Protein phosphorylation/dephosphorylation of
tyrosine residues is an important regulatory mechanism in cell growth and differentiation. Previously it has been reported that
RC-160, an octapeptide analog of
somatostatin, and [D-Trp6]
LHRH, an agonist of
luteinizing hormone-releasing hormone (
LHRH), stimulate receptor-mediated activity of
tyrosine phosphatases (PTP) and reverse growth promotion of the
tyrosine kinase (PTK) class of oncogenes in
tumor cells. The effect of
RC-160 and [D-Trp6]
LHRH on
protein phosphorylation was further examined in surgical specimens of human
carcinomas.
Protein extracts of human ovarian, liver, breast and prostate
tumor samples were preincubated with
epidermal growth factor (
EGF) (10(-7) M) with or without [D-Trp6]
LHRH or
RC-160 (10(-6) M) at 25 degrees C for 2 h, followed by incubation for 10 min with [gamma-32p]
ATP. SDS-PAGE, Western blotting, autoradiography and densitometry were then used to quantify the phosphorylation level of individual
protein bands. It was found that
EGF enhanced, and [D-Trp6]
LHRH and
RC-160 reduced phosphorylation of a prominent 300-kDa band. Two
proteins (65 and 60 kDa), involved in growth control in tumor cell lines, were also identified in this study. The homology of substrate phosphorylation between induced PTK and PTP in the presence of
hormones provided evidence that these substrates might be identical or related in
tumors. These findings, along with the previous cell culture results, suggest that many solid
tumors may respond to treatment with analogues of
somatostatin and
LHRH. Collectively, the results further support the hypothesis that these 60-, 65- and 300-kDa
protein substrates may be involved in growth-message transduction.