants were screened with a NSCLC diagnosis between May 2005 to December 2019. Of these, 661 patients had a body weight recorded at cancer diagnosis and at least one body weight pre-diagnosis. The primary outcome was weight change over time. Other measures included defining associations between weight change and tumor pathology, stage of disease, and other clinical characteristics. Results: NSCLC patients without cachexia at cancer diagnosis were weight stable up to 12-months pre-diagnosis (+1.0±0.3% body weight; p = 0.01). Weight loss post-diagnosis was driven by stage of disease (stage 1/2: 0.4±0.5% body weight vs. stage 3/4: 4.8±0.9% body weight p = 0.0008) and treatment with chemotherapy (p < 0.0001). NSCLC patients with cachexia at cancer diagnosis lost weight pre-diagnosis (8.8±0.5% body weight; p < 0.0001), with 25% of this weight loss occurring between 6- and 12-months pre-diagnosis. Assessing weight change over 12 months opposed to 6 months resulted in a 32% increase in NSCLC patients with cachexia at cancer diagnosis (p = 0.006). By 12-months post-diagnosis, 62% of surviving late-stage and 45% of early-stage NSCLC patients had met the criteria for cachexia. Patients on average remained free of weight loss for 150±6.4 days or had pre-cachexia for 146±7.2 days prior to transitioning to a more advanced stage of the syndrome. Conclusions: Significant weight loss in NSCLC patients initiated as far back as 12 months prior to a cancer diagnosis as opposed to a shorter 6-month interval currently used to define patients with cachexia. Therefore, detection of earlier weight loss should be prioritized in high-risk NSCLC screening candidates. Finally, advanced NSCLC patients develop cachexia post-diagnosis irrespective of previous weight history, highlighting the impact of treatment iatrogenicity and/or refractory disease on cachexia development.

Cachexia is Prevalent in Patients With Hepatocellular Carcinoma and Associated With Worse Prognosis

BACKGROUND & AIMS

Cancer cachexia is a wasting syndrome associated with functional impairment and reduced survival that impacts up to 50% of patients with gastrointestinal cancers. However, data are limited on the prevalence and clinical significance of cachexia in patients with hepatocellular carcinoma (HCC).

METHODS

We performed a retrospective cohort study of patients diagnosed with HCC at 2 United States health systems between 2008 and 2018. Patient weights were recorded 6 months prior to and at time of HCC diagnosis. Cachexia was defined as >5% weight loss (or >2% weight loss if body mass index <20 kg/m²), and precachexia was defined as 2% to 5% weight loss. We used multivariable logistic regression models to identify correlates of cachexia and multivariable Cox proportional hazard models to identify factors associated with overall survival.

RESULTS

Of 604 patients with HCC, 201 (33.3%) had precachexia and 143 (23.7%) had cachexia at diagnosis, including 19.0%, 23.5%, 34.7%, and 34.0% of patients with Barcelona Clinic Liver Cancer stages 0/A, B, C, and D, respectively. Patients with cachexia were less likely to receive HCC treatment (odds ratio, 0.38; 95% confidence interval, 0.21-0.71) and had worse survival than those with precachexia or stable weight (11.3 vs 20.4 vs 23.5 months, respectively; P < .001). Cachexia remained independently associated with worse survival (hazard ratio, 1.43; 95% confidence interval, 1.11-1.84) after adjusting for age, sex, race, ethnicity, Child Pugh class, alpha-fetoprotein, Barcelona Clinic Liver Cancer stage, and HCC treatment.

CONCLUSIONS

Nearly 1 in 4 patients with HCC present with cachexia, including many with compensated cirrhosis or early stage tumors. The presence of cancer-associated weight loss appears to be an early and independent predictor of worse outcomes in patients with HCC.

