[SalesForce] Retrieving All column names from a DataExtension

I am retrieving data from a data extension using RetrieveRequest. Following is a snippet of my approach:

RetrieveRequest retrieveRequest = new RetrieveRequest();
retrieveRequest.setObjectType(dataExtensionId);
String[] properties = new String[]{"hospital_long_name", "hospital_address", "client_name", "email_address"};
    for(String property : properties) {
       retrieveRequest.getProperties().add(property);
    }

RetrieveRequestMsg reqMessage = new RetrieveRequestMsg();
reqMessage.setRetrieveRequest(retrieveRequest);
RetrieveResponseMsg resMessage = exactTargetService.retrieve(reqMessage);

With the above approach, as you can see I am only able to retrieve 4 properties from my data extension. i.e ("hospital_long_name", "hospital_address", "client_name", "email_address")
I checked the documentation and I was unable to find a method through which I could just fetch all of the columns in a data extension without mentioning them explicitly in my request call.

Thanks for any suggestion.

Based on Kelly's suggestion , I tried something like this:

RetrieveRequest request = new RetrieveRequest();
request.setObjectType("DataExtensionObject[DE_" +key+"]");
SimpleFilterPart filterPart = new SimpleFilterPart();
filterPart.setProperty("CustomerKey");
SimpleOperators operand = SimpleOperators.EQUALS;
filterPart.setSimpleOperator(operand);
filterPart.getValue().add("DE_"+key);
request.setFilter(filterPart);  
RetrieveRequestMsg reqMessage = new RetrieveRequestMsg();
reqMessage.setRetrieveRequest(request);
RetrieveResponseMsg resMessage = exactTargetService.retrieve(reqMessage);

But it gave me an error saying "Object reference not set to an instance of an object".
Am I doing something wrong here?

Following is the SOAP request envelope. (Only Body Part):

<soap:Body>
    <RetrieveRequestMsg xmlns="http://exacttarget.com/wsdl/partnerAPI"
        xmlns:ns2="urn:fault.partner.exacttarget.com">
        <RetrieveRequest>
            <ObjectType>DataExtension</ObjectType>
            <Filter xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
                xsi:type="SimpleFilterPart">
                <Property>CustomerKey</Property>
                <SimpleOperator>equals</SimpleOperator>
                <Value>DE_Appointment_Reminder_H2882</Value>
            </Filter>
        </RetrieveRequest>
    </RetrieveRequestMsg>
</soap:Body>

Following is the SOAP response envelope (Only Body Part):

<soap:Body>
    <RetrieveResponseMsg xmlns="http://exacttarget.com/wsdl/partnerAPI">
        <OverallStatus>Error: Object reference not set to an instance of an
            object.</OverallStatus>
        <RequestID>f4c96918-276f-489d-84df-dea695c63383</RequestID>
    </RetrieveResponseMsg>
</soap:Body>