Objective:To determine lysophosphatidic acid receptor 6 (LPAR6) expression in patients with mycosis fungoides (MF) , a variant of cutaneous T-cell lymphoma (CTCL) , and to investigate its role and mechanism of action in the development and prognosis of CTCL.Methods:A total of 110 patients with confirmed MF were collected from Department of Dermatology, Peking University First Hospital from 2011 to 2020, including 24 with large-cell transformation (LCT) and 25 with non-large cell transformation (NLCT) in the discovery cohort, and 24 with LCT and 37 with NLCT in the validation cohort. RNA sequencing and RT-PCR were conducted to determine the LPAR6 expression in patients in the discovery cohort and validation cohort respectively. LPAR6 expression was compared between patients with LCT and those with NLCT, and its effect on the prognosis of patients was evaluated. Two LPAR6-overexpressing CTCL cell lines MyLa and Sz4 were constructed to evaluate the effect of LPAR6 overexpression on proliferative activity of MyLa and Sz4 cells, with the cells normally expressing LPAR6 as the control group; after the treatment with LPAR6-related ligand lysophosphatidic acid (LPA) , 2S-OMPT, adenosine triphosphate (ATP) or adenosine (ADO) , the effects of LPAR6 activation on the proliferative activity and apoptosis of LPAR6-overexpressing MyLa and Sz4 cells were evaluated by the MTS method and flow cytometry respectively. Log-rank test was used for prognostic analysis, and t test or Mann-Whitney U test was used for comparisons between two groups. Results:As RNA sequencing showed, LPAR6 was one of the significantly underexpressed genes in the LCT group in the discovery cohort; in the validation cohort, LPAR6 expression (median[ Q1, Q3]) was significantly lower in the LCT group (204.90[81.90, 512.70]) than in the NLCT group (809.40[417.50, 1 829.20], U= 242.00, P= 0.002) ; in the two cohorts, the underexpression of LPAR6 was significantly associated with increased risk of poor prognosis (both P < 0.01) . Cell proliferation assay showed no significant difference in the proliferative activity of MyLa or Sz4 cells between the LPAR6 overexpression group and control group at 0, 24, 48 and 72 hours during the experiment (all P > 0.05) ; 48 hours after activation of LPAR6 by LPA, 2S-OMPT, ATP and ADO in MyLa cells, the LPAR6 overexpression group showed significantly decreased cellular proliferative activity (1.38 ± 0.01, 1.04 ± 0.01, 1.09 ± 0.03, 1.23 ± 0.01, respectively) compared the control group (1.73 ± 0.04, 1.23 ± 0.01, 1.24 ± 0.01, 1.42 ± 0.03, t= 30.33, 18.38, 4.78, 5.75, respectively, all P < 0.05) , but significantly increased cell apoptosis rate (17.93% ± 0.88%, 17.75% ± 0.35%, 23.97% ± 0.57%, 31.44% ± 0.34%, respectively) compared the control group (3.98% ± 0.03%, 7.81% ± 0.59%, 11.95% ± 0.85%, 12.02% ± 0.48%, t= 15.93, 14.49, 11.74, 33.01, respectively, all P < 0.05) ; 48 hours after activation of LPAR6 by 2S-OMPT and ADO in Sz4 cells, compared with the control group, the LPAR6 overexpression group also showed significantly decreased cellular proliferative activity (2S-OMPT: 1.29 ± 0.04 vs. 1.48 ± 0.01; ADO: 1.27 ± 0.01 vs. 1.51 ± 0.02; both P < 0.05) , but significantly increased cell apoptosis rate (2S-OMPT: 41.70% ± 0.70% vs. 29.35% ± 0.55%; ADO: 37.05% ± 0.15% vs. 24.60% ± 1.00%; both P < 0.05) . Conclusions:LPAR6 was underexpressed in the patients with LCT, and its underexpression was significantly associated with increased risk of poor prognosis. In vitro activation of LPAR6 could inhibit the proliferation of CTCL cells and promote their apoptosis, suggesting that the decrease of LPAR6 expression may be one of the important mechanisms underlying disease progression in patients with LCT.