LIFR-α-dependent adipocyte signaling in obesity limits adipose expansion contributing to fatty liver disease

The role of chronic adipose inflammation in diet-induced obesity (DIO) and its sequelae including fatty liver disease remains unclear. Leukemia inhibitory factor (LIF) induces JAK-dependent adipocyte lipolysis and altered adipo/cytokine expression, suppressing in vivo adipose expansion in normal and obese mouse models. To characterize LIF receptor (LIFR-α)-dependent cytokine signaling in DIO, we created an adipocyte-specific LIFR knockout mouse model (Adipoq-Cre;LIFR^fl/fl). Differentiated adipocytes derived from this model blocked LIF-induced triacylglycerol lipolysis. Adipoq-Cre;LIFR^fl/fl mice on a high-fat diet (HFD) displayed reduced adipose STAT3 activation, 50% expansion in adipose, 20% body weight increase, and a 75% reduction in total hepatic triacylglycerides compared with controls. To demonstrate that LIFR-α signals adipocytes through STAT3, we also created an Adipoq-Cre;STAT3^fl/fl model that showed similar findings when fed a HFD as Adipoq-Cre;LIFR^fl/fl mice. These findings establish the importance of obesity-associated LIFR-α/JAK/STAT3 inflammatory signaling in adipocytes, blocking further adipose expansion in DIO contributing to ectopic liver triacylglyceride accumulation.

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LIFR-α signaling induces adipocyte lipolysis, restricting adipose expansion in DIO

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LIFR-α signaling requires STAT3 for adipocyte lipolysis

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LIFR-α/JAK/STAT3 lipolysis signaling in adipocytes promotes hepatic steatosis

Cholesterol Stabilizes TAZ in Hepatocytes to Promote Experimental Non-alcoholic Steatohepatitis

Incomplete understanding of how hepatosteatosis transitions to fibrotic non-alcoholic steatohepatitis (NASH) has limited therapeutic options. Two molecules that are elevated in hepatocytes in human NASH liver are cholesterol, whose mechanistic link to NASH remains incompletely understood, and TAZ, a transcriptional regulator that promotes fibrosis but whose mechanism of increase in NASH is unknown. We now show that increased hepatocyte cholesterol upregulates TAZ and promotes fibrotic NASH. ASTER-B/C-mediated internalization of plasma membrane cholesterol activates soluble adenylyl cyclase (sAC; ADCY10), triggering a calcium-RhoA-mediated pathway that suppresses β-TrCP/proteasome-mediated TAZ degradation. In mice fed with a cholesterol-rich NASH-inducing diet, hepatocyte-specific silencing of ASTER-B/C, sAC, or RhoA decreased TAZ and ameliorated fibrotic NASH. The cholesterol-TAZ pathway is present in primary human hepatocytes, and associations among liver cholesterol, TAZ, and RhoA in human NASH liver are consistent with the pathway. Thus, hepatocyte cholesterol contributes to fibrotic NASH by increasing TAZ, suggesting new targets for therapeutic intervention.

Ostreolysin A and anthrolysin O use different mechanisms to control movement of cholesterol from the plasma membrane to the endoplasmic reticulum

Recent studies using two cholesterol-binding bacterial toxin proteins, perfringolysin O (PFO) and domain 4 of anthrolysin O (ALOD4), have shown that cholesterol in the plasma membranes (PMs) of animal cells resides in three distinct pools. The first pool comprises mobile cholesterol, accessible to both PFO and ALOD4, that is rapidly transported to the endoplasmic reticulum (ER) to signal cholesterol excess and maintain cholesterol homeostasis. The second is a sphingomyelin (SM)-sequestered pool inaccessible to PFO and ALOD4 but that becomes accessible by treatment with SM-degrading sphingomyelinase (SMase). The third is an essential pool also inaccessible to PFO and ALOD4 that cannot be liberated by SMase treatment. The accessible cholesterol pool can be trapped on PMs of live cells by nonlytic ALOD4, blocking its transport to the ER. However, studies of the two other pools have been hampered by a lack of available tools. Here, we used ostreolysin A (OlyA), which specifically binds SM/cholesterol complexes in membranes, to study the SM-sequestered cholesterol pool. Binding of nonlytic OlyA to SM/cholesterol complexes in PMs of live cells depleted the accessible PM cholesterol pool detectable by ALOD4. Consequently, transport of accessible cholesterol from PM to ER ceased, thereby activating SREBP transcription factors and increasing cholesterol synthesis. Thus, OlyA and ALOD4 both control movement of PM cholesterol, but through different lipid-binding mechanisms. We also found that PM-bound OlyA was rapidly internalized into cells, whereas PM-bound ALOD4 remained on the cell surface. Our findings establish OlyA and ALOD4 as complementary tools to investigate cellular cholesterol transport.

