Submitted on April 2, 2007
Accepted on June 18, 2007
Dexamethasone Modulates ErbB Tyrosine Kinase Expression and Signaling Through Multiple And Redundant Mechanisms In Cultured Rat Hepatocytes
Lawrence Allen Scheving1*, Renee Buchanan2, Michael A. Krause2, Xiuqi Zhang3, Mary C. Stevenson3, and William E. Russell4
1 Division of Endocrinology, Department of Pediatrics, Vanderbilt University, Nashville, Tennessee, United States; Digestive Disease Research Center, United States
2 Division of Endocrinology, Department of Pediatrics, Vanderbilt University, Nashville, Tennessee, United States
3 Division of Endocrinology, Department of Pediatrics, Vanderbilt University, Nashville, Tennessee, United States; Nashville, Tennessee, United States
4 Division of Endocrinology, Department of Pediatrics, Vanderbilt University, Nashville, Tennessee, United States; Department of Cell and Developmental Biology, Vanderbilt University, United States; Digestive Disease Research Center, United States; Vanderbilt Diabetes Center, United States; Vanderbilt Ingram Cancer Center, United States
* To whom correspondence should be addressed. E-mail: lawrence.a.scheving{at}vanderbilt.edu.
Glucocorticoids paradoxically exert both stimulatory and inhibitory effects on the proliferation of cultured rat hepatocytes. We studied the effects of dexamethasone, a synthetic glucocorticoid, on the proliferation of cultured rat hepatocytes. The timing of growth factor addition modified the action of high dose dexamethasone (10-6M) on DNA synthesis. When we added transforming growth factor-
(TGF) at the time of plating, 10-6M dexamethasone weakly stimulated DNA synthesis by 26% relative to cells cultured in dexamethasone-free media. When we delayed growth factor addition until 24-48 h after plating, 10-6M dexamethasone inhibited DNA synthesis by 50%. Using immunologic methods, we analyzed the expression and signaling patterns of the ErbB kinases in dexamethasone-treated cells. High dose dexamethasone stabilized the expression of EGFr and ErbB3, and it suppressed the de novo expression in ErbB2 that occurs during the third and fourth day of culture in 10-8M dexamethasone. High dose dexamethasone by 72 h suppressed basal and epidermal growth factor (EGF)-associated phosphorylation of ERK and AKT. The reduction in ERK 1/2 phosphorylation correlated with suppression of a culture-dependent increase in Son-of sevenless 1 (Sos1) and ERK1/2 expression. High dose dexamethasone in hepatocytes stabilized or upregulated several inhibitory effectors of EGFr/ErbB2 and ERK, including receptor-associated late transducer (RALT) and MAPK Phosphatase-1 (MKP-1), respectively. Thus, 10-6M dexamethasone exerts a time dependent and redundant inhibitory effect on EGFr-mediated proliferative signaling in hepatocytes, targeting not only the ErbB proteins but also their various positive and negative effectors.
