Objective To evaluate the effect of aminolevulinic acid-based photodynamic therapy (ALA-PDT) on the expression of protein kinase D1 (PKD1) in a cutaneous squamous cell carcinoma cell line A431,and to explore the mechanism underlying ALA-PDT-induced apoptosis of A431 ceils.Methods A431 cells were cultured in vitro,and cell counting kit-8 (CCK-8) assay was performed to select the optimal combination of ALA concentration and PDT dose with the strongest proliferation inhibitory effect.A431 ceils at exponential growth phase were randomly divided into 4 groups:control group receiving no treatment,ALA group treated with ALA solution alone,PDT group treated with PDT alone,and ALA-PDT group treated firstly with ALA solution and then with PDT.After 12-,24-,36-and 48-hour additional culture,CCK-8 assay was conducted to evaluate the cellular proliferation inhibition,and the apoptosis rate at the time point of the strongest proliferation inhibitory effect was measured by flow cytometry.RT-PCR was performed to determine the expression of protein kinase D1 gene (PRKD1) in A431 cells at different time points after the ALA-PDT treatment,and Western blot analysis to measure protein expression of PKD 1 and its phosphorylation at Tyr463 (pTyr463) and Ser916 (pSer916) in A431 cells.Results The combination of ALA at the concentration of 1.5 mmol/L with PDT at an irradiation dose of 2 J/cm2 was optimal due to its strongest proliferation inhibitory effect.After 12-,24-,36-and 48-hour additional culture,there were significant differences in the proliferation inhibition rate among the 4 groups (F =39.56,P ＜ 0.05).At 24 hours after the treatment,the ALA-PDT group showed significantly higher proliferation inhibition rate (46.26％ ± 1.25％) compared with the ALA group (14.65％ ± 0.33％,P ＜ 0.05),PDT group (14.96％ ± 0.68％,P ＜ 0.05) and control group (11.98％ ± 0.32％,P ＜ 0.05),as well as compared with that at 12 hours (P ＜ 0.05).At 24 hours after the treatment,the apoptosis rate significantly differed among the 4 groups (F =16.32,P ＜ 0.05),and the ALA-PDT group showed a significantly higher apoptosis rate (41.92％ ± 3.23％) compared with the control group (4.67％ ± 0.88％,P ＜ 0.05),ALA group (7.02％ ± 1.52％,P ＜ 0.05) and PDT group (8.37％ ± 0.59％,P ＜ 0.05).At 0,6,12,24,36 and 48 hours after the treatment,there were significant differences in the mRNA expression of PRKD 1 among the 4 groups (F =22.24,P ＜ 0.05),and the mRNA expression of PRKD1 at 24 hours was significantly lower than that at 0,6,12 hours (all P ＜ 0.05),but was not significantly different from that at 36 and 48 hours (both P ＞ 0.05).No significant difference in the Ser916-phosphorylated PKD1 expression was found among the 4 groups (F =1.53,P ＞ 0.05),while there were significant differences in the expression of PKD1 and Tyr463-phosphorylated PKD 1 among the 4 groups (F =10.04,8.27,both P ＜ 0.05).Additionally,the ALA-PDT group showed significantly lower expression of PKD 1 and Tyr463-phosphorylated PKD 1 compared with the control group,ALA group and PDT group (all P ＜ 0.05).Conclusion PKD1 may be involved in the photochemical process of A431 cell apoptosis induced by ALA-PDT,and may promote the occurrence of squamous cell carcinoma by Tyr463 phosphorylation.