Cachexia-associated adipose loss induced by tumor-secreted leukemia inhibitory factor is counterbalanced by decreased leptin

Cachexia syndrome consists of adipose and muscle loss, often despite normal food intake. We hypothesized that cachexia-associated adipose wasting is driven in part by tumor humoral factors that induce adipocyte lipolysis. We developed an assay to purify secreted factors from a cachexia-inducing colon cancer line that increases lipolysis in adipocytes and identified leukemia inhibitory factor (LIF) by mass spectrometry. Recombinant LIF induced lipolysis in vitro. Peripheral LIF administered to mice caused >50% loss of adipose tissue and >10% reduction in body weight despite only transient hypophagia due to decreasing leptin. LIF-injected mice lacking leptin (ob/ob) resulted in persistent hypophagia and loss of adipose tissue and body weight. LIF's peripheral role of initiating lipolysis in adipose loss was confirmed in pair-fed ob/ob mouse studies. Our studies demonstrate that (a) LIF is a tumor-secreted factor that promotes cachexia-like adipose loss when administered peripherally, (b) LIF directly induces adipocyte lipolysis, (c) LIF has the ability to sustain adipose and body weight loss through an equal combination of peripheral and central contributions, and (d) LIF's central effect is counterbalanced by decreased leptin signaling, providing insight into cachexia's wasting, despite normophagia.

Identification of surface residues on Niemann-Pick C2 essential for hydrophobic handoff of cholesterol to NPC1 in lysosomes

Water-soluble Niemann-Pick C2 (NPC2) and membrane-bound NPC1 are cholesterol-binding lysosomal proteins required for export of lipoprotein-derived cholesterol from lysosomes. The binding site in NPC1 is located in its N-terminal domain (NTD), which projects into the lysosomal lumen. Here we perform alanine-scanning mutagenesis to identify residues in NPC2 that are essential for transfer of cholesterol to NPC1(NTD). Transfer requires three residues that form a patch on the surface of NPC2. We previously identified a patch of residues on the surface of NPC1(NTD) that are required for transfer. We present a model in which these two surface patches on NPC2 and NPC1(NTD) interact, thereby opening an entry pore on NPC1(NTD) and allowing cholesterol to transfer without passing through the water phase. We refer to this transfer as a hydrophobic handoff and hypothesize that this handoff is essential for cholesterol export from lysosomes.

Structure of N-terminal domain of NPC1 reveals distinct subdomains for binding and transfer of cholesterol

LDL delivers cholesterol to lysosomes by receptor-mediated endocytosis. Exit of cholesterol from lysosomes requires two proteins, membrane-bound Niemann-Pick C1 (NPC1) and soluble NPC2. NPC2 binds cholesterol with its isooctyl side chain buried and its 3beta-hydroxyl exposed. Here, we describe high-resolution structures of the N-terminal domain (NTD) of NPC1 and complexes with cholesterol and 25-hydroxycholesterol. NPC1(NTD) binds cholesterol in an orientation opposite to NPC2: 3beta-hydroxyl buried and isooctyl side chain exposed. Cholesterol transfer from NPC2 to NPC1(NTD) requires reorientation of a helical subdomain in NPC1(NTD), enlarging the opening for cholesterol entry. NPC1 with point mutations in this subdomain (distinct from the binding subdomain) cannot accept cholesterol from NPC2 and cannot restore cholesterol exit from lysosomes in NPC1-deficient cells. We propose a working model wherein after lysosomal hydrolysis of LDL-cholesteryl esters, cholesterol binds NPC2, which transfers it to NPC1(NTD), reversing its orientation and allowing insertion of its isooctyl side chain into the outer lysosomal membranes.

