Role of breast tumour kinase in the in vitro differentiation of HaCaT cells

BACKGROUND

Breast tumour kinase (BRK) is a newly identified non-receptor protein tyrosine kinase from a metastatic breast tumour. Its biological functions are still under extensive investigation. The mouse homologue Sik (Src-related intestinal kinase) has been implicated in mouse keratinocyte differentiation; however, not much is known about the functions of BRK in human cutaneous biology.

OBJECTIVES

Using HaCaT cells as an experimental model, to explore the mutual relationships between BRK and differentiation of human keratinocytes.

METHODS

Archival paraffin blocks of normal and pathological skin were retrieved for examining the in vivo distribution of BRK. Its expression and subcellular localization were examined via indirect immunofluorescence, and quantitative changes were analysed by Northern and Western blots. The kinase activity of BRK was determined by its autophosphorylation and phosphorylation of exogenous substrate in the in vitro kinase assay. Using a retroviral infection method, we established stably transfected HaCaT cells expressing vector, wild-type BRK or a kinase-defective mutant (K219M). Expression of the differentiation marker keratin 10 (K10) was compared among these cells using semiquantitative reverse transcription-polymerase chain reaction. Results Histochemical examination showed that BRK was expressed exclusively in suprabasal keratinocytes. Its distribution was both cytoplasmic and intranuclear. An enhanced regional suprabasal expression pattern was observed in the confluent areas of cell cultures. The expression of BRK transcript and protein was up-regulated in prolonged confluence culture in a serum-dependent manner. Its kinase activity was activated shortly after the addition of calcium and ionomycin and returned to the basal level within 30 min. Overexpression of wild-type BRK moderately promoted the expression of K10 transcript while the kinase-defective BRK mutant exerted a prominent suppressive effect.

CONCLUSIONS

The in vivo distribution of BRK and its up-regulation during in vitro differentiation of HaCaT cells, together with the activation of its kinase activity by calcium/ionomycin and its influence on K10 expression, all indicate a role for BRK in the complex process of keratinocyte differentiation.

Opposite functions for E2F1 and E2F4 in human epidermal keratinocyte differentiation

Proteins of the retinoblastoma family (pRb, p107, and p130) modulate cell proliferation, a function related to their capacity to control the activity of the E2F transcription factor family. The Rb proteins also control cell differentiation in different tissues. We have recently described their involvement in human epidermal keratinocyte differentiation (Paramio, J. M., Lain, S., Segrelles, C., Lane, E. B. , and Jorcano, J. L. (1998) Oncogene 17, 949-957). Here we show that E2F proteins are also involved in this process. We found that E2F1 and E2F4 are expressed differentially during the in vitro differentiation of human epidermal keratinocytes, with the former uniformly present throughout the process, whereas the second is predominantly expressed at the onset of differentiation. This pattern is also observed in human skin by confocal microscopy. Electrophoretic mobility shift assays and immunoprecipitation experiments demonstrated that the complexes formed by E2F1 and E2F4 and Rb family proteins vary throughout in vitro keratinocyte differentiation. In agreement with this observation, several E2F-responsive genes are differentially regulated during this process. To test the functional implications of these observations, we transfected HaCaT keratinocytes with plasmids coding for E2F1 and E2F4. Transfected cells display opposite in vitro differentiation properties. Although E2F1-transfected cells are unable to differentiate, E2F4-transfected cells show an increased differentiation rate compared with Neo-transfected control cells. Our data demonstrate that the differential and coordinated expression and interaction of E2F and Rb proteins modulate the process of epidermal differentiation and provide clear evidence that members of the E2F family of transcription factors play specific and opposite roles during cell differentiation.