Objective      To investigate whether all-trans retinoic acid (ATRA) could enhance the killing effect of ultrasound microbubble carrying herpes simplex virus thymidine kinase (HSV-TK) on hepatocellular carcinoma

in vivo

.       Methods      Nude mice were inoculated subcutaneously with SMMC-7721 tumor ce

lls. The anti-tumor effect of ATRA at different doses was tested by measuring the tumor size to obtain a dose without anti-tumor effect. SMMC-7721 tumor cells transplanted nude mice were randomized into 4 groups

PBS treatment group

ATRA treatment group

HSV-TK treatment group

combined treatment group. The corresponding treatments were given by tail injection at 0.2 ml/time

once per 3 d

for 15 d. The later 2 groups received an ultrasonic irradiation at 3 000 Hz

2 W/cm

2

for 5 min. ATRA was given by intraperitoneal injection at a dose of 1 mg/(kg·d) for 15 d. Plasmid gene was test by Western blotting. The tumors were removed for histopathological analysis. Protein Cx32 was detected by immunohistochemistry.       Results      ATRA showed no anti-tumor effect at a dose of 1 mg/kg

but it showed anti-tumor effect when the dose beyond 4 mg/kg. The inhibitory rate of tumor growth was 0 in PBS-treat group

(7.46±7.25)% in ATRA treatment group

(37.65±3.87)% in HSV-TK treatment group

and (59.04±3.08)% in combined treatment group (

F

=116.709

P

<

0.05; For combined treat group vs PBS treatment group

ATRA treatment group

HSV-TK treatment group

t

=59.04

51.55

and 21.39

respectively

P

<

0.05). HE staining showed that extensive necrosis was observed in combined treatment group when compared with the other groups. The expression of protein Cx32 were increased in the groups with ATRA treatment and mainly located in the cell membrane.       Conclusion      ATRA enhances the killing effect of ultrasound microbubble carrying HSV-TK on hepatocellular carcinoma

in vivo

which may be associated with promoting the expression of Cx32 by ATRA.