Purified NPC1 protein: II. Localization of sterol binding to a 240-amino acid soluble luminal loop

Defects in Niemann-Pick, Type C-1 protein (NPC1) cause cholesterol, sphingolipids, phospholipids, and glycolipids to accumulate in lysosomes of liver, spleen, and brain. In cultured fibroblasts, NPC1 deficiency causes lysosomal retention of lipoprotein-derived cholesterol after uptake by receptor-mediated endocytosis. NPC1 contains 1278 amino acids that form 13 membrane-spanning helices and three large loops that project into the lumen of lysosomes. We showed earlier that NPC1 binds cholesterol and oxysterols. Here we localize the binding site to luminal loop-1, a 240-amino acid domain with 18 cysteines. When produced in cultured cells, luminal loop-1 was secreted as a soluble dimer. This loop bound [(3)H]cholesterol (K(d), 130 nM) and [(3)H]25-hydroxycholesterol (25-HC, K(d), 10 nM) with one sterol binding site per dimer. Binding of both sterols was competed by oxysterols (24-, 25-, and 27-HC). Unlabeled cholesterol competed strongly for binding of [(3)H]cholesterol, but weakly for [(3)H]25-HC binding. Binding of [(3)H]cholesterol but not [(3)H]25-HC was inhibited by detergents. We also studied NPC2, a soluble protein whose deficiency causes a similar disease phenotype. NPC2 bound cholesterol, but not oxysterols. Epicholesterol and cholesteryl sulfate competed for [(3)H]cholesterol binding to NPC2, but not NPC1. Glutamine 79 in luminal loop-1 of NPC-1 is important for sterol binding; a Q79A mutation abolished binding of [(3)H]cholesterol and [(3)H]25-HC to full-length NPC1. Nevertheless, the Q79A mutant restored cholesterol transport to NPC1-deficient Chinese hamster ovary cells. Thus, the sterol binding site on luminal loop-1 is not essential for NPC1 function in fibroblasts, but it may function in other cells where NPC1 deficiency produces more complicated lipid abnormalities.

Purified NPC1 protein. I. Binding of cholesterol and oxysterols to a 1278-amino acid membrane protein

The Niemann-Pick, Type C1 protein (NPC1) is required for the transport of lipoprotein-derived cholesterol from lysosomes to endoplasmic reticulum. The 1278-amino acid, polytopic membrane protein has not been purified, and its mechanism of action is unknown. Unexpectedly, we encountered NPC1 in a search for a membrane protein that binds 25-hydroxycholesterol (25-HC) and other oxysterols. A 25-HC-binding protein was purified more than 14,000-fold from rabbit liver membranes and identified as NPC1 by mass spectroscopy. We prepared recombinant human NPC1 and confirmed its ability to bind oxysterols, including those with a hydroxyl group on the 24, 25, or 27 positions. Hydroxyl groups on the 7, 19, or 20 positions failed to confer binding. Recombinant human NPC1 also bound [(3)H]cholesterol in a reaction inhibited by Nonidet P-40 above its critical micellar concentration. Low concentrations of unlabeled 25-HC abolished binding of [(3)H]cholesterol, but the converse was not true, i.e. unlabeled cholesterol, even at high concentrations, did not abolish binding of [(3)H]25-HC. NPC1 is not required for the known regulatory actions of oxysterols. Thus, in NPC1-deficient fibroblasts 25-HC blocked the processing of sterol regulatory element-binding proteins and activated acyl-CoA:cholesterol acyltransferase in a normal fashion. The availability of assays to measure NPC1 binding in vitro may further the understanding of ways in which oxysterols regulate intracellular lipid transport.